[PATCH] drm/i915/dp: Limit max compressed bpp to 18 when forcing DSC

[Ankit Nautiyal <ankit.k.nautiyal@intel.com>]

While forcing DSC when it is not actually required, the max compressed
bpp from the source gets picked for compression for some pipe bpps.
Apparently, when DSC is not required, forcing DSC to the maximum compressed
bpp seems to cause underruns.

To avoid this, limit the max compressed bpp to 18 while forcing DSC. This
value works across platforms with different pipe bpps and prevents
underruns.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 28fa5e2d0c33..0ce29cefaa13 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -2021,6 +2021,15 @@ static int dsc_src_max_compressed_bpp(struct intel_dp *intel_dp)
 {
 	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
 
+	/*
+	 * Forcing DSC and using the platform's max compressed bpp is seen to cause
+	 * underruns. Since DSC isn't needed in these cases, limit the
+	 * max compressed bpp to 18, which is a safe value across platforms with different
+	 * pipe bpps.
+	 */
+	if (intel_dp->force_dsc_en)
+		return 18;
+
 	/*
 	 * Max Compressed bpp for Gen 13+ is 27bpp.
 	 * For earlier platform is 23bpp. (Bspec:49259).
-- 
2.45.2