[PATCH v3 03/10] drm/i915/dp_mst: Limit M/N ratio for MST

[Suraj Kandpal <suraj.kandpal@intel.com>]

From: Ankit Nautiyal <ankit.k.nautiyal@intel.com>

The hardware does not support DisplayPort configurations where the
ceiling of the Link M/Link N ratio exceeds the limit. Enforce this
constraint for MST as well.

The compute config path needs no change: MST goes through
intel_dp_compute_config_limits() too, so the link config filter set up
there already excludes the configurations violating the ratio limit, and
MST fails there if none is left.

What is still missing is pruning such modes, so add the check to
mst_connector_mode_valid_ctx(), reusing the helper the SST mode_valid
uses. It walks the link configurations and only rejects the mode if none
of them can satisfy the ratio, rather than evaluating it at a single
link rate.

v2:
-Drop the checks in the MST link config helpers, redundant now that the
constraint is applied while computing the link config limits. (Imre)
-Share the mode_valid helper with SST instead of evaluating the ratio at
one link rate, since the link symbol clock is not monotonic in the link
rate and there is no single best case configuration.
v3:
-Now that intel_dp_mode_valid_m_n checks for all configs just call
that (Sashiko)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp_mst.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
index 6a869d0f6ffc..430ba1230c5a 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
@@ -1564,6 +1564,11 @@ mst_connector_mode_valid_ctx(struct drm_connector *_connector,
 					   min_link_bpp_x16,
 					   bw_overhead_flags);
 
+	if (!intel_dp_mode_valid_m_n(intel_dp, mode->clock)) {
+		*status = MODE_CLOCK_HIGH;
+		return 0;
+	}
+
 	/*
 	 * TODO:
 	 * - Also check if compression would allow for the mode
-- 
2.34.1