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

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, explained below:
1. For DSI AUO panel(1920x1200 @60) required pixclk is 157100 KHZ
2. glk_calc_cdclk returns 79200 KHZ for this pixclk, For 2PPC it
   will be 158400 KHZ
3. Practically 100% of the cdclk can’t be achieved, so 99% of 158400
   KHZ  = 156816 which is less than the desired pixlclk and causes
   panel corruption.

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
v5: Added details of non-working scenario in commit message

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>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1491397463-13637-1-git-send-email-madhav.chauhan@intel.com

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;