[PATCH v3 10/10] drm/msm/dsi: Prevent signed BPG offsets from bleeding into adjacent bits

[Marijn Suijten]

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")
Signed-off-by: Marijn Suijten <marijn.suijten@xxxxxxxxxxxxxx>
---
 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 46032c576a59..c5c2d70ac27d 100644
--- a/drivers/gpu/drm/msm/dsi/dsi_host.c
+++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
@@ -1804,7 +1804,11 @@ 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 */
-- 
2.38.0