GPLv2 and GPLv3

Open Source

"Free as in freedom - not as in free beer". Free beer is nice, but freedom is even nicer.

I have been working with companies from different sections including consumer electronics, military applications, automotive and aeronautics. One common question, regardless of section, is "Can we really use Open Source in our product?". The answer is usually Yes, you can, but....

One common misunderstanding is to interpret Open Source as in free beer. This is kind of true for some Open Source, but that is nothing you can take for granted. The "rules for how the code may be used is specified by its license.

Of those who think they had understood the difference, there is a common misunderstanding that no Open Source software is free and does not belong in any commercial products. Both misunderstandings are of course wrong, but you have to make sure that you understand the licenses you are using.

Before you start to work with any source code (not only Open Source) you always have to take the license into consideration. If do your homework you can avoid surprises and practical implications that otherwise can cause you a delayed project or legal inconveniences.

In short, you have to know what you are doing, and that should not differ from other parts of your development.

Open Source Licenses

"Open source licenses are licenses that comply with the Open Source Definition — in brief, they allow software to be freely used, modified, and shared. To be approved by the Open Source Initiative (also known as the OSI), a license must go through the Open Source Initiative's license review process."

This text is taken from the Open Source Initiative webpage [4], which is an organization that works with defining criterea for Open Source and certificate licenses that comply with OSD (Open Source Definition).

Open Source Definition

Many licenses [5] are certified and may have different requirements for its users, but they all comply with these "rules":

Free Redistribution

The license shall not restrict any party from selling or giving away the software as a component of an aggregate software distribution containing programs from several different sources. The license shall not require a royalty or other fee for such sale.

Source Code

The program must include source code, and must allow distribution in source code as well as compiled form. Where some form of a product is not distributed with source code, there must be a well-publicized means of obtaining the source code for no more than a reasonable reproduction cost, preferably downloading via the Internet without charge. The source code must be the preferred form in which a programmer would modify the program. Deliberately obfuscated source code is not allowed. Intermediate forms such as the output of a preprocessor or translator are not allowed.

Derived Works

The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.

Integrity of The Author's Source Code

The license may restrict source-code from being distributed in modified form only if the license allows the distribution of "patch files" with the source code for the purpose of modifying the program at build time. The license must explicitly permit distribution of software built from modified source code. The license may require derived works to carry a different name or version number from the original software.

No Discrimination Against Persons or Groups

The license must not discriminate against any person or group of persons.

No Discrimination Against Fields of Endeavor

The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.

Distribution of License

The rights attached to the program must apply to all to whom the program is redistributed without the need for execution of an additional license by those parties.

License Must Not Be Specific to a Product

The rights attached to the program must not depend on the program's being part of a particular software distribution. If the program is extracted from that distribution and used or distributed within the terms of the program's license, all parties to whom the program is redistributed should have the same rights as those that are granted in conjunction with the original software distribution.

License Must Not Restrict Other Software

The license must not place restrictions on other software that is distributed along with the licensed software. For example, the license must not insist that all other programs distributed on the same medium must be open-source software.

License Must Be Technology-Neutral

No provision of the license may be predicated on any individual technology or style of interface.

GPL

GPL, or General Public License, is one of the most common Open Source Licenses you will find out there. At least version 2, GPLv2, is something you will encounter for sure if you intend to build an embedded Linux system as the kernel [6] itself is using this license.

GPLv2

So what do you need to comply with GPLv2 code? Basically, you need to provide the source code for all GPLv2 licensed code. Yes, that includes all your modifications too, and this part could seem scary at the first glare.

But will you need to make any changes? Probably. If you want to run Linux on your system you will probably have to make some adaptions to the Linux kernel specific for your board, those changes will follow the GPLv2 license and should be provided as well.

The license is stated as follows:

"The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. "

GPLv2, Sec.3 [2]

If any of those changes is your top-secret algorithm then you have done it wrong by design anyway. Please note that no installation information is required at all which makes it more sufficient for embedded devices.

Tivoization

Tivoization, TiVO ran GPLv2 only Linux kernel, but had HW signature keys that made it possible to only run signed kernels. Even if the TiVO did provide the kernel code, the TiVO customers could not build and install the firmware.

The Free Software Foundation(FSF) found this objectionable as it violates one of the purposes the GPLv2 license had. So FSF ended up with GPLv3 to solve this.

GPLv3

/media/gplv3.png

(Yes, this logo is under Public Domain [7] )

One big difference between v2 and v3 is this part

" “Installation Information” for a User Product means any methods, procedures, authorization keys, or other information required to install and execute modified versions of a covered work in that User Product from a modified version of its Corresponding Source. The information must suffice to ensure that the continued functioning of the modified object code is in no case prevented or interfered with solely because modification has been made. "

GPLv3, Sec .6 [1]

Which states that "Installation information" must be provided together with the source code, in short, you have to provide instruction for an end-user to build and replace GPLv3 parts of your product. But there are also a few exceptions. Most of them is more or less hard to make any use of in a real world product.

Exception 1

It only requred for "User Products". It is hard to say if this is an exception or not as most of the products that use GPLv3 is user products. But the license states that it only affects User products. Consult your lawyer as it is not entirely clear what a "User product" really are.

Exception 2

Only if device is sold or offered for long-term lease. As with all lega stuff, things are a bit unclear. Does remote access or temporary possessions qualify for example?

Please note that even long term lease need you to provide installation information.

Exception 3

If non-modifiable by anyone, you don't have to give the information how to reinstalling or modify binaries.

If you want to be able to update your software, then you will probably need to provide the "installation information".

Exception 4

You can void any warranties if binaries are modified, but you can't prevent reinstallation of modigied binaries.

Conclusion

There is a reason why an author of code chose to use any particular license, and it is important (both for principal and legal reasons) to respect that. Some licenses are more or less appropriate for specific products, but the general rule I follow is to avoid any GPLv3 (both GPLv3 and LGPLv3) licensed software in any embedded system as it hard to be fully compliant. The installation information is often something that companies want to keep for themself, with all rights.

What is my opinion about this? Well, I do like to have the freedom to install whatever software I want in the products I own, but there are circumstances where I'm not sure if it is a good idea when it comes to safety and liability. If I buy a car in second hand, I don't want the software for my airbag or braking system to be "fixed" by some random guy. I think that the Open Source has limited use in too restrictive licenses, and that is somehow counterproductive for the Open Source itself.

