[PATCH] drm/i915: Do not explicilty enable FEC in DP_TP_CTL for UHBR rates

[Chaitanya Kumar Borah]

In case of UHBR rates, we do not need to explicitly enable FEC by writing
to DP_TP_CTL register.
For MST use-cases, intel_dp_mst_find_vcpi_slots_for_bpp() takes care of
setting fec_enable to false. However, it gets overwritten in
intel_dp_dsc_compute_config(). This change keeps fec_enable false across
MST and SST use-cases for UHBR rates.

While at it, add a comment explaining why we don't enable FEC in eDP v1.5.

v2: Correct logic to cater to SST use-cases (Jani)

Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah at intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 49a37b996530..de30eaacd9b4 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -2298,9 +2298,15 @@ int intel_dp_dsc_compute_config(struct intel_dp *intel_dp,
 		&pipe_config->hw.adjusted_mode;
 	int ret;
 
+	/*
+	 * Though eDP v1.5 supports FEC with DSC, unlike DP, it is optional.
+	 * Since, FEC is a bandwidth overhead, continue to not enable it for
+	 * eDP. Until, there is a good reason to do so.
+	 */
 	pipe_config->fec_enable = pipe_config->fec_enable ||
 		(!intel_dp_is_edp(intel_dp) &&
-		 intel_dp_supports_fec(intel_dp, connector, pipe_config));
+		 intel_dp_supports_fec(intel_dp, connector, pipe_config) &&
+		 !intel_dp_is_uhbr(pipe_config));
 
 	if (!intel_dp_supports_dsc(connector, pipe_config))
 		return -EINVAL;
-- 
2.25.1