v4l2 kernel module debugging methods

v4l2 kernel module debugging methods

[Ran Shalit]

Hello,

I would like to ask a general question regarding methods to debug a
v4l2 device driver.
Since I assume that the kernel driver will probably won't work in
first try after coding everything inside the device driver...

1. Do you think qemu/kgdb debugger is a good method for the device
driver debugging , or is it plain printing ?

2. Is there a simple way to display the image of a YUV-like buffer in memory ?

Any other methods, tips, about validation, testing and developing a
v4l2 device is appreciated.

Best Regards,
Ran

Re: v4l2 kernel module debugging methods

[Nicolas Dufresne]

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Le dimanche 06 décembre 2015 à 00:00 +0200, Ran Shalit a écrit :
> Hello,
> 
> I would like to ask a general question regarding methods to debug a
> v4l2 device driver.
> Since I assume that the kernel driver will probably won't work in
> first try after coding everything inside the device driver...
> 
> 1. Do you think qemu/kgdb debugger is a good method for the device
> driver debugging , or is it plain printing ?
> 
> 2. Is there a simple way to display the image of a YUV-like buffer in
> memory ?

Most Linux distribution ships GStreamer. You can with GStreamer read
and display a raw YUV images (you need to know the specific format)
using videoparse element.

  gst-launch-1.0 filesrc location=my.yuv ! videoparse format=yuy2 width=320 height=240 ! imagefreeze ! videoconvert ! autovideosink

You could also encode and store to various formats, replacing the
imagefreeze ... section with an encoder and a filesink. Note that
videoparse unfortunatly does not allow passing strides array or
offsets. So it will work only if you set the width/height to padded
width/height.

regards,
Nicolas

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Re: v4l2 kernel module debugging methods

[Ran Shalit]

On Sun, Dec 6, 2015 at 2:23 AM, Nicolas Dufresne
<nicolas.dufresne@collabora.com> wrote:
> Le dimanche 06 décembre 2015 à 00:00 +0200, Ran Shalit a écrit :
>> Hello,
>>
>> I would like to ask a general question regarding methods to debug a
>> v4l2 device driver.
>> Since I assume that the kernel driver will probably won't work in
>> first try after coding everything inside the device driver...
>>
>> 1. Do you think qemu/kgdb debugger is a good method for the device
>> driver debugging , or is it plain printing ?
>>
>> 2. Is there a simple way to display the image of a YUV-like buffer in
>> memory ?
>
> Most Linux distribution ships GStreamer. You can with GStreamer read
> and display a raw YUV images (you need to know the specific format)
> using videoparse element.
>
>   gst-launch-1.0 filesrc location=my.yuv ! videoparse format=yuy2 width=320 height=240 ! imagefreeze ! videoconvert ! autovideosink
>
> You could also encode and store to various formats, replacing the
> imagefreeze ... section with an encoder and a filesink. Note that
> videoparse unfortunatly does not allow passing strides array or
> offsets. So it will work only if you set the width/height to padded
> width/height.
>
> regards,
> Nicolas

Hi Nicolas,

Thank you for the comment.
As someone expreinced with v4l2 device driver, do you recommened using
debugging technique such as qemu (or kgdb) or do you rather use plain
printing ?

Thank you very much,
Ran

Re: v4l2 kernel module debugging methods

[Nicolas Dufresne]

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Le jeudi 10 décembre 2015 à 23:46 +0200, Ran Shalit a écrit :
> Thank you for the comment.
> As someone expreinced with v4l2 device driver, do you recommened
> using
> debugging technique such as qemu (or kgdb) or do you rather use plain
> printing ?

I never used that, printing I used. You should also run v4l2-
compliance. It's a test suite, part of v4l-utils.

Nicolas

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Re: v4l2 kernel module debugging methods

[Fabien DESSENNE]