V4L2 and media controller

V4L2 and media controller

The media infrastructure in the kernel is a giant beast handling many different types of devices involving different busses and electrical interfaces. Providing an interface to handle the complexity of the hardware is not an easy task. Most devices have multiple ICs with different communication protocols... so the device drivers tends to be very complex as well.

Video For Linux 2 (V4L2) is the interface for such media devices. V4L2 is the second version of V4L and is not really compatible with V4L, even if there is a compatibility mode but the support is more than often incomplete. The name Video4Linux is a counterpart to Video4Windows, but is not technically related to it at all.

Here is an example of what a system may look like (borrowed from the Linux kernel documentation)

/media/typical_media_device.svg

Media controller

System-on-Chips (SoC) devices often provides wide range of hardware blocks that can be interconneced in a variety of ways to obtain the desired functionality. To configure these hardware blocks, the kernel provides the Media Controller kernel API which expose detailed information about the media device and let them to be interconnected in a dynamic and complex way at runtime, all from userspace.

Each hardware block, called entity, in the media controller framework has one or more source and sink pads. The API let the user link source to sink pads and set the format of pads.

Here is a the topology exported from my sabresd with an imx219 (camera module) connected:

/media/media-ctl-graph.png

Let's go through the entities in the picture. All these entities is of course specific for the iMX6 SoC. (Partly taken from the kernel documentation)

imx219 1-0010

This is the camera sensor. The sensor is controlled with I2C commands and the data stream is over the MIPI CSI-2 interface. The name tells us that the sensor is connected to I2C bus 1. The device has the address 0x10.

The entity has one source pad.

imx6-mipi-csi2

This is the MIPI CSI-2 receiver entity. It has one sink pad to receive the MIPI CSI-2 stream (usually from a MIPI CSI-2 camera sensor). It has four source pads, corresponding to the four MIPI CSI-2 demuxed virtual channel outputs. Multiple source pads can be enabled to independently stream from multiple virtual channels.

ipuX_csiY_mux

This is the video multiplexers. They have two or more sink pads to select from either camera sensors with a parallel interface, or from MIPI CSI-2 virtual channels from imx6-mipi-csi2 entity. They have a single source pad that routes to a CSI (ipuX_csiY entities).

ipuX_csiY

These are the CSI entities. They have a single sink pad receiving from either a video mux or from a MIPI CSI-2 virtual channel as described above.

ipuX_vdic

The VDIC carries out motion compensated de-interlacing, with three motion compensation modes: low, medium, and high motion. The mode is specified with the menu control V4L2_CID_DEINTERLACING_MODE. The VDIC has two sink pads and a single source pad.

ipuX_ic_prp

This is the IC pre-processing entity. It acts as a router, routing data from its sink pad to one or both of its source pads.

The direct sink pad receives from an ipuX_csiY direct pad. With this link the VDIC can only operate in high motion mode.

ipuX_ic_prpenc

This is the IC pre-processing encode entity. It has a single sink pad from ipuX_ic_prp, and a single source pad. The source pad is routed to a capture device node, with a node name of the format "ipuX_ic_prpenc capture".

This entity performs the IC pre-process encode task operations: color-space conversion, resizing (downscaling and upscaling), horizontal and vertical flip, and 90/270 degree rotation. Flip and rotation are provided via standard V4L2 controls.

Like the ipuX_csiY IDMAC source, this entity also supports simple de-interlace without motion compensation, and pixel reordering.

ipuX_ic_prpvf

This is the IC pre-processing viewfinder entity. It has a single sink pad from ipuX_ic_prp, and a single source pad. The source pad is routed to a capture device node, with a node name of the format "ipuX_ic_prpvf capture".

This entity is identical in operation to ipuX_ic_prpenc, with the same resizing and CSC operations and flip/rotation controls. It will receive and process de-interlaced frames from the ipuX_vdic if ipuX_ic_prp is receiving from ipuX_vdic.

Capture video stream from sensor

In order to capture a video stream from the sensor we need to:

  1. Create links between the needed entities
  2. Configure pads to hold the correct image format

To do this, we use the media-ctl [1] tool.

Configure pads

We also need to configure each pad to the right format. This image sensor is ouput in raw bayer format (SRGGB8).

export fmt=SRGGB8_1X8/640x480
media-ctl --set-v4l2 "'imx219 1-0010':0[fmt:$fmt field:none]"
media-ctl --set-v4l2 "'imx6-mipi-csi2':1[fmt:$fmt field:none]"
media-ctl --set-v4l2 "'ipu1_csi0_mux':5[fmt:$fmt field:none]"
media-ctl --set-v4l2 "'ipu1_csi0':1[fmt:$fmt field:none]"

Stream to framebuffer

Now a full pipe is created from imx219 to the video0 device.

GStreamer is a handy multimedia framework that we can use to test the full chain

gst-launch-1.0 -vvv v4l2src device=/dev/video0 io-mode=dmabuf blocksize=76800 ! "video/x-bayer,format=rggb,width=640,height=480,framerate=30/1" ! queue ! bayer2rgbneon ! videoconvert ! fbdevsink sync=false
/media/imx219.jpg

Parsing command line options

Parsing command line options

Parsing command line options is something allmost every command or applications needs to handle in some way, and there is too many home-made argument parsers out there. As so many programs needs to parse options from the command line, this facility is encapsulated in a standard library function getopt(2).

The GNU C library provides an even more sophisticated API for parsing the command line, argp(), and is described in the glibc manual [1]. However, this function is not portable.

There is also many libraries that provides such facilities, but lets keep us to what the glibc library provides.

Command line options

A typical UNIX command takes options in the following form

command [option] arguments

The options has the form of a hyphen (-) followed by a unique character and a possible argument. If the options take an argument, it may be separated from that argument by a white space. When multiple options is specified, those can be grouped after a single hyphen, and the last option in the group may be the only one that takes an argument.

Example on single option

ls -l

Example on grouped options

ls -lI *hidden* .

In the example above, the -l (long listing format) does not takes an argument, but -I (Ignore) takes *hidden* as argument.

Long options

It is not unusual that a command allows both a short (-I) and a long (--ignore) option syntax. A long option begins with two hyphens, and the option itself is identified using a word. If the options take an argument, it may be separated from that argument by a =.

To parse such options, use the getopt_long(2) glibc function, or the (nonportable) argp().

Example using getopt_long()

getopt_long() is quite simple to use. First we create a struct option and defines the following elements: * name is the name of the long option.

