For DP 1.4 and above, Display Stream compression can be
enabled only if Forward Error Correctin can be performed.

Check if the sink supports FEC using the helper.

v2: Mention External DP where ever FEC is mentioned
in the code.Check return status of dpcd reads. (Gaurav)
- Do regular mode check even if FEC is not supported. (manasi)

v3: Do not perform any dpcd writes in the atomic
check phase. (DK, Manasi)

v4: Use debug level logging for scenario where sink does
not support a feature. (DK)

v5: Correct commit message. rebase.

v6: pass single field instead of an array for helper
function. (manasi)

Cc: Gaurav K Singh <gaurav.k.si...@intel.com>
Cc: Ville Syrjala <ville.syrj...@linux.intel.com>
Cc: Jani Nikula <jani.nik...@linux.intel.com>
Cc: Manasi Navare <manasi.d.nav...@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandi...@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.sriva...@intel.com>
Reviewed-by: Manasi Navare <manasi.d.nav...@intel.com>
---
 drivers/gpu/drm/i915/intel_dp.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 8ae7cf3d4ee1..a344be555dd6 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -680,7 +680,7 @@ intel_dp_mode_valid(struct drm_connector *connector,
                        dsc_slice_count =
                                
drm_dp_dsc_sink_max_slice_count(intel_dp->dsc_dpcd,
                                                                true);
-               } else {
+               } else if (drm_dp_sink_supports_fec(intel_dp->fec_capable)) {
                        dsc_max_output_bpp =
                                intel_dp_dsc_get_output_bpp(max_link_clock,
                                                            max_lanes,
@@ -690,7 +690,8 @@ intel_dp_mode_valid(struct drm_connector *connector,
                                intel_dp_dsc_get_slice_count(intel_dp,
                                                             target_clock,
                                                             mode->hdisplay);
-               }
+               } else
+                       DRM_DEBUG_KMS("Sink device does not Support FEC\n");
        }
 
        if ((mode_rate > max_rate && !(dsc_max_output_bpp && dsc_slice_count)) 
||
@@ -2063,6 +2064,13 @@ static bool intel_dp_dsc_compute_config(struct intel_dp 
*intel_dp,
        if (pipe == PIPE_A && !intel_dp_is_edp(intel_dp))
                return false;
 
+       /* DSC not supported if external DP sink does not support FEC */
+       if (!intel_dp_is_edp(intel_dp) &&
+           !drm_dp_sink_supports_fec(intel_dp->fec_capable)) {
+               DRM_DEBUG_KMS("Sink does not support Forward Error Correction, 
disabling Display Compression\n");
+               return false;
+       }
+
        /* DSC not supported for DSC sink BPC < 8 */
        if (limits->max_bpp < 3 * DP_DSC_MIN_SUPPORTED_BPC) {
                DRM_DEBUG_KMS("No DSC support for less than 8bpc\n");
-- 
2.17.1

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