[PATCH v2] drm/i915: Reduce Data Link N value for 1 lane DP->hdmi converters

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From: Clint Taylor <clinton.a.taylor@xxxxxxxxx>

Several major vendor USB-C->HDMI converters fail to recover a 5.4 GHz 1 lane
signal if the Data Link N is greater than 0x80000.
Patch detects when 1 lane 5.4 GHz signal is being used and makes the maximum
value 20 bit instead of the maximum specification supported 24 bit value.

v2: Detect specific device that exhibits the issue.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@xxxxxxxxx>
Cc: Jani Nikula <jani.nikula@xxxxxxxxx>
Cc: Anusha Srivatsa <anusha.srivatsa@xxxxxxxxx>

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93578

Signed-off-by: Clint Taylor <clinton.a.taylor@xxxxxxxxx>
---
 drivers/gpu/drm/i915/i915_drv.h      |    3 ++-
 drivers/gpu/drm/i915/i915_reg.h      |    2 ++
 drivers/gpu/drm/i915/intel_display.c |   20 ++++++++++++++------
 drivers/gpu/drm/i915/intel_dp.c      |   14 ++++++++++++--
 drivers/gpu/drm/i915/intel_dp_mst.c  |    3 ++-
 5 files changed, 32 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index a5947a4..6869df9 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -580,7 +580,8 @@ struct intel_link_m_n {
 
 void intel_link_compute_m_n(int bpp, int nlanes,
 			    int pixel_clock, int link_clock,
-			    struct intel_link_m_n *m_n);
+			    struct intel_link_m_n *m_n,
+			    bool reduced_n);
 
 /* Interface history:
  *
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 04c8f69..838d8d5 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -4869,6 +4869,8 @@ enum {
 
 #define  DATA_LINK_M_N_MASK	(0xffffff)
 #define  DATA_LINK_N_MAX	(0x800000)
+/* Maximum N value useable on some DP->HDMI converters */
+#define  DATA_LINK_REDUCED_N_MAX (0x80000)
 
 #define _PIPEA_DATA_N_G4X	0x70054
 #define _PIPEB_DATA_N_G4X	0x71054
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 010e5dd..143c7ac 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -6139,7 +6139,7 @@ static int ironlake_fdi_compute_config(struct intel_crtc *intel_crtc,
 	pipe_config->fdi_lanes = lane;
 
 	intel_link_compute_m_n(pipe_config->pipe_bpp, lane, fdi_dotclock,
-			       link_bw, &pipe_config->fdi_m_n);
+			       link_bw, &pipe_config->fdi_m_n, false);
 
 	ret = ironlake_check_fdi_lanes(dev, intel_crtc->pipe, pipe_config);
 	if (ret == -EINVAL && pipe_config->pipe_bpp > 6*3) {
@@ -6315,9 +6315,10 @@ static int intel_crtc_compute_config(struct intel_crtc *crtc,
 }
 
 static void compute_m_n(unsigned int m, unsigned int n,
-			uint32_t *ret_m, uint32_t *ret_n)
+			uint32_t *ret_m, uint32_t *ret_n,
+			uint32_t max_link_n)
 {
-	*ret_n = min_t(unsigned int, roundup_pow_of_two(n), DATA_LINK_N_MAX);
+	*ret_n = min_t(unsigned int, roundup_pow_of_two(n), max_link_n);
 	*ret_m = div_u64((uint64_t) m * *ret_n, n);
 	intel_reduce_m_n_ratio(ret_m, ret_n);
 }
@@ -6325,16 +6326,23 @@ static void compute_m_n(unsigned int m, unsigned int n,
 void
 intel_link_compute_m_n(int bits_per_pixel, int nlanes,
 		       int pixel_clock, int link_clock,
-		       struct intel_link_m_n *m_n)
+		       struct intel_link_m_n *m_n,
+		       bool reduced_n)
 {
+	uint32_t max_link_n = DATA_LINK_N_MAX;
 	m_n->tu = 64;
 
+	if ((reduced_n) && (nlanes == 1) && (link_clock >= 540000))
+		max_link_n = DATA_LINK_REDUCED_N_MAX;
+
 	compute_m_n(bits_per_pixel * pixel_clock,
 		    link_clock * nlanes * 8,
-		    &m_n->gmch_m, &m_n->gmch_n);
+		    &m_n->gmch_m, &m_n->gmch_n,
+		    max_link_n);
 
 	compute_m_n(pixel_clock, link_clock,
-		    &m_n->link_m, &m_n->link_n);
+		    &m_n->link_m, &m_n->link_n,
+		    max_link_n);
 }
 
 static inline bool intel_panel_use_ssc(struct drm_i915_private *dev_priv)
diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index fd96a6c..9c96f5f 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -1624,6 +1624,8 @@ static int intel_dp_compute_bpp(struct intel_dp *intel_dp,
 	int common_rates[DP_MAX_SUPPORTED_RATES] = {};
 	int common_len;
 	uint8_t link_bw, rate_select;
+	char id[6];
+	bool reduced_n = false;
 
 	common_len = intel_dp_common_rates(intel_dp, common_rates);
 
@@ -1750,10 +1752,17 @@ static int intel_dp_compute_bpp(struct intel_dp *intel_dp,
 	DRM_DEBUG_KMS("DP link bw required %i available %i\n",
 		      mode_rate, link_avail);
 
+	/* Quirk to detect DP->HDMI converter */
+	drm_dp_downstream_id(&intel_dp->aux, id);
+	if (!strncmp(id , "7737" ,4)) {
+	      reduced_n = true;
+	}
+
 	intel_link_compute_m_n(bpp, lane_count,
 			       adjusted_mode->crtc_clock,
 			       pipe_config->port_clock,
-			       &pipe_config->dp_m_n);
+			       &pipe_config->dp_m_n,
+			       reduced_n);
 
 	if (intel_connector->panel.downclock_mode != NULL &&
 		dev_priv->drrs.type == SEAMLESS_DRRS_SUPPORT) {
@@ -1761,7 +1770,8 @@ static int intel_dp_compute_bpp(struct intel_dp *intel_dp,
 			intel_link_compute_m_n(bpp, lane_count,
 				intel_connector->panel.downclock_mode->clock,
 				pipe_config->port_clock,
-				&pipe_config->dp_m2_n2);
+				&pipe_config->dp_m2_n2,
+				reduced_n);
 	}
 
 	/*
diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
index 094cbdc..3303767 100644
--- a/drivers/gpu/drm/i915/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/intel_dp_mst.c
@@ -75,7 +75,8 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
 	intel_link_compute_m_n(bpp, lane_count,
 			       adjusted_mode->crtc_clock,
 			       pipe_config->port_clock,
-			       &pipe_config->dp_m_n);
+			       &pipe_config->dp_m_n,
+			       false);
 
 	pipe_config->dp_m_n.tu = slots;
 
-- 
1.7.9.5

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