[PATCH] drm/i915/glk: limit pixel clock to 99% of cdclk workaround

[Jani Nikula <jani.nikula@intel.com>]

From: Madhav Chauhan <madhav.chauhan@intel.com>

As per BSPEC, valid cdclk values for glk are 79.2, 158.4, 316.8 Mhz.
Practically we can achive only 99% of these cdclk values (HW team
checking on this). So cdclk should be calculated for the given pixclk as
per that otherwise it may lead to screen corruption for some scenarios.

v2: Rebased to new CDLCK code framework
v3: Addressed review comments from Ander/Jani
    - Add comment in code about 99% usage of CDCLK
    - Calculate max dot clock as well with 99% limit
v4 by Jani:
    - drop superfluous whitespace change
    - rewrite code comments to clarify

Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
---
 drivers/gpu/drm/i915/intel_cdclk.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_cdclk.c b/drivers/gpu/drm/i915/intel_cdclk.c
index dd3ad52b7dfe..763010f8ad89 100644
--- a/drivers/gpu/drm/i915/intel_cdclk.c
+++ b/drivers/gpu/drm/i915/intel_cdclk.c
@@ -1071,9 +1071,15 @@ static int bxt_calc_cdclk(int max_pixclk)
 
 static int glk_calc_cdclk(int max_pixclk)
 {
-	if (max_pixclk > 2 * 158400)
+	/*
+	 * FIXME: Avoid using a pixel clock that is more than 99% of the cdclk
+	 * as a temporary workaround. Use a higher cdclk instead. (Note that
+	 * intel_compute_max_dotclk() limits the max pixel clock to 99% of max
+	 * cdclk.)
+	 */
+	if (max_pixclk > DIV_ROUND_UP(2 * 158400 * 99, 100))
 		return 316800;
-	else if (max_pixclk > 2 * 79200)
+	else if (max_pixclk > DIV_ROUND_UP(2 * 79200 * 99, 100))
 		return 158400;
 	else
 		return 79200;
@@ -1664,7 +1670,11 @@ static int intel_compute_max_dotclk(struct drm_i915_private *dev_priv)
 	int max_cdclk_freq = dev_priv->max_cdclk_freq;
 
 	if (IS_GEMINILAKE(dev_priv))
-		return 2 * max_cdclk_freq;
+		/*
+		 * FIXME: Limiting to 99% as a temporary workaround. See
+		 * glk_calc_cdclk() for details.
+		 */
+		return 2 * max_cdclk_freq * 99 / 100;
 	else if (INTEL_INFO(dev_priv)->gen >= 9 ||
 		 IS_HASWELL(dev_priv) || IS_BROADWELL(dev_priv))
 		return max_cdclk_freq;
-- 
2.1.4

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