linux-kernel - Re: [PATCH v3 10/10] drm/msm/dsi: Prevent signed BPG offsets from bleeding into adjacent bits

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Message-ID: <408bf6a2-47fe-3eaf-7235-578d33a576c9@linaro.org>
Date:   Sun, 9 Oct 2022 22:14:16 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Marijn Suijten <marijn.suijten@...ainline.org>,
        phone-devel@...r.kernel.org
Cc:     ~postmarketos/upstreaming@...ts.sr.ht,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...ainline.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Martin Botka <martin.botka@...ainline.org>,
        Jami Kettunen <jami.kettunen@...ainline.org>,
        Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, Vinod Koul <vkoul@...nel.org>,
        Douglas Anderson <dianders@...omium.org>,
        Vladimir Lypak <vladimir.lypak@...il.com>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 10/10] drm/msm/dsi: Prevent signed BPG offsets from
 bleeding into adjacent bits

On 09/10/2022 21:53, Marijn Suijten wrote:
> The bpg_offset array contains negative BPG offsets which fill the full 8
> bits of a char thanks to two's complement: this however results in those
> bits bleeding into the next field when the value is packed into DSC PPS
> by the drm_dsc_helper function, which only expects range_bpg_offset to
> contain 6-bit wide values.  As a consequence random slices appear
> corrupted on-screen (tested on a Sony Tama Akatsuki device with sdm845).
> 
> Use AND operators to limit these two's complement values to 6 bits,
> similar to the AMD and i915 drivers.
> 
> Fixes: b9080324d6ca ("drm/msm/dsi: add support for dsc data")
> Signed-off-by: Marijn Suijten <marijn.suijten@...ainline.org>


Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>

Side note: the DSC params tables are more or less common between amd, 
i916 and msm drivers. It might be worth moving them to the DSC helpers 
from the individual drivers. This would mean such masks handling can go 
into the helper too.

-- 
With best wishes
Dmitry