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

[clinton.a.taylor@intel.com]

From: Clint Taylor <clinton.a.taylor@intel.com>

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.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>

Signed-off-by: Clint Taylor <clinton.a.taylor@intel.com>
---
 drivers/gpu/drm/i915/i915_reg.h      |    2 ++
 drivers/gpu/drm/i915/intel_display.c |   15 +++++++++++----
 2 files changed, 13 insertions(+), 4 deletions(-)

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..6e1fdd2 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -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);
 }
@@ -6327,14 +6328,20 @@ static void compute_m_n(unsigned int m, unsigned int n,
 		       int pixel_clock, int link_clock,
 		       struct intel_link_m_n *m_n)
 {
+	uint32_t max_link_n = DATA_LINK_N_MAX;
 	m_n->tu = 64;
 
+	if ((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)
-- 
1.7.9.5

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