[PATCH v5] drm/i915/display: Use the recomended min_hblank values

[Arun R Murthy]

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

Bspec: 74379
Signed-off-by: Arun R Murthy <arun.r.murthy at intel.com>
---
Changes in v5:
- EDITME: describe what is new in this series revision.
- EDITME: use bulletpoints and terse descriptions.
- Link to v4: https://lore.kernel.org/r/20250728-min_hblank-v4-1-b9d83bf03025@intel.com
---
 drivers/gpu/drm/i915/display/intel_dp.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 54d88f24b6893e1f8009f00dda87d8dae5dfc137..1cc19f3e7729c25d2066f42601a18bdfa03b2721 100644
--- 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: Driver doesn't support compressed bpp 6.
+		 * Its unusual to have 6bpc with YCBCR420, for which the spec suggests
+		 * a minimum hblank value of 2(Hence this condition is omitted).
+		 */
+		drm_WARN(display->drm, (crtc_state->dsc.compression_enable &&
+			 crtc_state->output_format == INTEL_OUTPUT_FORMAT_YCBCR420 &&
+			 crtc_state->dsc.compressed_bpp_x16 == 6),
+			 "Unusual to have compressed bpp 6 with ycbcr420\n");
+		min_hblank = min(3, min_hblank);
+	} else {
+		min_hblank = min(10, min_hblank);
+	}
+
 	crtc_state->min_hblank = min_hblank;
 
 	return 0;

---
base-commit: eb0c2b25072b5c56e8c41d7099050bf53bebaa14
change-id: 20250728-min_hblank-263ceb40508c

Best regards,
-- 
Arun R Murthy <arun.r.murthy at intel.com>