drm/i915/display: Use the recomended min_hblank values

Use recommended values as per wa_14021694213 to compare with the
calculated value and choose minimum of them.

v2: corrected checkpatch warning and retain the restriction for
min_hblank (Jani)
v3: use calculated value to compare with recomended value and choose
minimum of them (Imre)
v4: As driver supported min bpc is 8, omit the condition check for
bpc6 with ycbcr420. Added a note for the same (Imre)
v5: Add a warn for the unexpected case of 6bpc + uhbr + ycbcr420
v6: Reworded the comments and check fo anything < compressed bpp 8(Imre)
v7: Fix checkpatch warning. (Ankit)

Bspec: 74379
Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://lore.kernel.org/r/20250730-min_hblank-v7-1-179360220ced@intel.com

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 54d88f24b6893e1f8009f00dda87d8dae5dfc137..625036c47bdf2c3aeec92d9a582b391b765191a4 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -3182,7 +3182,26 @@ int intel_dp_compute_min_hblank(struct intel_crtc_state *crtc_state,
         */
        min_hblank = min_hblank - 2;
 
-       min_hblank = min(10, min_hblank);
+       /*
+        * min_hblank formula is undergoing a change, to avoid underrun use the
+        * recomended value in spec to compare with the calculated one and use the
+        * minimum value
+        */
+       if (intel_dp_is_uhbr(crtc_state)) {
+               /*
+                * Note: Bspec requires a min_hblank of 2 for YCBCR420
+                * with compressed bpp 6, but the minimum compressed bpp
+                * supported by the driver is 8.
+                */
+               drm_WARN_ON(display->drm,
+                           (crtc_state->dsc.compression_enable &&
+                            crtc_state->output_format == INTEL_OUTPUT_FORMAT_YCBCR420 &&
+                            crtc_state->dsc.compressed_bpp_x16 < fxp_q4_from_int(8)));
+               min_hblank = min(3, min_hblank);
+       } else {
+               min_hblank = min(10, min_hblank);
+       }
+
        crtc_state->min_hblank = min_hblank;
 
        return 0;