Re: Kernel error "Unknown pixelformat 0x00000000" occurs when I start capture video

From: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
To: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	linux-media@vger.kernel.org
Subject: Re: Kernel error "Unknown pixelformat 0x00000000" occurs when I start capture video
Date: Tue, 8 Jan 2019 16:49:16 -0200	[thread overview]
Message-ID: <20190108164916.55aa9b93@coco.lan> (raw)
In-Reply-To: <CABXGCsOMdyzXACZa9T1OdUmDhNPDK=cX+DfBCAnY2A4aozCFHA@mail.gmail.com>

Em Tue, 8 Jan 2019 21:11:41 +0500
Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com> escreveu:

> On Tue, 8 Jan 2019 at 20:57, Laurent Pinchart
> <laurent.pinchart@ideasonboard.com> wrote:
> >
> > Thank you.
> >
> > Your device exposes five formats: YUY2 (YUYV), YV12 (YVU420), NV12, P010 and
> > BGR3 (BGR24). They are all supported by V4L2 and the uvcvideo driver except
> > for the P010 format. This would be easy to fix in the uvcvideo driver if it
> > wasn't for the fact that the P010 format isn't support by V4L2. Adding support
> > for it isn't difficult, but I don't have time to do this myself at the moment.
> > Would you consider volunteering if I guide you ? :-)
> >  
> 
> Sure, I'd be happy to help. What is required of me?

It shouldn't be hard. 

First, you need to add the new format at include/uapi/linux/videodev2.h,
like this one:

	#define V4L2_PIX_FMT_YUV420M v4l2_fourcc('Y', 'M', '1', '2') /* 12  YUV420 planar */

Please put it together with the other YUV formats.

As the fourcc "P010" was not used on Linux yet, you could use it,
e. g., something like:

	#define V4L2_PIX_FMT_YUV_P10 v4l2_fourcc('P', '0', '1', '0') /* 10  YUV420 planar */

You need then to document it. Each V4L2 format should have a description, 
like this:

	https://linuxtv.org/downloads/v4l-dvb-apis-new/uapi/v4l/pixfmt-yuv420m.html

This is generated via a text file (using ReST syntax). On the above example,
it is produced by this file:

	https://git.linuxtv.org/media_tree.git/tree/Documentation/media/uapi/v4l/pixfmt-yuv420m.rst

Writing it would take a little more time, but, provided that you don't 
copy what's written from external docs, you could take a look at the
Internet for the P010 descriptions, and use the pixfmt-yuy420m.rst file
as the basis for a new pixfmt-p010.rst file.

This will produce a patch similar to this one:

	https://git.linuxtv.org/media_tree.git/commit/?id=5df082e2312c

Finally, you need to teach the uvc driver that it should report P010
format, instead of 0x00000000, by adding it at uvc_format_desc uvc_fmts,
with the corresponding UVC GUID. We usually do this on a separate patch.
Something like this:

	https://git.linuxtv.org/media_tree.git/commit/?id=6ea0d588d35b

Once you have the patches, submit it to the ML :-)

You could take a look at the LinuxTV wiki page about how to submit,
at the developer's section:

	https://linuxtv.org/wiki/index.php/Developer_section

In particular, take a look at "Submitting Your Work" section there.

Thanks,
Mauro