linux-kernel - Re: [PATCH v3 09/14] media: uapi: Add V4L2_CID_STATELESS_HEVC_ENTRY_POINT

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Message-ID: <me5p1hpnrarnqqaefoofrtqb898afl0rsn@4ax.com>
Date:   Mon, 28 Feb 2022 09:47:18 +0000
From:   John Cox <jc@...esim.co.uk>
To:     Jernej Škrabec <jernej.skrabec@...il.com>
Cc:     mchehab@...nel.org, ezequiel@...guardiasur.com.ar,
        p.zabel@...gutronix.de, gregkh@...uxfoundation.org,
        mripard@...nel.org, paul.kocialkowski@...tlin.com, wens@...e.org,
        jonas@...boo.se, nicolas@...fresne.ca,
        Benjamin Gaignard <benjamin.gaignard@...labora.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-staging@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        kernel@...labora.com, knaerzche@...il.com
Subject: Re: [PATCH v3 09/14] media: uapi: Add V4L2_CID_STATELESS_HEVC_ENTRY_POINT_OFFSET control

On Sat, 26 Feb 2022 19:17:46 +0100, you wrote:

>Dne sobota, 26. februar 2022 ob 18:59:18 CET je Jernej Škrabec napisal(a):
>> Hi!
>> 
>> Dne petek, 25. februar 2022 ob 20:30:20 CET je Jernej Škrabec napisal(a):
>> > Hi!
>> > 
>> > Dne petek, 25. februar 2022 ob 17:45:55 CET je Benjamin Gaignard 
>napisal(a):
>> > > The number of 'entry point offset' could be very variable.
>> > > Rather than use a large static array define a v4l2 dynamic array
>> > > of integer control.
>> > 
>> > I suggest we should be more specific and say U32 (V4L2_CTRL_TYPE_U32).
>> > 
>> > > The number of entry point offsets is reported by the elems field.
>> 
>> I did few more tests and these are my findings:
>> 1. dynamic array can't be set over size, specified in .dims array
>> 2. entry point offsets are per slice, so if we make slices dynamic array, 
>then 
>> entry points would become two dimensional dynamic array
>> 3. num_entry_point_offsets must be part of slice control, because it can be 
>> zero, but size of array can't be
>
>Clarification: num_entry_point_offsets is needed in any case, even without entry 
>point list.

I thought that at least one decoder wanted the entry points or have we
decided that we don't? Whilst they aren't required for decode, what they
give you, if your h/w supports it, is the ability to decode multiple
lines of CTBs in parallel (with a 2 CTB offset between lines), so they
aren't useless.

If we do decided we want the entry point offset control, now or later,
then num_entry_points in the slice_param allows you to determine which
offsets apply to which slices without the complexity of a variable array
of variable arrays.

But whether or not we end up supporting entry point offsets, the Pi h/w
requires me to generate 1 table entry for each CTB line and
num_entry_points is the parameter I need for that, so I really want it
in the slice_params.

Regards

John Cox

>> 4. fortunately, not setting entry points doesn't impact decoding 
>correctness. 
>> This is in line what John told me about them.
>> 
>> Hans, can you comment points 1-3? I might misunderstand point 1.
>> 
>> In short, it seems like we don't really need entry points, even if they are 
>> used in BSP library. In both cases, I got fluster score 119/138 (10-bit 
>> excluded), so we can just drop this patch, although I'm a bit uneasy not 
>> setting entry points...
>> 
>> Best regards,
>> Jernej
>> 
>> > > 
>> > > Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...labora.com>
>> > > ---
>> > >  .../userspace-api/media/v4l/ext-ctrls-codec.rst          | 9 +++++++++
>> > >  include/media/hevc-ctrls.h                               | 1 +
>> > >  2 files changed, 10 insertions(+)
>> > > 
>> > > diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst b/
>> > Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>> > > index 44a268a948c0..71f7dc1c1ccd 100644
>> > > --- a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>> > > +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>> > > @@ -3128,6 +3128,15 @@ enum v4l2_mpeg_video_hevc_size_of_length_field -
>> > >  
>> > >      \normalsize
>> > >  
>> > > +``V4L2_CID_STATELESS_HEVC_ENTRY_POINT_OFFSETS (integer)``
>> > 
>> > Here you have OFFSETS (plural) ...
>> > 
>> > > +    Specifies the i-th entry point offset in bytes and is represented by
>> > > +    offset_len_minus1 plus 1 bits.
>> > 
>> > You probably mean entry_point_offset_minus1? offset_len_minus1 just tells 
>how 
>> > much bits need to be read for each element and it's not important for 
>actual 
>> > decoding.
>> > 
>> > > +    This control is a dynamically sized array. The number of entry 
>point
>> > > +    offsets is reported by the ``elems`` field.
>> > > +    This bitstream parameter is defined according to :ref:`hevc`.
>> > > +    They are described in section 7.4.7.1 "General slice segment header
>> > > +    semantics" of the specification.
>> > > +
>> > >  ``V4L2_CID_STATELESS_HEVC_SCALING_MATRIX (struct)``
>> > >      Specifies the HEVC scaling matrix parameters used for the scaling 
>> > process
>> > >      for transform coefficients.
>> > > diff --git a/include/media/hevc-ctrls.h b/include/media/hevc-ctrls.h
>> > > index 3016c1abb1d0..3f8a67924df3 100644
>> > > --- a/include/media/hevc-ctrls.h
>> > > +++ b/include/media/hevc-ctrls.h
>> > > @@ -20,6 +20,7 @@
>> > >  #define V4L2_CID_STATELESS_HEVC_DECODE_PARAMS	(V4L2_CID_CODEC_BASE + 
>> > 1012)
>> > >  #define V4L2_CID_STATELESS_HEVC_DECODE_MODE	(V4L2_CID_CODEC_BASE + 
>> > 1015)
>> > >  #define V4L2_CID_STATELESS_HEVC_START_CODE	(V4L2_CID_CODEC_BASE 
>+ 
>> 1016)
>> > > +#define V4L2_CID_STATELESS_HEVC_ENTRY_POINT_OFFSET (V4L2_CID_CODEC_BASE 
>+ 
>> > 1017)
>> > 
>> > ... and here you have OFFSET (singlular). I suggest plural form to be used 
>> in 
>> > all places, including subject line of this commit.
>> > 
>> > Additionally, it would be nice if control is initialized, like so:
>> > https://github.com/jernejsk/linux-1/commit/
>> > f938e162cd8dd77c9f6f1b248d80144840a37bce
>> > 
>> > Best regards,
>> > Jernej
>> > 
>> > >  
>> > >  /* enum v4l2_ctrl_type type values */
>> > >  #define V4L2_CTRL_TYPE_HEVC_SPS 0x0120
>> > > -- 
>> > > 2.32.0
>> > > 
>> > > 
>> > 
>> > 
>> > 
>> 
>> 
>> 
>