linux-kernel - Re: [PATCH v3 1/5] msm/drm/dsi: Round up DSC hdisplay calculation

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Message-ID: <aki3mrbsmpn4to74kx3p2sv2xv24ep5wnfu6yp4ehqncwamdmd@yfnpixtyet5n>
Date:   Fri, 19 May 2023 23:33:02 +0200
From:   Marijn Suijten <marijn.suijten@...ainline.org>
To:     Jessica Zhang <quic_jesszhan@...cinc.com>
Cc:     Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/5] msm/drm/dsi: Round up DSC hdisplay calculation

On 2023-05-19 14:17:26, Jessica Zhang wrote:
> Currently, when compression is enabled, hdisplay is reduced via integer
> division. This causes issues for modes where the original hdisplay is
> not a multiple of 3.

The "issue" probably being some kind of underflow, because the stream
size is too small compared to how much data we actually send?

> To fix this, use DIV_ROUND_UP to divide hdisplay.
> 
> Reported-by: Marijn Suijten <marijn.suijten@...ainline.org>
> Fixes: f3a99460406b ("drm/msm/dsi: update hdisplay calculation for dsi_timing_setup")
> Signed-off-by: Jessica Zhang <quic_jesszhan@...cinc.com>

Yes, downstream has as very clear:

    dsc->pclk_per_line =  DIV_ROUND_UP(total_bytes_per_intf, 3);

which is used for width_final in SDE, and this is one of the mandatory
fixes on a 1096-pixels-wide panel, which is not a multiple of 3.

Reviewed-by: Marijn Suijten <marijn.suijten@...ainline.org>

> ---
>  drivers/gpu/drm/msm/dsi/dsi_host.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
> index 9223d7ec5a73..18d38b90eb28 100644
> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> @@ -952,7 +952,7 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
>  		 * pulse width same
>  		 */
>  		h_total -= hdisplay;
> -		hdisplay = msm_dsc_get_bytes_per_line(msm_host->dsc) / 3;
> +		hdisplay = DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc), 3);
>  		h_total += hdisplay;
>  		ha_end = ha_start + hdisplay;
>  	}
> 
> -- 
> 2.40.1
>