[PATCH] drm/i915/dp: Fix the math in intel_dp_link_required

[Herton R. Krzesinski]

From: Adam Jackson <ajax at redhat.com>

The previous code was confused about units, which is pretty reasonable
given that the units themselves are confusing.

Signed-off-by: Adam Jackson <ajax at redhat.com>
Signed-off-by: Keith Packard <keithp at keithp.com>
BugLink: https://bugs.launchpad.net/bugs/899598
(backported from commit cd9dde44f47501394b9f0715b6a36a92aa74c0d0)
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.com>
---
 drivers/gpu/drm/i915/intel_dp.c |   25 +++++++++++++++++++++----
 1 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 3ede21d..d4be591 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -174,16 +174,33 @@ intel_dp_link_clock(uint8_t link_bw)
 		return 162000;
 }
 
-/* I think this is a fiction */
+/*
+ * The units on the numbers in the next two are... bizarre.  Examples will
+ * make it clearer; this one parallels an example in the eDP spec.
+ *
+ * intel_dp_max_data_rate for one lane of 2.7GHz evaluates as:
+ *
+ *     270000 * 1 * 8 / 10 == 216000
+ *
+ * The actual data capacity of that configuration is 2.16Gbit/s, so the
+ * units are decakilobits.  ->clock in a drm_display_mode is in kilohertz -
+ * or equivalently, kilopixels per second - so for 1680x1050R it'd be
+ * 119000.  At 18bpp that's 2142000 kilobits per second.
+ *
+ * Thus the strange-looking division by 10 in intel_dp_link_required, to
+ * get the result in decakilobits instead of kilobits.
+ */
+
 static int
 intel_dp_link_required(struct drm_device *dev, struct intel_dp *intel_dp, int pixel_clock)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
+	int bpp = 24;
 
 	if (is_edp(intel_dp))
-		return (pixel_clock * dev_priv->edp.bpp + 7) / 8;
-	else
-		return pixel_clock * 3;
+		bpp = dev_priv->edp.bpp;
+
+	return (pixel_clock * bpp + 9) / 10;
 }
 
 static int
-- 
1.7.5.4