[Intel-gfx] [RFC][PATCH] drm/i915: Make data/link N value power of two

[ville.syrjala at linux.intel.com]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

The BIOS uses power of two values for the data/link N value.

Follow suit to make the Zotac DP to dual-HDMI dongle work.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=49402
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
Warning: I've only tested this with the Zotac dongle hooked to my 1080p
Samsumg telly. Whether or not it works with other displays is an open
question.

 drivers/gpu/drm/i915/intel_display.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 74156e2..fea9c99 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -4151,18 +4151,29 @@ intel_reduce_ratio(uint32_t *num, uint32_t *den)
 	}
 }
 
+static void compute_m_n(int m, int n, uint32_t *ret_m, uint32_t *ret_n)
+{
+	if (n >= 1 << 23)
+		*ret_n = 1 << 23;
+	else
+		*ret_n = roundup_pow_of_two(n);
+	*ret_m = div_u64((uint64_t) m * *ret_n, n);
+	intel_reduce_ratio(ret_m, ret_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)
 {
 	m_n->tu = 64;
-	m_n->gmch_m = bits_per_pixel * pixel_clock;
-	m_n->gmch_n = link_clock * nlanes * 8;
-	intel_reduce_ratio(&m_n->gmch_m, &m_n->gmch_n);
-	m_n->link_m = pixel_clock;
-	m_n->link_n = link_clock;
-	intel_reduce_ratio(&m_n->link_m, &m_n->link_n);
+
+	compute_m_n(bits_per_pixel * pixel_clock,
+		    link_clock * nlanes * 8,
+		    &m_n->gmch_m, &m_n->gmch_n);
+
+	compute_m_n(pixel_clock, link_clock,
+		    &m_n->link_m, &m_n->link_n);
 }
 
 static inline bool intel_panel_use_ssc(struct drm_i915_private *dev_priv)
-- 
1.8.1.5