Re: [PATCH v3 15/22] staging: media: tegra-video: tegra20: simplify format align calculations

["Luca Ceresoli" <luca.ceresoli@bootlin.com>]

Hello Svyatoslav,

On Tue Oct 7, 2025 at 6:02 PM CEST, Svyatoslav Ryhel wrote:
> пн, 6 жовт. 2025 р. о 21:55 Luca Ceresoli <luca.ceresoli@bootlin.com> пише:
>>
>> Hello Svyatoslav,
>>
>> On Thu Oct 2, 2025 at 8:20 AM CEST, Svyatoslav Ryhel wrote:
>> >> > > > 12 represents amount of bits used per pixel, 8 for Y plane, 2 for U
>> >> > > > plane and 2 for V plane, total is 12. "but explainable with a comment
>> >> > > > and improve-able later" why then we cannot use 12 with a comment? this
>> >> > > > is all arbitrary. Downstream is not wrong from this perspective, you
>> >> > > > don't take into account that YUV420 is planar and it uses 3 planes a
>> >> > > > whole Y plane and 1/4 of U and V which in total results in wigth + 2 *
>> >> > > > 1/4 width which is width * 3/2
>> >> > >
>> >> > > Yes -- but AIUI, the only thing the bpp value is used for the bytesperline calculation. When we add the special case for planar formats, which doesn't use the bpp value, then the value 12 is never used anywhere. We should at least have a comment saying it is unused. (At that point, we could just hardcode the bpp values in the fmt_align function -- but I don't mind either way.)
>> >> > >
>> >> > https://ffmpeg.org/pipermail/ffmpeg-user/2023-June/056488.html
>> >>
>> >> I understand very well that for YUV420, each pixel has 12 bits of color information. But how many bits of color information each pixel has is not useful in the context of this driver. The number of bytes per line is not related to how many bits of color information each pixel has for planar formats.
>> >
>> > No, it has direct impact. This is how buffer size / image size is
>> > calculated since we place each plane consecutive. And bytes per line
>> > is used specifically in image size calculation. This is common part
>> > with non-planar formats. Then since Tegra provides a dedicated
>> > channels/buffers for each plane, configuration of planar format
>> > includes an additional step with calculation for each plane.
>>
>> Sorry, I haven't followed the discussion in detail, but I tested you series
>> on Tegra20 VIP and capture does not work, with a SIGSEGV in
>> gstreamer. Bisecting pointed to this as the first commit where the issue
>> happens.
>>
>> I compared the input and output values of tegra20_fmt_align() at this
>> commit and at the previous one, and this is the result:
>>
>>                        before this patch     with this patch
>>   At function entry:
>>   bpp                        1                     12
>>   pix->width                 640                   640
>>   pix->height                480                   480
>>
>>   On return:
>>   pix->bytesperline          640                   960
>>   pix->sizeimage             460800                460800
>>
>> I hope these info will help.
>
> Which command did you use? I have tested with ffmpeg and
> yuv422/yuv420p and it worked perfectly fine.

I have a simple testing script that runs these commands, with
VNODE="/dev/video0":

v4l2-ctl -d ${VNODE} --set-ctrl horizontal_flip=1 --set-ctrl vertical_flip=1

gst-launch-1.0 -ve v4l2src device=${VNODE} num-buffers=500 \
  ! video/x-raw,width=640,height=480,framerate=50/1,format=I420 \
  ! videorate drop-only=true skip-to-first=true \
  ! video/x-raw,framerate=50/4 \
  ! queue \
  ! avenc_mpeg4 \
  ! mp4mux \
  ! filesink location=/tmp/grab.mp4

Luca

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com