linux-kernel - Re: [Freedreno] [PATCH 1/5] drm/msm/dsi: Remove useless math in DSC calculation

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Date:   Tue, 4 Oct 2022 07:33:49 -0700
From:   Abhinav Kumar <quic_abhinavk@...cinc.com>
To:     Marijn Suijten <marijn.suijten@...ainline.org>,
        <phone-devel@...r.kernel.org>, Rob Clark <robdclark@...il.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Vinod Koul <vkoul@...nel.org>
CC:     Marek Vasut <marex@...x.de>, <freedreno@...ts.freedesktop.org>,
        Douglas Anderson <dianders@...omium.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jami Kettunen <jami.kettunen@...ainline.org>,
        Vladimir Lypak <vladimir.lypak@...il.com>,
        <linux-arm-msm@...r.kernel.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        <dri-devel@...ts.freedesktop.org>,
        Javier Martinez Canillas <javierm@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Martin Botka <martin.botka@...ainline.org>,
        <~postmarketos/upstreaming@...ts.sr.ht>,
        Daniel Vetter <daniel@...ll.ch>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...ainline.org>,
        Alex Deucher <alexander.deucher@....com>,
        Sean Paul <sean@...rly.run>, <linux-kernel@...r.kernel.org>
Subject: Re: [Freedreno] [PATCH 1/5] drm/msm/dsi: Remove useless math in DSC
 calculation



On 10/1/2022 12:08 PM, Marijn Suijten wrote:
> Multiplying a value by 2 and adding 1 to it always results in a value
> that is uneven, and that 1 gets truncated immediately when performing
> integer division by 2 again.  There is no "rounding" possible here.
> 
> Fixes: b9080324d6ca ("drm/msm/dsi: add support for dsc data")
> Signed-off-by: Marijn Suijten <marijn.suijten@...ainline.org>
> ---
>   drivers/gpu/drm/msm/dsi/dsi_host.c | 7 +------
>   1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
> index 8e4bc586c262..e05bae647431 100644
> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> @@ -1864,12 +1864,7 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
>   	data = 2048 * (dsc->rc_model_size - dsc->initial_offset + num_extra_mux_bits);
>   	dsc->slice_bpg_offset = DIV_ROUND_UP(data, groups_total);
>   
> -	/* bpp * 16 + 0.5 */
> -	data = dsc->bits_per_pixel * 16;
> -	data *= 2;
> -	data++;
> -	data /= 2;
> -	target_bpp_x16 = data;
> +	target_bpp_x16 = dsc->bits_per_pixel * 16;
>   
Since this patch is titled, "remove useless math", even the 
target_bpp_x16 math looks redundant to me,

first we do

target_bpp_x16 = dsc->bits_per_pixel * 16;

then in the next line we do

data = (dsc->initial_xmit_delay * target_bpp_x16) / 16;

the *16 and /16 will cancel out here.

Instead we can just do

data = (dsc->initial_xmit_delay * dsc->drm->bits_per_pixel) ?

>   	data = (dsc->initial_xmit_delay * target_bpp_x16) / 16;
>   	final_value =  dsc->rc_model_size - data + num_extra_mux_bits;