linux-kernel - Re: [PATCH v2 4/4] Documentation: media: Document V4L2_H264_DECODE_PARAM_FLAG

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Message-ID: <11ea89d2-e4f1-4c3a-a2c7-f99170781fba@gmail.com>
Date: Sat, 30 Aug 2025 14:29:48 +0300
From: Dmitry Osipenko <digetx@...il.com>
To: Paul Kocialkowski <paulk@...-base.io>,
 Nicolas Dufresne <nicolas.dufresne@...labora.com>
Cc: linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
 Mauro Carvalho Chehab <mchehab@...nel.org>, Hans Verkuil
 <hverkuil@...all.nl>, Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v2 4/4] Documentation: media: Document
 V4L2_H264_DECODE_PARAM_FLAG_PFRAME/BFRAME

29.08.2025 17:15, Paul Kocialkowski пишет:
>>> +      - All submitted slices for the frame are P slices. This is a compability
>>> +        flag required for decoders that only support decoding such frames, but
>>> +        should not be required for slice-based decoders.
>> Seems to match the comment in Tegra driver, and related to a hardware
>> limitation. Shall we also recommend not to use this unless similar limitation
>> exists ?
> I think the flag should only be allowed for frame-based decode mode and indeed
> it would be good to say that drivers should only check this flag if they have
> such limitations.
> 
> Userspace on the other hand cannot really know if it will be used or not so it
> should set the flags when applicable.

IIRC, Tegra dec driver doesn't support decoding frames consisting of
multiple slices, it can only decode frames consisting of a single slice.
Tegra dec can handle multi-slice frames using a "macroblock by
macroblock" decoding mode with a lot of CPU processing, like classic old
hw decoders did it, this is not supported by upstream driver.