drm/i915/dp: Fix DSC line buffer depth programming

Fix the calculation of the DSC line buffer depth. This is limited both
by the source's and sink's maximum line buffer depth, but the former one
was not taken into account. On all Intel platform's the source's maximum
buffer depth is 13, so the overall limit is simply the minimum of the
source/sink's limit, regardless of the DSC version.

This leaves the DSI DSC line buffer depth calculation as-is, trusting
VBT.

On DSC version 1.2 for sinks reporting a maximum line buffer depth of 16
the line buffer depth was incorrectly programmed as 0, leading to a
corruption in color gradients / lines on the decompressed screen image.

Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Manasi Navare <navaremanasi@chromium.org>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240416221010.376865-2-imre.deak@intel.com

drivers/gpu/drm/i915/display/intel_dp.c

@@ -89,6 +89,9 @@
 #define DP_DSC_MAX_ENC_THROUGHPUT_0		340000
 #define DP_DSC_MAX_ENC_THROUGHPUT_1		400000
 
+/* Max DSC line buffer depth supported by HW. */
+#define INTEL_DP_DSC_MAX_LINE_BUF_DEPTH		13
+
 /* DP DSC FEC Overhead factor in ppm = 1/(0.972261) = 1.028530 */
 #define DP_DSC_FEC_OVERHEAD_FACTOR		1028530
 
@@ -1705,7 +1708,6 @@ static int intel_dp_dsc_compute_params(const struct intel_connector *connector,
 {
 	struct drm_i915_private *i915 = to_i915(connector->base.dev);
 	struct drm_dsc_config *vdsc_cfg = &crtc_state->dsc.config;
-	u8 line_buf_depth;
 	int ret;
 
 	/*
@@ -1734,20 +1736,14 @@ static int intel_dp_dsc_compute_params(const struct intel_connector *connector,
 			connector->dp.dsc_dpcd[DP_DSC_DEC_COLOR_FORMAT_CAP - DP_DSC_SUPPORT] &
 			DP_DSC_RGB;
 
-	line_buf_depth = drm_dp_dsc_sink_line_buf_depth(connector->dp.dsc_dpcd);
-	if (!line_buf_depth) {
+	vdsc_cfg->line_buf_depth = min(INTEL_DP_DSC_MAX_LINE_BUF_DEPTH,
+				       drm_dp_dsc_sink_line_buf_depth(connector->dp.dsc_dpcd));
+	if (!vdsc_cfg->line_buf_depth) {
 		drm_dbg_kms(&i915->drm,
 			    "DSC Sink Line Buffer Depth invalid\n");
 		return -EINVAL;
 	}
 
-	if (vdsc_cfg->dsc_version_minor == 2)
-		vdsc_cfg->line_buf_depth = (line_buf_depth == DSC_1_2_MAX_LINEBUF_DEPTH_BITS) ?
-			DSC_1_2_MAX_LINEBUF_DEPTH_VAL : line_buf_depth;
-	else
-		vdsc_cfg->line_buf_depth = (line_buf_depth > DSC_1_1_MAX_LINEBUF_DEPTH_BITS) ?
-			DSC_1_1_MAX_LINEBUF_DEPTH_BITS : line_buf_depth;
-
 	vdsc_cfg->block_pred_enable =
 		connector->dp.dsc_dpcd[DP_DSC_BLK_PREDICTION_SUPPORT - DP_DSC_SUPPORT] &
 		DP_DSC_BLK_PREDICTION_IS_SUPPORTED;

include/drm/display/drm_dsc.h

@@ -40,9 +40,6 @@
 #define DSC_PPS_RC_RANGE_MINQP_SHIFT		11
 #define DSC_PPS_RC_RANGE_MAXQP_SHIFT		6
 #define DSC_PPS_NATIVE_420_SHIFT		1
-#define DSC_1_2_MAX_LINEBUF_DEPTH_BITS		16
-#define DSC_1_2_MAX_LINEBUF_DEPTH_VAL		0
-#define DSC_1_1_MAX_LINEBUF_DEPTH_BITS		13
 
 /**
  * struct drm_dsc_rc_range_parameters - DSC Rate Control range parameters