linux-kernel - Re: [PATCH v1] media: v4l2-core: Initialize h264 scaling matrix

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Date:   Mon, 21 Feb 2022 16:12:51 +0100
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Ezequiel Garcia <ezequiel@...guardiasur.com.ar>,
        Hans Verkuil <hverkuil-cisco@...all.nl>
Cc:     kernel@...labora.com, yunfei.dong@...iatek.com,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] media: v4l2-core: Initialize h264 scaling matrix

Il 18/02/22 19:42, Nicolas Dufresne ha scritto:
> In the final H264 API, it is not required to set scaling matrix if
> they are not present in the bitstream. A flag was added in order to let
> the driver know. The downside is that it leaves the default control
> value to 0, which isn't valid. As per the spec (see formulas 7-8/7-9),
> when the scaling matrix are absent from the bitstream, flat values
> of 16 should be used. This improves this control semantic in a way
> that the control value are always valid. Drivers can then use
> the scaling_matrix control values without having to check its presence.
> Same method was employed for MPEG2_QUANTISATION.
> 
> This fixes issues with MTK VCODEC H264 decoder when using GStreamer.
> GStreamer does not set this control if its not present in the bitstream.
> As MTK VDCODEC was using the initialized to 0 values, the frames ended
> up completely gray.
> 
> Fixes: 54889c51b833d236 ("media: uapi: h264: Rename and clarify PPS_FLAG_SCALING_MATRIX_PRESENT")
> Signed-off-by: Nicolas Dufresne <nicolas.dufresne@...labora.com>

Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>