Re: v4l2 kernel module debugging methods

[Fabien DESSENNE <fabien.dessenne@st.com>]

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