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

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>
Reviewed-by: Naga Venkata Srikanth V <nagavenkata.srikanth.v@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241030084100.1361689-1-ankit.k.nautiyal@intel.com

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index b40964d654972503a5b5bfa2cdf51720d7d1de20..053a9a4182e732a3698c9dc58b3471950891cda9 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -2039,6 +2039,15 @@ 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).