  • has_arg
    is: no_argument (or 0) if the option does not take an argu‐ ment; required_argument (or 1) if the option requires an argu‐ ment; or optional_argument (or 2) if the option takes an optional argument.
  • flag
    specifies how results are returned for a long option. If flag is NULL, then getopt_long() returns val. (For example, the calling program may set val to the equivalent short option character.) Otherwise, getopt_long() returns 0, and flag points to a variable which is set to val if the option is found, but left unchanged if the option is not found.
  • val
    is the value to return, or to load into the variable pointed
    to by flag.

The last element of the array has to be filled with zeros,

The next step is to iterate through all options and take care of the arguments.

Example code

Example code

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>

struct arguments
{
    int a;
    int b;
    int c;
    int area;
    int perimeter;
};


void print_usage() {
    printf("Usage: triangle [Ap] -a num -b num -c num\n");
}

int main(int argc, char *argv[]) {
    int opt= 0;
    struct arguments arguments;

    /* Default values. */
    arguments.a = -1;
    arguments.b = -1;
    arguments.c = -1;
    arguments.area = 0;
    arguments.perimeter = 0;


    static struct option long_options[] = {
        {"area",      no_argument,       0,  'A' },
        {"perimeter", no_argument,       0,  'p' },
        {"hypotenuse",required_argument, 0,  'c' },
        {"opposite",  required_argument, 0,  'a' },
        {"adjecent",  required_argument, 0,  'b' },
        {0,           0,                 0,  0   }
    };

    int long_index =0;
    while ((opt = getopt_long(argc, argv,"Apa:b:c:",
                   long_options, &long_index )) != -1) {
        switch (opt) {
             case 'A':
                 arguments.area = 1;
                 break;
             case 'p':
                arguments.perimeter = 1;
                 break;
             case 'a':
                 arguments.a = atoi(optarg);
                 break;
             case 'b':
                 arguments.b = atoi(optarg);
                 break;
             case 'c':
                 arguments.c = atoi(optarg);
                 break;
             default: print_usage();
                 exit(EXIT_FAILURE);
        }
    }


    if (arguments.a == -1 || arguments.b == -1 || arguments.c == -1) {
        print_usage();
        exit(EXIT_FAILURE);
    }

    if (arguments.area) {
        arguments.area = (arguments.a*arguments.b)/2;
        printf("Area: %d\n",arguments.area);
    }

    if (arguments.perimeter) {
        arguments.perimeter = arguments.a + arguments.b + arguments.c;
        printf("Perimeter: %d\n",arguments.perimeter);
    }

    return 0;
}

Example of usages

Full example with short options

[13:49:00]marcus@little:~/tmp/cmdline$ ./getopt  -Ap -a 3 -b 4 -c 5
Area: 6
Perimeter: 12

Missing -c option

[14:07:37]marcus@little:~/tmp/cmdline$ ./getopt  -Ap -a 3 -b 4
Usage: triangle [Ap] -a num -b num -c num

Full example with long options

[14:09:38]marcus@little:~/tmp/cmdline$ ./getopt  --area --perimeter --opposite 3 --adjecent 4 --hypotenuse 5
Area: 6
Perimeter: 12

Invalid options

[14:10:14]marcus@little:~/tmp/cmdline$ ./getopt  --area --perimeter --opposite 3 --adjecent 4 -j=3
./getopt: invalid option -- 'j'
Usage: triangle [Ap] -a num -b num -c num

Full example with mixed syntaxes

[14:09:38]marcus@little:~/tmp/cmdline$ ./getopt  -A --perimeter --opposite=3 -b4 -c 5
Area: 6
Perimeter: 12

Variants

getopt_long_only() is like getopt_long(), but '-' as well as "--" can indicate a long option. If an option that starts with '-' (not "--") doesn't match a long option, but does match a short option, it is parsed as a short option instead.

Example using argp()

argp() is a more flexible and powerful than getopt() with friends, but it is not part of the POSIX standard and is therefr not portable between different POSIX-compatible operating systems. However, argp() provides a few interresting features that getopt() does not.

These features include automatically producing output in response to the ‘--help’ and ‘--version’ options, as described in the GNU coding standards. Using argp makes it less likely that programmers will neglect to implement these additional options or keep them up to date.

The implementation is pretty much straigt forwards and similiar to getopt() with a few notes.

const char *argp_program_version = Triangle 1.0";
const char *argp_program_bug_address = "<marcus.folkesson@combitech.se>";

Is used in automatic generation for the --help and --version options.

struct argp_option

This structure specifies a single option that an argp parser understands, as well as how to parse and document that option. It has the following fields:

  • const char *name
    The long name for this option, corresponding to the long option --name; this field may be zero if this option only has a short name. To specify multiple names for an option, additional entries may follow this one, with the OPTION_ALIAS flag set. See Argp Option Flags.
  • int key
    The integer key provided by the current option to the option parser. If key has a value that is a printable ASCII character (i.e., isascii (key) is true), it also specifies a short option ‘-char’, where char is the ASCII character with the code key.
  • const char *arg
    If non-zero, this is the name of an argument associated with this option, which must be provided (e.g., with the --name=value or -char value syntaxes), unless the OPTION_ARG_OPTIONAL flag (see Argp Option Flags) is set, in which case it may be provided.
  • int flags
    Flags associated with this option, some of which are referred to above. See Argp Option Flags.
  • const char *doc
    A documentation string for this option, for printing in help messages.

If both the name and key fields are zero, this string will be printed tabbed left from the normal option column, making it useful as a group header. This will be the first thing printed in its group. In this usage, it’s conventional to end the string with a : character.

