[PATCH 2/8] drm/i915/dp: Limit m/n ratio to 10 for DP SST

[Ankit Nautiyal]

The hardware cannot support DisplayPort configurations where the
ceiling of the Link M/Link N ratio exceeds 10. This limitation has
always existed, but it typically wasn't encountered without the use of
joiners and DSC.

With higher resolutions and combinations involving joiners and DSC,
this constraint can now be hit in certain scenarios.

Introduce a check during link rate configuration to ensure
the computed M/N ratio does not exceed the hardware limit. If no valid
link rate satisfies this constraint, the mode will be rejected.

Note: This change applies the check only for SST. Support for MST will
be added in a subsequent commit.

v2:
-Move the M/N ratio check to the link rate configuration phase instead
of during M/N computation. (Ville)
-Prune modes that cannot be supported even with highest link rate due to
M/N ratio restriction.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal at intel.com>
---
 drivers/gpu/drm/i915/display/intel_display.c |  1 -
 drivers/gpu/drm/i915/display/intel_display.h |  3 ++
 drivers/gpu/drm/i915/display/intel_dp.c      | 33 +++++++++++++++++++-
 3 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index 0726b25ed5a1..6b20ac9eca36 100644
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -2506,7 +2506,6 @@ static void compute_m_n(u32 *ret_m, u32 *ret_n,
 	intel_reduce_m_n_ratio(ret_m, ret_n);
 }
 
-static
 void intel_display_get_link_m_n(u32 *link_m, u32 *link_n,
                                 u32 pixel_clock,
                                 u32 link_clock)
diff --git a/drivers/gpu/drm/i915/display/intel_display.h b/drivers/gpu/drm/i915/display/intel_display.h
index 37e2ab301a80..08834f91410a 100644
--- a/drivers/gpu/drm/i915/display/intel_display.h
+++ b/drivers/gpu/drm/i915/display/intel_display.h
@@ -559,5 +559,8 @@ bool assert_port_valid(struct intel_display *display, enum port port);
 
 bool intel_scanout_needs_vtd_wa(struct intel_display *display);
 int intel_crtc_num_joined_pipes(const struct intel_crtc_state *crtc_state);
+void intel_display_get_link_m_n(u32 *link_m, u32 *link_n,
+                                u32 pixel_clock,
+                                u32 link_clock);
 
 #endif
diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 6d4e0fa0c054..c6ca25a94968 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -1407,6 +1407,28 @@ bool intel_dp_has_dsc(const struct intel_connector *connector)
 	return true;
 }
 
+static int
+intel_dp_get_max_m_n_ratio(void)
+{
+	return 10;
+}
+
+static bool
+intel_dp_can_support_m_n(int pixel_clock,
+			 int link_rate)
+{
+	int max_m_n_ratio = intel_dp_get_max_m_n_ratio();
+	u32 link_m, link_n;
+	int m_n_ratio;
+
+	intel_display_get_link_m_n(&link_m, &link_n,
+				   pixel_clock, link_rate);
+
+	m_n_ratio = DIV_ROUND_UP(link_m, link_n);
+
+	return m_n_ratio <= max_m_n_ratio;
+}
+
 static enum drm_mode_status
 intel_dp_mode_valid(struct drm_connector *_connector,
 		    const struct drm_display_mode *mode)
@@ -1518,6 +1540,9 @@ intel_dp_mode_valid(struct drm_connector *_connector,
 	if (status != MODE_OK)
 		return status;
 
+	if (!intel_dp_can_support_m_n(target_clock, max_rate))
+		return MODE_CLOCK_HIGH;
+
 	return intel_mode_valid_max_plane_size(display, mode, num_joined_pipes);
 }
 
@@ -1783,6 +1808,9 @@ intel_dp_compute_link_config_wide(struct intel_dp *intel_dp,
 			    link_rate > limits->max_rate)
 				continue;
 
+			if (!intel_dp_can_support_m_n(clock, link_rate))
+				continue;
+
 			for (lane_count = limits->min_lane_count;
 			     lane_count <= limits->max_lane_count;
 			     lane_count <<= 1) {
@@ -1790,7 +1818,6 @@ intel_dp_compute_link_config_wide(struct intel_dp *intel_dp,
 									 link_rate,
 									 lane_count);
 
-
 				if (mode_rate <= link_avail) {
 					pipe_config->lane_count = lane_count;
 					pipe_config->pipe_bpp = bpp;
@@ -1977,6 +2004,10 @@ static int dsc_compute_link_config(struct intel_dp *intel_dp,
 		if (link_rate < limits->min_rate || link_rate > limits->max_rate)
 			continue;
 
+		if (!intel_dp_can_support_m_n(adjusted_mode->clock,
+					      link_rate))
+			continue;
+
 		for (lane_count = limits->min_lane_count;
 		     lane_count <= limits->max_lane_count;
 		     lane_count <<= 1) {
-- 
2.45.2