[PATCH 6.1 0286/1146] drm/msm/dsi: Prevent signed BPG offsets from bleeding into adjacent bits

Greg Kroah-Hartman
2022-12-28T15:36:04Z

From: Marijn Suijten <marijn.suijten@somainline.org>

[ Upstream commit cc84b66be223d36a3d10d59d68ba647e72db3099 ]

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")
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/508941/
Link: https://lore.kernel.org/r/20221026182824.876933-11-marijn.suijten@somainline.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/gpu/drm/msm/dsi/dsi_host.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
index c229cf6eeb9c..89aadd3b3202 100644
--- a/drivers/gpu/drm/msm/dsi/dsi_host.c
+++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
@@ -1782,7 +1782,11 @@ static int dsi_populate_dsc_params(struct msm_dsi_host *msm_host, struct drm_dsc
for (i = 0; i < DSC_NUM_BUF_RANGES; i++) {
dsc->rc_range_params[i].range_min_qp = min_qp[i];
dsc->rc_range_params[i].range_max_qp = max_qp[i];
- dsc->rc_range_params[i].range_bpg_offset = bpg_offset[i];
+ /*
+ * Range BPG Offset contains two's-complement signed values that fill
+ * 8 bits, yet the registers and DCS PPS field are only 6 bits wide.
+ */
+ dsc->rc_range_params[i].range_bpg_offset = bpg_offset[i] & DSC_RANGE_BPG_OFFSET_MASK;
}

dsc->initial_offset = 6144; /* Not bpp 12 */
--
2.35.1