Example code

Example code with little more comments

#include <stdlib.h>
#include <argp.h>

const char *argp_program_version = "Triangle 1.0";
const char *argp_program_bug_address = "<marcus.folkesson@combitech.se>";

/* Program documentation. */
static char doc[] = "Triangle example";

/* A description of the arguments we accept. */
static char args_doc[] = "ARG1 ARG2";

/* The options we understand. */
static struct argp_option options[] = {
    {"area",        'A',    0,  0,  "Calculate area"},
    {"perimeter",   'p',    0,  0,  "Calculate perimeter"},
    {"hypotenuse",  'c',    "VALUE",  0,  "Specify hypotenuse of the triangle"},
    {"opposite",    'b',    "VALUE",  0,  "Specify opposite of the triangle"},
    {"adjecent",    'a',    "VALUE",  0,  "Specify adjecent of the triangle"},
    { 0 }
};

/* Used by main to communicate with parse_opt. */
struct arguments
{
    int a;
    int b;
    int c;
    int area;
    int perimeter;
};

/* Parse a single option. */
static error_t parse_opt (int key, char *arg, struct argp_state *state)
{
    struct arguments *arguments = (struct arguments*)state->input;

    switch (key) {
        case 'a':
            arguments->a = atoi(arg);
            break;
        case 'b':
            arguments->b = atoi(arg);
            break;
        case 'c':
            arguments->c = atoi(arg);
            break;
        case 'p':
            arguments->perimeter = 1;
            break;
        case 'A':
            arguments->area = 1;
            break;

        default:
            return ARGP_ERR_UNKNOWN;
    }
    return 0;
}

/* Our argp parser. */
static struct argp argp = { options, parse_opt, args_doc, doc };

int
main (int argc, char **argv)
{
    struct arguments arguments;

    /* Default values. */
    arguments.a = -1;
    arguments.b = -1;
    arguments.c = -1;
    arguments.area = 0;
    arguments.perimeter = 0;

    /* Parse our arguments; every option seen by parse_opt will
     *      be reflected in arguments. */
    argp_parse (&argp, argc, argv, 0, 0, &arguments);


    if (arguments.a == -1 || arguments.b == -1 || arguments.c == -1) {
        exit(EXIT_FAILURE);
    }

    if (arguments.area) {
        arguments.area = (arguments.a*arguments.b)/2;
        printf("Area: %d\n",arguments.area);
    }

    if (arguments.perimeter) {
        arguments.perimeter = arguments.a + arguments.b + arguments.c;
        printf("Perimeter: %d\n",arguments.perimeter);
    }

    return EXIT_SUCCESS;
}

Example of usages

This application gives the same output as the getopt() usage, with the following extra features:

The options --help, --usage and --version is automaically generated

[15:53:04]marcus@little:~/tmp/cmdline$ ./argp --help
Usage: argp [OPTION...] ARG1 ARG2
Triangle example

  -a, --adjecent=VALUE       Specify adjecent of the triangle
  -A, --area                 Calculate area
  -b, --opposite=VALUE       Specify opposite of the triangle
  -c, --hypotenuse=VALUE     Specify hypotenuse of the triangle
  -p, --perimeter            Calculate perimeter
  -?, --help                 Give this help list
      --usage                Give a short usage message
  -V, --version              Print program version

Mandatory or optional arguments to long options are also mandatory or optional
for any corresponding short options.

Report bugs to <marcus.folkesson@combitech.se>.

Version information

[15:53:08]marcus@little:~/tmp/cmdline$ ./argp --version
Triangle 1.0

Conclusion

Parsing command line options is simple. argp() provides a log of features that I really appreciate.

When portability is no issue, I allways go for argp() as, besides the extra features, the interface is more appealing.

Embedded Linux Conference 2019

Embedded Linux Conference 2019

Here we go again! This trip got exited even before it begun. I checked my passport the day before we should leave and noticed that my passport has expired. Outch. Fortunataly I was able to get a temporary passport at the airport. I must admit that I'm not traveling that often and do not have these 'must-checks' in my muscle memory..

This time we were heading Lyon in France. The weather is not the best but at least it is not freezing cold as in Sweden as it is this time of the year.

The conference

The conference this year is good as usual. Somehow, my attendence focus has switched from the technical talks to actually connect and talk to people. Of course, I have a group of people that I allways meet (it is mostly the same people that shows up on the conferences, after all), but I have met far more people than I used to. Am I beginning to be social? Anyway, as said before, it is fun to have a face on the patches I've reviewed or got comments on.

The talks

I mostly go for the "heavy technical" talks, but the talk I appreciated most this year had a very low technical level. The talk was performed by Gardena [1] that is doing gardening tools. Yes, water hoses and stuff. They described their journey from a product family that historically have no software at all, to a full blown embedded Linux system with all the legal implications that you can encounter with open source licenses. Gardena became sued for breaking the GPL license, which could be a very costly story. What Gardena did was absolutely the best way to handle it, and it was really nice to hear about. The consensus is that Gardena now have a public github account [2] containing the software for their Garden Gateway products [3]. Support for the SoC that is used were using is not only published, but also mainlined(!!).

Gardena hired contractors from Denx [4] for mainlining U-Boot and Linux kernel and also hired the maintainer of the radiochip that they were using. Thanks to this, all open parts of their product is mainlined and Gardena even managed to get the radio certified, which have helped at least two other companies.

Hiring the right folks for the right tasks was really the best thing Gardena could do. The radiochip maintainer fixed their problem in 48 man-hour, something that could take months to fix for Gardena. The estimated cost of all these "mainlining work" was only 10% of their budget, which is really nothing. It also made it possible for Gardena to put their products on the market in time. One big bonus is that the maintainence is far mor easy when the code is mainlined.

This is also how we work on Combitech. Linux is not part of our customers "core competence", and it really should not be. Linux is our core competence, that is why our customers let us take care of "our thing", IOW Linux development.

But why does all this make me so happy? First of all, the whole Open Source Community is really a big thing to me. It has influenced both my career choices and my view on software. In fact, I'm not even sure that I had enjoyed programming without Open Source.

So, Gardena, Keep up the good work!

/media/elce2019.jpg

ligsegfault.so

libsegfault.so

The dynamic linker [1] in a Linux system is using several environment variables to customize it's behavior. The most commonly used is probably LD_LIBRARY_PATH which is a list of directories where it search for libraries at execution time. Another variable I use quite often is LD_TRACE_LOADED_OBJECTS to let the program list its dynamic dependencies, just like ldd(1).

For example, consider the following output

$ LD_TRACE_LOADED_OBJECTS=1 /bin/bash
    linux-vdso.so.1 (0x00007ffece29e000)
    libreadline.so.7 => /usr/lib/libreadline.so.7 (0x00007fc9b82d1000)
    libdl.so.2 => /usr/lib/libdl.so.2 (0x00007fc9b80cd000)
    libc.so.6 => /usr/lib/libc.so.6 (0x00007fc9b7d15000)
    libncursesw.so.6 => /usr/lib/libncursesw.so.6 (0x00007fc9b7add000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fc9b851f000)
    libtinfo.so.6 => /usr/lib/libtinfo.so.6 (0x00007fc9b78b1000)

LD_PRELOAD

LD_PRELOAD is a list of additional shared objects that should be loaded before all other dynamic dependencies. When the loader is resolving symbols, it sequentially walk through the list of dynamic shared objects and takes the first match. This makes it possible to overide functions in other shared objects and change the behavior of the application completely.

