[Intel-gfx] [PATCH] drm/i915: Ensure DSC has enough BW and stays within HW limits

[Stanislav Lisovskiy]

We have currently the issue with some BPPs when using DSC.
According the HW team the reason is that single VDSC engine instance,
has some BW limitations which have to be accounted, so whenever
we approach around 90% of the CDCLK, second VDSC engine have to be
used. Also that always means using 2 slices, however in our current code
amount of slices is calculated for some reason independently of
whether we need to enable 2nd VDSC engine or not, thus leading to
some logical issues, when according to pixel clock needs we need to enable
2nd VDSC engine, however as we calculated previously that we can use
only single slice, we can't do that and fail.
So we need to fix that, so that amount of VDSC engines enabled should depend
on amount of slices and amount of slices should also depend on BW requirements.
Lastly we didn't have BPP limitation for ADLP/MTL/DG2 implemented which says
that DSC output BPP's can only be chosen within range of 8 to 27(BSpec 49259).
This all applied together allows to fix existing FIFO underruns, which we
have in many DSC tests.

BSpec: 49259
HSDES: 18027167222

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index aee93b0d810e..e3680ae95b83 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -687,6 +687,12 @@ u32 intel_dp_dsc_nearest_valid_bpp(struct drm_i915_private *i915, u32 bpp, u32 p
 	/* From XE_LPD onwards we support from bpc upto uncompressed bpp-1 BPPs */
 	if (DISPLAY_VER(i915) >= 13) {
 		bits_per_pixel = min(bits_per_pixel, pipe_bpp - 1);
+
+		/* According to BSpec, 27 is the max DSC output bpp */
+		bits_per_pixel = min(bits_per_pixel, (u32)27);
+
+		/* According to BSpec, 8 is the min DSC output bpp */
+		bits_per_pixel = max(bits_per_pixel, (u32)8);
 	} else {
 		/* Find the nearest match in the array of known BPPs from VESA */
 		for (i = 0; i < ARRAY_SIZE(valid_dsc_bpp) - 1; i++) {
@@ -771,6 +777,9 @@ u8 intel_dp_dsc_get_slice_count(struct intel_dp *intel_dp,
 		min_slice_count = DIV_ROUND_UP(mode_clock,
 					       DP_DSC_MAX_ENC_THROUGHPUT_1);
 
+	if (mode_clock >= ((i915->display.cdclk.max_cdclk_freq * 85) / 100))
+		min_slice_count = max(min_slice_count, (u8)2);
+
 	max_slice_width = drm_dp_dsc_sink_max_slice_width(intel_dp->dsc_dpcd);
 	if (max_slice_width < DP_DSC_MIN_SLICE_WIDTH_VALUE) {
 		drm_dbg_kms(&i915->drm,
@@ -1597,16 +1606,8 @@ int intel_dp_dsc_compute_config(struct intel_dp *intel_dp,
 	 * is greater than the maximum Cdclock and if slice count is even
 	 * then we need to use 2 VDSC instances.
 	 */
-	if (adjusted_mode->crtc_clock > dev_priv->display.cdclk.max_cdclk_freq ||
-	    pipe_config->bigjoiner_pipes) {
-		if (pipe_config->dsc.slice_count > 1) {
-			pipe_config->dsc.dsc_split = true;
-		} else {
-			drm_dbg_kms(&dev_priv->drm,
-				    "Cannot split stream to use 2 VDSC instances\n");
-			return -EINVAL;
-		}
-	}
+	if (pipe_config->bigjoiner_pipes || pipe_config->dsc.slice_count > 1)
+		pipe_config->dsc.dsc_split = true;
 
 	ret = intel_dp_dsc_compute_params(&dig_port->base, pipe_config);
 	if (ret < 0) {
-- 
2.37.3