Hi Ran,

On 12/10/2015 10:46 PM, Ran Shalit wrote:
> On Sun, Dec 6, 2015 at 2:23 AM, Nicolas Dufresne
> <nicolas.dufresne@collabora.com> wrote:
>> Le dimanche 06 décembre 2015 à 00:00 +0200, Ran Shalit a écrit :
>>> Hello,
>>>
>>> I would like to ask a general question regarding methods to debug a
>>> v4l2 device driver.
>>> Since I assume that the kernel driver will probably won't work in
>>> first try after coding everything inside the device driver...
>>>
>>> 1. Do you think qemu/kgdb debugger is a good method for the device
>>> driver debugging , or is it plain printing ?
>>>
>>> 2. Is there a simple way to display the image of a YUV-like buffer in
>>> memory ?
>>
>> Most Linux distribution ships GStreamer. You can with GStreamer read
>> and display a raw YUV images (you need to know the specific format)
>> using videoparse element.
>>
>>    gst-launch-1.0 filesrc location=my.yuv ! videoparse format=yuy2 width=320 height=240 ! imagefreeze ! videoconvert ! autovideosink
>>
>> You could also encode and store to various formats, replacing the
>> imagefreeze ... section with an encoder and a filesink. Note that
>> videoparse unfortunatly does not allow passing strides array or
>> offsets. So it will work only if you set the width/height to padded
>> width/height.
>>
>> regards,
>> Nicolas
>
> Hi Nicolas,
>
> Thank you for the comment.
> As someone expreinced with v4l2 device driver, do you recommened using
> debugging technique such as qemu (or kgdb) or do you rather use plain
> printing ?
>
> Thank you very much,
> Ran


You can get several level of logs.
First, you can enable v4l2 core debug logs. Example:
  echo 3 > /sys/class/video4linux/video0/debug
More details: 
https://www.kernel.org/doc/Documentation/video4linux/v4l2-framework.txt

If the tested driver uses the v4l2_mem2mem framework you can get M2M 
logs with:
  echo Y > /sys/module/v4l2_mem2mem/parameters/debug

Get also internal vb2 logs with:
  echo 3 > /sys/module/videobuf2_core/parameters/debug

And optionally more logs using the dynamic debug feature: 
(https://www.kernel.org/doc/Documentation/dynamic-debug-howto.txt). Example:
  echo "module my_driver +p" > /sys/kernel/debug/dynamic_debug/control

Fabien

Re: v4l2 kernel module debugging methods

[Ran Shalit]

On Fri, Dec 11, 2015 at 10:50 AM, Fabien DESSENNE
<fabien.dessenne@st.com> wrote:
> Hi Ran,
>
> On 12/10/2015 10:46 PM, Ran Shalit wrote:
>> On Sun, Dec 6, 2015 at 2:23 AM, Nicolas Dufresne
>> <nicolas.dufresne@collabora.com> wrote:
>>> Le dimanche 06 décembre 2015 à 00:00 +0200, Ran Shalit a écrit :
>>>> Hello,
>>>>
>>>> I would like to ask a general question regarding methods to debug a
>>>> v4l2 device driver.
>>>> Since I assume that the kernel driver will probably won't work in
>>>> first try after coding everything inside the device driver...
>>>>
>>>> 1. Do you think qemu/kgdb debugger is a good method for the device
>>>> driver debugging , or is it plain printing ?
>>>>
>>>> 2. Is there a simple way to display the image of a YUV-like buffer in
>>>> memory ?
>>>
>>> Most Linux distribution ships GStreamer. You can with GStreamer read
>>> and display a raw YUV images (you need to know the specific format)
>>> using videoparse element.
>>>
>>>    gst-launch-1.0 filesrc location=my.yuv ! videoparse format=yuy2 width=320 height=240 ! imagefreeze ! videoconvert ! autovideosink
>>>
>>> You could also encode and store to various formats, replacing the
>>> imagefreeze ... section with an encoder and a filesink. Note that
>>> videoparse unfortunatly does not allow passing strides array or
>>> offsets. So it will work only if you set the width/height to padded
>>> width/height.
>>>
>>> regards,
>>> Nicolas
>>
>> Hi Nicolas,
>>
>> Thank you for the comment.
>> As someone expreinced with v4l2 device driver, do you recommened using
>> debugging technique such as qemu (or kgdb) or do you rather use plain
>> printing ?
>>
>> Thank you very much,
>> Ran
>
>
> You can get several level of logs.
> First, you can enable v4l2 core debug logs. Example:
>   echo 3 > /sys/class/video4linux/video0/debug
> More details:
> https://www.kernel.org/doc/Documentation/video4linux/v4l2-framework.txt
>
> If the tested driver uses the v4l2_mem2mem framework you can get M2M
> logs with:
>   echo Y > /sys/module/v4l2_mem2mem/parameters/debug
>
> Get also internal vb2 logs with:
>   echo 3 > /sys/module/videobuf2_core/parameters/debug
>
> And optionally more logs using the dynamic debug feature:
> (https://www.kernel.org/doc/Documentation/dynamic-debug-howto.txt). Example:
>   echo "module my_driver +p" > /sys/kernel/debug/dynamic_debug/control
>
> Fabien

Thank you very much for the helpful ideas.

Regards,
Ran