Consider the following example

$ LD_PRELOAD=/usr/lib/libSegFault.so LD_TRACE_LOADED_OBJECTS=1 /bin/bash
    linux-vdso.so.1 (0x00007ffc73f61000)
    /usr/lib/libSegFault.so (0x00007f131c234000)
    libreadline.so.7 => /usr/lib/libreadline.so.7 (0x00007f131bfe6000)
    libdl.so.2 => /usr/lib/libdl.so.2 (0x00007f131bde2000)
    libc.so.6 => /usr/lib/libc.so.6 (0x00007f131ba2a000)
    libncursesw.so.6 => /usr/lib/libncursesw.so.6 (0x00007f131b7f2000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f131c439000)
    libtinfo.so.6 => /usr/lib/libtinfo.so.6 (0x00007f131b5c6000)

Here we have preloaded libSegFault and it is listed in second place. In the first place we have linux-vdso.so.1 which is a Virtual Dynamic Shared Object provided by the Linux kernel. The VDSO deserves it's own separate blog post, it is a cool feature that maps kernel code into the a process's context as a .text segment in a virtual library.

libSegFault.so

libSegFault.so is part of glibc [2] and comes with your toolchain. The library is for debugging purpose and is activated by preload it at runtime. It does not actually overrides functions but register signal handlers in a constructor (yes, you can execute code before main) for specified signals. By default only SIGEGV (see signal(7)) is registered. These registered handlers print a backtrace for the applicaton when the signal is delivered. See its implementation in [3].

Set the environment variable SEGFAULT_SIGNALS to explicit select signals you want to register a handler for.

/media/libsegfault.png

This is an useful feature for debug purpose. The best part is that you don't have to recompile your code.

libSegFault in action

Our application

Consider the following in real life application taken directly from the local nuclear power plant:

void handle_uranium(char *rod)
{
    *rod = 0xAB;
}

void start_reactor()
{
    char *rod = 0x00;
    handle_uranium(rod);
}

int main()
{
    start_reactor();
}

The symptom

We are seeing a segmentation fault when operate on a particular uranium rod, but we don't know why.

Use libSegFault

Start the application with libSegFault preloaded and examine the dump:

$ LD_PRELOAD=/usr/lib/libSegFault.so ./powerplant
*** Segmentation fault
Register dump:

 RAX: 0000000000000000   RBX: 0000000000000000   RCX: 0000000000000000
 RDX: 00007ffdf6aba5a8   RSI: 00007ffdf6aba598   RDI: 0000000000000000
 RBP: 00007ffdf6aba480   R8 : 000055d2ad5e16b0   R9 : 00007f98534729d0
 R10: 0000000000000008   R11: 0000000000000246   R12: 000055d2ad5e14f0
 R13: 00007ffdf6aba590   R14: 0000000000000000   R15: 0000000000000000
 RSP: 00007ffdf6aba480

 RIP: 000055d2ad5e1606   EFLAGS: 00010206

 CS: 0033   FS: 0000   GS: 0000

 Trap: 0000000e   Error: 00000006   OldMask: 00000000   CR2: 00000000

 FPUCW: 0000037f   FPUSW: 00000000   TAG: 00000000
 RIP: 00000000   RDP: 00000000

 ST(0) 0000 0000000000000000   ST(1) 0000 0000000000000000
 ST(2) 0000 0000000000000000   ST(3) 0000 0000000000000000
 ST(4) 0000 0000000000000000   ST(5) 0000 0000000000000000
 ST(6) 0000 0000000000000000   ST(7) 0000 0000000000000000
 mxcsr: 1f80
 XMM0:  00000000000000000000000000000000 XMM1:  00000000000000000000000000000000
 XMM2:  00000000000000000000000000000000 XMM3:  00000000000000000000000000000000
 XMM4:  00000000000000000000000000000000 XMM5:  00000000000000000000000000000000
 XMM6:  00000000000000000000000000000000 XMM7:  00000000000000000000000000000000
 XMM8:  00000000000000000000000000000000 XMM9:  00000000000000000000000000000000
 XMM10: 00000000000000000000000000000000 XMM11: 00000000000000000000000000000000
 XMM12: 00000000000000000000000000000000 XMM13: 00000000000000000000000000000000
 XMM14: 00000000000000000000000000000000 XMM15: 00000000000000000000000000000000

Backtrace:
./powerplant(+0x606)[0x55d2ad5e1606]
./powerplant(+0x628)[0x55d2ad5e1628]
./powerplant(+0x639)[0x55d2ad5e1639]
/usr/lib/libc.so.6(__libc_start_main+0xea)[0x7f9852ec6f6a]
./powerplant(+0x51a)[0x55d2ad5e151a]

Memory map:

55d2ad5e1000-55d2ad5e2000 r-xp 00000000 00:13 14897704                   /home/marcus/tmp/segfault/powerplant
55d2ad7e1000-55d2ad7e2000 r--p 00000000 00:13 14897704                   /home/marcus/tmp/segfault/powerplant
55d2ad7e2000-55d2ad7e3000 rw-p 00001000 00:13 14897704                   /home/marcus/tmp/segfault/powerplant
55d2ada9c000-55d2adabd000 rw-p 00000000 00:00 0                          [heap]
7f9852c8f000-7f9852ca5000 r-xp 00000000 00:13 13977863                   /usr/lib/libgcc_s.so.1
7f9852ca5000-7f9852ea4000 ---p 00016000 00:13 13977863                   /usr/lib/libgcc_s.so.1
7f9852ea4000-7f9852ea5000 r--p 00015000 00:13 13977863                   /usr/lib/libgcc_s.so.1
7f9852ea5000-7f9852ea6000 rw-p 00016000 00:13 13977863                   /usr/lib/libgcc_s.so.1
7f9852ea6000-7f9853054000 r-xp 00000000 00:13 13975885                   /usr/lib/libc-2.26.so
7f9853054000-7f9853254000 ---p 001ae000 00:13 13975885                   /usr/lib/libc-2.26.so
7f9853254000-7f9853258000 r--p 001ae000 00:13 13975885                   /usr/lib/libc-2.26.so
7f9853258000-7f985325a000 rw-p 001b2000 00:13 13975885                   /usr/lib/libc-2.26.so
7f985325a000-7f985325e000 rw-p 00000000 00:00 0
7f985325e000-7f9853262000 r-xp 00000000 00:13 13975827                   /usr/lib/libSegFault.so
7f9853262000-7f9853461000 ---p 00004000 00:13 13975827                   /usr/lib/libSegFault.so
7f9853461000-7f9853462000 r--p 00003000 00:13 13975827                   /usr/lib/libSegFault.so
7f9853462000-7f9853463000 rw-p 00004000 00:13 13975827                   /usr/lib/libSegFault.so
7f9853463000-7f9853488000 r-xp 00000000 00:13 13975886                   /usr/lib/ld-2.26.so
7f9853649000-7f985364c000 rw-p 00000000 00:00 0
7f9853685000-7f9853687000 rw-p 00000000 00:00 0
7f9853687000-7f9853688000 r--p 00024000 00:13 13975886                   /usr/lib/ld-2.26.so
7f9853688000-7f9853689000 rw-p 00025000 00:13 13975886                   /usr/lib/ld-2.26.so
7f9853689000-7f985368a000 rw-p 00000000 00:00 0
7ffdf6a9b000-7ffdf6abc000 rw-p 00000000 00:00 0                          [stack]
7ffdf6bc7000-7ffdf6bc9000 r--p 00000000 00:00 0                          [vvar]
7ffdf6bc9000-7ffdf6bcb000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]

