drm/i915/backlight: setup and cache pwm alternate increment value

This will also be needed later on when setting up the alternate
increment in backlight enable.

Cc: Shawn Lee <shawn.c.lee@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/9984b20bc59aee90b83caf59ce91f3fb122c9627.1474281249.git.jani.nikula@intel.com

drivers/gpu/drm/i915/intel_drv.h

@@ -263,6 +263,7 @@ struct intel_panel {
 		bool enabled;
 		bool combination_mode;	/* gen 2/4 only */
 		bool active_low_pwm;
+		bool alternate_pwm_increment;	/* lpt+ */
 
 		/* PWM chip */
 		bool util_pin_active_low;	/* bxt+ */

drivers/gpu/drm/i915/intel_panel.c

@@ -1242,10 +1242,10 @@ static u32 bxt_hz_to_pwm(struct intel_connector *connector, u32 pwm_freq_hz)
  */
 static u32 spt_hz_to_pwm(struct intel_connector *connector, u32 pwm_freq_hz)
 {
-	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
+	struct intel_panel *panel = &connector->panel;
 	u32 mul;
 
-	if (I915_READ(SOUTH_CHICKEN1) & SPT_PWM_GRANULARITY)
+	if (panel->backlight.alternate_pwm_increment)
 		mul = 128;
 	else
 		mul = 16;
@@ -1261,9 +1261,10 @@ static u32 spt_hz_to_pwm(struct intel_connector *connector, u32 pwm_freq_hz)
 static u32 lpt_hz_to_pwm(struct intel_connector *connector, u32 pwm_freq_hz)
 {
 	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
+	struct intel_panel *panel = &connector->panel;
 	u32 mul, clock;
 
-	if (I915_READ(SOUTH_CHICKEN2) & LPT_PWM_GRANULARITY)
+	if (panel->backlight.alternate_pwm_increment)
 		mul = 16;
 	else
 		mul = 128;
@@ -1414,6 +1415,13 @@ static int lpt_setup_backlight(struct intel_connector *connector, enum pipe unus
 	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
 	struct intel_panel *panel = &connector->panel;
 	u32 pch_ctl1, pch_ctl2, val;
+	bool alt;
+
+	if (HAS_PCH_LPT(dev_priv))
+		alt = I915_READ(SOUTH_CHICKEN2) & LPT_PWM_GRANULARITY;
+	else
+		alt = I915_READ(SOUTH_CHICKEN1) & SPT_PWM_GRANULARITY;
+	panel->backlight.alternate_pwm_increment = alt;
 
 	pch_ctl1 = I915_READ(BLC_PWM_PCH_CTL1);
 	panel->backlight.active_low_pwm = pch_ctl1 & BLM_PCH_POLARITY;