linux-kernel - Re: [PATCH 4/5] drm/msm/dpu1: Account for DSC's bits_per

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Date:   Tue, 4 Oct 2022 17:35:46 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Marijn Suijten <marijn.suijten@...ainline.org>
Cc:     phone-devel@...r.kernel.org, Rob Clark <robdclark@...il.com>,
        Vinod Koul <vkoul@...nel.org>,
        ~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>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Sean Paul <sean@...rly.run>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Alex Deucher <alexander.deucher@....com>,
        Douglas Anderson <dianders@...omium.org>,
        Vladimir Lypak <vladimir.lypak@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, freedreno@...ts.freedesktop.org
Subject: Re: [PATCH 4/5] drm/msm/dpu1: Account for DSC's bits_per_pixel having
 4 fractional bits

On Sat, 1 Oct 2022 at 22:08, Marijn Suijten
<marijn.suijten@...ainline.org> wrote:
>
> According to the comment this DPU register contains the bits per pixel
> as a 6.4 fractional value, conveniently matching the contents of
> bits_per_pixel in struct drm_dsc_config which also uses 4 fractional
> bits.  However, the downstream source this implementation was
> copy-pasted from has its bpp field stored _without_ fractional part.
>
> This makes the entire convoluted math obsolete as it is impossible to
> pull those 4 fractional bits out of thin air, by somehow trying to reuse
> the lowest 2 bits of a non-fractional bpp (lsb = bpp % 4??).
>
> The rest of the code merely attempts to keep the integer part a multiple
> of 4, which is rendered useless thanks to data |= dsc->bits_per_pixel <<
> 12; already filling up those bits anyway (but not on downstream).
>
> Fixes: c110cfd1753e ("drm/msm/disp/dpu1: Add support for DSC")
> Signed-off-by: Marijn Suijten <marijn.suijten@...ainline.org>

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

> ---
>  drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c | 11 ++---------
>  1 file changed, 2 insertions(+), 9 deletions(-)

-- 
With best wishes
Dmitry