At a first glance, the information may feel overwelming, but lets go through the most importat lines.

The backtrace lists the call chain when the the signal was delivered to the application. The first entry is on top of the stack

Backtrace:
./powerplant(+0x606)[0x55d2ad5e1606]
./powerplant(+0x628)[0x55d2ad5e1628]
./powerplant(+0x639)[0x55d2ad5e1639]
/usr/lib/libc.so.6(__libc_start_main+0xea)[0x7f9852ec6f6a]
./powerplant(+0x51a)[0x55d2ad5e151a]

Here we can see that the last executed instruction is at address 0x55d2ad5e1606. The tricky part is that the address is not absolute in the application, but virtual for the whole process. In other words, we need to calculate the address to an offset within the application's .text segment. If we look at the Memory map we see three entries for the powerplant application:

55d2ad5e1000-55d2ad5e2000 r-xp 00000000 00:13 14897704                   /home/marcus/tmp/segfault/powerplant
55d2ad7e1000-55d2ad7e2000 r--p 00000000 00:13 14897704                   /home/marcus/tmp/segfault/powerplant
55d2ad7e2000-55d2ad7e3000 rw-p 00001000 00:13 14897704                   /home/marcus/tmp/segfault/powerplant

Why three? Most ELF files (application or library) has at least three memory mapped sections: - .text, The executable code - .rodata, read only data - .data, read/write data

With help of the permissions it is possible to figure out which mapping correspond to each section.

The last mapping has rw- as permissions and is probably our .data section as it allows both write and read. The middle mapping has r-- and is a read only mapping - probably our .rodata section. The first mapping has r-x which is read-only and executable. This must be our .text section!

Now we can take the address from our backtrace and subtract with the offset address for our .text section: 0x55d2ad5e1606 - 0x55d2ad5e1000 = 0x606

Use addr2line to get the corresponding line our source code

$ addr2line -e ./powerplant -a 0x606
    0x0000000000000606
    /home/marcus/tmp/segfault/main.c:3

If we go back to the source code, we see that line 3 in main.c is

*rod = 0xAB;

Here we have it. Nothing more to say.

Conclusion

libSegFault.so has been a great help over a long time. The biggest benefit is that you don't have to recompile your application when you want to use the feature. However, you cannot get the line number from addr2line if the application is not compiled with debug symbols, but often it is not that hard to figure out the context out from a dissassembly of your application.

Embedded Linux Conference 2018

Embedded Linux Conference 2018

Ok, time for another conference. This time in Edinburgh, Scottland. My travel is limited to Edinburgh, but this city has a lot of things to see, including Edinburgh Castle, the Royal Botanic Garden, the clock that is always is 3 minutes wrong [1] and lots of more. A sidenote, yes I've tried Haggis as it is a must-try-thing and so should you. But be prepared to buy a backup-meal.

The conference this year has a few talks already on sunday. I'm going for Michael Kerrisk talks about cgroups (control groups) [2]. Michael is a great speaker as usual. His book, The Linux Programming Interface [3] is the only book that you need about system programming as it cover everything.

I was listen to a talk about CGROUPS that i appreciated a lot.

Control groups (cgroups)

cgroups is a hierchy of of processes with several controllers applied to it. These controllers restrict resources such as memory usage and cpu utilisation for a certain group.

Worth to tell is that there are currently two version of cgroups, cgroupv1 and cgroupv2. These are documented in Documentation/cgroup-v1/* and Documentation/admin-guide/cgroup-v2.rst respectively. I've been using cgroupv1 on some system configurations and I can just say that I'm glad that we have a v2. There is no consistency between controllers in v1 and the support for multiple hierchies is just messy, just to mention a few issus with v1. v2 is different. It has a design in mind (v1 was more of a design by implementation) and a set of design-rules. Even better - it has maintainers that make sure the design rules are followed. With all that in mind we hopefully won't end up with a v3... :-)

However, v2 does not have all functionality that v1 provides, but it is on its way into the kernel. v1 and v2 can coexist though, as long as they do not use the same controllers.

The conference

I enjoyed the conference and met a lot of interresting people. It is really fun to put a face on those patches I have reviewed on email :-)

/media/edinburgh.jpg

BEER2RST - my first attempt with golang

BEER2RST - my first attempt with golang

I'm using Beersmith [1] a (non-free...) software to create my beer recipes. It is written in Java and runs well on Linux. One of the biggest benefits with using Beersmith is that my brewing house [2] is taking exported recipes as input and setup mash schemes automatically - really comfortable.

I brew beer several times a month and always takes notes of each brewing, both methods and results. The goal is to get reproducible results each time or to make small improvements. Instead of having all these notes in separate locations, it would be nice if I instead collected all of my brewing as blog posts.

With that said, this blog will probably evolve to contain non-technical posts as well in the near future.

In these posts, I also want to publish my recipes somehow. Beersmith is able to export recipes in a non overcomplicaded XML format and is quite straight forward to parse. All posts that I'm writing is in reStructuredText [3] format, so I need to create a tool that read the XML and export the recipes in reStructuredText format.

