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

[Srivatsa, Anusha]

>-----Original Message-----
>From: Taylor, Clinton A
>Sent: Wednesday, March 22, 2017 4:28 PM
>To: Intel-gfx at lists.freedesktop.org
>Cc: Taylor, Clinton A <clinton.a.taylor at intel.com>; Nikula, Jani
><jani.nikula at intel.com>; Srivatsa, Anusha <anusha.srivatsa at intel.com>
>Subject: [PATCH] drm/i915: Reduce Data Link N value for 1 lane DP->hdmi
>converters
>
>From: Clint Taylor <clinton.a.taylor at 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.

Reviewed-by: Anusha Srivatsa <anusha.srivatsa at intel.com>
>Cc: Jani Nikula <jani.nikula at intel.com>
>Cc: Anusha Srivatsa <anusha.srivatsa at intel.com>
>
>Signed-off-by: Clint Taylor <clinton.a.taylor at 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