linux-kernel - [PATCH v2 6/7] drm/msm/dpu1: Account for DSC's bits_per

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Date:   Wed,  5 Oct 2022 20:16:56 +0200
From:   Marijn Suijten <marijn.suijten@...ainline.org>
To:     phone-devel@...r.kernel.org, Rob Clark <robdclark@...il.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Vinod Koul <vkoul@...nel.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>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        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,
        David Airlie <airlied@...ux.ie>
Subject: [PATCH v2 6/7] drm/msm/dpu1: Account for DSC's bits_per_pixel having 4 fractional bits

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")
Reviewed-by: Abhinav Kumar <quic_abhinavk@...cinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Reviewed-by: Vinod Koul <vkoul@...nel.org>
Signed-off-by: Marijn Suijten <marijn.suijten@...ainline.org>
---
 drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c | 11 ++---------
 1 file changed, 2 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c
index 46cc2afd2bb9..c63e6eef1ba6 100644
--- a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c
+++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_dsc.c
@@ -47,7 +47,7 @@ static void dpu_hw_dsc_config(struct dpu_hw_dsc *hw_dsc,
 			      u32 initial_lines)
 {
 	struct dpu_hw_blk_reg_map *c = &hw_dsc->hw;
-	u32 data, lsb, bpp;
+	u32 data;
 	u32 slice_last_group_size;
 	u32 det_thresh_flatness;
 	bool is_cmd_mode = !(mode & DSC_MODE_VIDEO);
@@ -61,14 +61,7 @@ static void dpu_hw_dsc_config(struct dpu_hw_dsc *hw_dsc,
 	data = (initial_lines << 20);
 	data |= ((slice_last_group_size - 1) << 18);
 	/* bpp is 6.4 format, 4 LSBs bits are for fractional part */
-	data |= dsc->bits_per_pixel << 12;
-	lsb = dsc->bits_per_pixel % 4;
-	bpp = dsc->bits_per_pixel / 4;
-	bpp *= 4;
-	bpp <<= 4;
-	bpp |= lsb;
-
-	data |= bpp << 8;
+	data |= (dsc->bits_per_pixel << 8);
 	data |= (dsc->block_pred_enable << 7);
 	data |= (dsc->line_buf_depth << 3);
 	data |= (dsc->simple_422 << 2);
-- 
2.38.0