First glance at Golang

/media/golang.png

To be honest, I'm not really a fan of high-level programming languages as I can't take them seriously. But I guess it is time to learn something new. What I have tested with Go so far is rather impressive. For example, I cannot imagine a simpler XML parsing (I'm used to libxml2).

I also like the gofmt [4] tool to make the code properly formatted. Every language should have such a tool. It is also easy so cross compile the application to different architectures by specify $GOOS and $GOARCH. I have only tested with ARM and it just works.

Imports

Most languages has a way to tell what functionality it should import and be usable to your file. Go is using import and has the following syntax

import (
    "encoding/xml"
    "flag"
    "fmt"
    "io/ioutil"
    "os"
    "strings"
)

What makes it really interesting is when you do this

import (
    "github.com/marcusfolkesson/tablewriter"
)

I.e. point to a repository. You only need to download the repository to your $GOPATH location and then it is useable

go get github.com/marcusfolkesson/tablewriter
Notes:
I need to create tables for print the recipes properly. I found tablewriter [5] that is printing ASCII-tables. Unfortunately, it does not create tables in reStructuredText format so I had to fork the project and implement support for that. Hopefully the changes will make it back to the original project. There is a pending pull request for that.

Unmarshal XML

I really liked how easy it was to unmarshal XML. Consider the following snip from the exported recipe

<HOP>
 <NAME>Northern Brewer</NAME>
 <VERSION>1</VERSION>
 <ORIGIN>Germany</ORIGIN>
 <ALPHA>8.5000000</ALPHA>
 <AMOUNT>0.0038459</AMOUNT>
 <USE>Boil</USE>
 <TIME>60.0000000</TIME>
 <NOTES>Also called Hallertauer Northern Brewers
Used for: Bittering and finishing both ales and lagers of all kinds
Aroma: Fine, dry, clean bittering hop.  Unique flavor.
Substitutes: Hallertauer Mittelfrueh, Hallertauer
Examples: Anchor Steam, Old Peculiar, </NOTES>
 <TYPE>Both</TYPE>
 <FORM>Pellet</FORM>
 <BETA>4.0000000</BETA>
 <HSI>35.0000000</HSI>
 <DISPLAY_AMOUNT>3.85 g</DISPLAY_AMOUNT>
 <INVENTORY>0.00 g</INVENTORY>
 <DISPLAY_TIME>60.0 min</DISPLAY_TIME>
</HOP>

The first step is to create a structure that should hold the values

type Hop struct {
    Name   string  `xml:"NAME"`
    Origin string  `xml:"ORIGIN"`
    Alpha  float64 `xml:"ALPHA"`
    Amount float64 `xml:"AMOUNT"`
    Use    string  `xml:"USE"`
    Time   float64 `xml:"TIME"`
    Notes  string  `xml:"NOTES"`
}

There is no need to create variables for all tags, just the ones you are interesting in.

Later on, read the file and unmarshal the XML

content, err := ioutil.ReadFile("beer.xml")
if err != nil {
        panic(err)
}

err = xml.Unmarshal(content, &hops)
if err != nil {
        panic(err)
}

The structure will now be populated with values from the XML file. Magical, isn't it?

Conclusion

I have only used tested Golang for a working day approximately, and I like it. I used to use Python for all kind of fast prototyping, but I think I will consider Golang the next time.

What I really like in comparison with Python are:

  • The fact that it compiles to a single binary is really nice, especially when you cross compile to a different architecture.
  • No need for interpreter
  • Static types! Dynamic type languages makes my brain hurt

The result can be found on my GitHub [6] account.

Lund Linux Conference 2018

Lund Linux Conference 2018

It is just two weeks from now til the Lund Linux Conference (LLC) [1] begins! LLC is a two-day conference with the same layout as the bigger Linux conferences - just smaller, but just as nice.

There will be talks about PCIe, The serial device bus, security in cars and a few more topics. My highlights this year is to hear about the XDP (eXpress Data Path) [2] to get really fast packet processing with standard Linux. For the last six months, XDP has had great progress and is a technically cool feature.

Here we are back in 2017:

/media/lund-linuxcon-2018.jpg

ath10k QCA6584 and Wireless network stack

ath10k QCA6584 and Wireless network stack

ATH10K is the mac80211 wireless driver for Qualcomm Atheros QCA988x family of chips, and I'm currently working [1] with the QCA6584 chip which is an automotive graded radio chip with PHY support for the abgn+ac modes. The connection interface to the chip is SDIO which is hardly supported for now, but my friend and kernelhacker, Erik Strömdahl [2] , has got his hands dirty and is currently working on it. There has been some progress, the chip now able to scan, connect, send and receive data. There is still some issues with the link speed but that is coming.

He is also the reason for why I got interested in the network part of the kernel which is quite... big.

Even only the wireless networking subsystem is quite big, and the first you meet when you start to dig is a bunch of terms thrown up in your face. I will try to briefly describe a few of these terms that is fundamental for wireless communication.

In this post will discuss the right side of the this figure:

/media/wireless-stack.png

IEEE 802.11

We will see 802.11 a lot of times, so the first thing is to know where these numbers comes from. IEEE 802.11 is a set of specifications for implementation of wireless networking over several frequency bands. The specifications cover layer 1 (Physical) and layer 2 (Data link) of the OSI model [3].

The Linux kernel MAC subsystem register ieee80211 compliant hardware device with

int ieee80211_register_hw(struct ieee80211_hw *hw)

found in .../net/mac80211/main.c

The Management Layer (MLME)

One more thing that we need to cover is the management layer, since all other layers somehow depend on it.

There are three components in the 802.11 management architecture: - The Physical Layer Management Entity (PLME) - The System Management Entity (SME) - The MAC Layer Management Entity (MLME)

The Management layer assist you in several ways. For instance, it handle things such as scanning, authentication, beacons, associations and much more.

Scanning

Scanning is simply looking for other 802.11 compliant devices in the air. There are two types of scanning; passive and active.

Passive scanning

When performing a passive scanning, the radio is listening passively for beacons, without transmitting packages, as it moves from channel to channel and records all devices that it receives beacons from. Higher frequency bands in the ieee802.11a standard does not allow to transmit anything unless you have heard an Access Point (AP) beacon. Passive scanning is therefore the only way to be aware of the surroundings.

Active scanning

Active scanning on the other hand, is transmitting Probe Request (IEEE80211_STYPE_PROBE_REQ) management packets. This type of scanning is also walking from channel to channel, sending these probe requests management packet for each channel.

These requests is handled by ieee80211_send_probe_req() in .../net/mac80211/util.c:

void ieee80211_send_probe_req(struct ieee80211_sub_if_data *sdata,
                  const u8 *src, const u8 *dst,
                  const u8 *ssid, size_t ssid_len,
                  const u8 *ie, size_t ie_len,
                  u32 ratemask, bool directed, u32 tx_flags,
                  struct ieee80211_channel *channel, bool scan)

Authentication

The authentication procedure sends a management frame of a authentication type (IEEE80211_STYPE_AUTH). There is not only one type of authentication but plenty of them. The ieee80211 specification does only specify one mandatory authentication type; the Open-system authentication (WLAN_AUTH_OPEN). Another common authentication type is Shared key authentication (WLAN_AUTH_SHARED_KEY).

These management frames is handled by ieee80211_send_auth() in .../net/mac80211/util.c:

void ieee80211_send_auth(struct ieee80211_sub_if_data *sdata,
             u16 transaction, u16 auth_alg, u16 status,
             const u8 *extra, size_t extra_len, const u8 *da,
             const u8 *bssid, const u8 *key, u8 key_len, u8 key_idx,
             u32 tx_flags)

Open system authentication

This is the most simple type of authentication, all clients that request authentication will be authenticated. No security is involved at all.

Shared key authentication

In this type of authentication the client and AP is using a shared key, also known as Wired Equivalent Privacy (WEP) key.

Association

The association is started when the station sends management frames of the type IEEE80211_STYPE_ASSOC_REQ. In the kernel code this is handled by ieee80211_send_assoc() in .../net/mac80211/mlme.c

static void ieee80211_send_assoc(struct ieee80211_sub_if_data *sdata)

Reassociation

When the station is roaming, i.e. moving between APs within an ESS (Extended Service Set), it also sends a reassociation request to a new AP of the type IEEE802_STYPE_REASSOC_REQ. Association and reassociation has so much in common that it is both handled by ieee80211_send_assoc().

MAC (Medium Access Control)

All ieee80211 devices needs to implement the Management Layer (MLME), but the implementation could be in device hardware or software. These types of devices are divided into Full MAC device (hardware implementation) and Soft MAC device (software implementation). Most devices today are soft MAC devices.

The MAC layer can be further broken down into two pieces: Upper MAC and Lower MAC. The upper part of the MAC handle the management aspect (all that we covered in the MLME section above), and the lower part handle the time critical operations such as ACK:ing received packets.

Linux does only handle the upper part of MAC, the lower part is operated in device hardware. What we can see in the figure is that the MAC layer is separating data packets from configuration/management packets. The data packets is forwarded to the network device and will travel the same path through the network layer as data packets from all other type of network devices.

The Linux wireless subsystem consists of two major parts, where this, mac80211, is one of them. cfg80211 is the other major part.

CFG80211

cfg80211 is a configuration management service for mac80211 compliant devices. Both Full MAC and Soft MAC devices needs to implement operations to be compatible with the cfg80211 configuration interface in order to let userspace application to configure the device.

The configuration may be done with on of two interfaces, wext and nl80211.

Wireless Extension, WEXT (Legacy)

This is the legacy and ugly way to configure wireless devices. It is still supported only for backward compatibility reasons. Users of this configuration interface are wireless-tools (iwconfig, iwlist).

nl80211

nl80211 on the other hand, is a new netlink interface intended to replace the Wireless Extension (wext) interface. Users of this interface is typically iw and wpa_supplicant.

Conclusion

The whole network stack of the Linux kernel is really complex and optimized for high throughput with low latencies. In this post we only covered what support for wireless devices has complemented the stack with, which is mainly the mac80211 layer for handle all device management, and cfg80211 layer to configure the MAC layer. Packets to wireless devices is divided into data packets and configuration/managment packets. The data packets follow the same path as for all network devices, and the management packets goes to the cfg80211 layer.

Linux driver for PhoenixRC adapter

Linux driver for PhoenixRC adapter

Update: Michael Larabel on Phoronix has written a post [3] about this driver. Go ahead and read it as well!

A few years ago I used to build multirotors, mostly quadcopters and tricopters. It is a fun hobby, both building and flying is incredible satisfying. The first multirotors i built was nicely made with CNC cutted details. They looked really nice and robust. However, with more than 100+ crashes under the belt, the last ones was made out of sticks and a food box. Easy to repair and just as fun to fly.

This hobby requires practice, and even if the most fun way to practice is by flying, it is really time consuming. A more time efficient way to practice is by a simulator, so I bought PhoenixRC [1], which is a flight simulator. It comes with an USB adapter that let you connect and fly with your own RC controller. I did not run the simulator so much. PhoenixRC is a Windows software and there was no driver for the adapter for Linux. The only instance of Windows I had was on a separate disk that layed on the shelf, but switching disk on your laptop each time you want to fly is simply not going to happened.

This new year eve (2017), my wife became ill and I got some time for my own. Far down into a box I found the adapter and plugged it into my Linux computer. Still no driver.

Reverse engineering the adapter

The reverse engineering was quite simple. It turns out that the adapter only has 1 configuration, 1 interface and 1 endpoint of in-interrupt type. This simply means that it only has an unidirectional communication path, initiated by sending an interrupt URB (USB Request Block) to the device. If you are not familiar with what configurations, interfaces and endpoints are in terms of USB, please google the USB standard specification.

The data from our URB was only 8 bytes. After some testing with my RC controller I got the following mapping between data and the channels on the controller:

data[0] = channel 1
data[1] = ? (Possibly a switch)
data[2] = channel 2
data[3] = channel 3
data[4] = channel 4
data[5] = channel 5
data[6] = channel 6
data[7] = channel 7

So I created a device driver that registered an input device with the following events:

Channel Event
1 ABS_X
2 ABS_Y
3 ABS_RX
4 ABS_RY
5 ABS_RUDDER
6 ABS_THROTTLE
7 ABS_MISC

Using a simulator

Heli-X [2] is an excellent cross platform flight simulator that runs perfect on Linux. Now I have spent several hours with a Goblin 700 Helicopter and it is just as fun as I rembembered.

/media/phoenixrc.jpg

Available in Linux kernel 4.17

Of course all code is submitted to the Linux kernel and should be merged in v4.17.