drm/i915/bxt: backlight clock gating workaround

Per bspec, "Backlight PWM may stop in the asserted state, causing
backlight to stay fully on. WA: Before disabling PWM, set CLKGATE_DIS_0
0x46530 bit 13 PWM1 Gating Dis (for PWM1) or bit 14 PWM2 Gating Dis (for
PWM2). The bits can remain set without harm." (There's no workaround
name for this.)

This fixes some Broxton backlight issues.
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[Jani: cleanup & commit message]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448958232-26520-3-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/i915_reg.h

@@ -2972,6 +2972,13 @@ enum skl_disp_power_wells {
 #define OGAMC1			_MMIO(0x30020)
 #define OGAMC0			_MMIO(0x30024)
 
+/*
+ * GEN9 clock gating regs
+ */
+#define GEN9_CLKGATE_DIS_0		_MMIO(0x46530)
+#define   PWM2_GATING_DIS		(1 << 14)
+#define   PWM1_GATING_DIS		(1 << 13)
+
 /*
  * Display engine regs
  */

drivers/gpu/drm/i915/intel_pm.c

@@ -66,6 +66,14 @@ static void bxt_init_clock_gating(struct drm_device *dev)
 	 */
 	I915_WRITE(GEN8_UCGCTL6, I915_READ(GEN8_UCGCTL6) |
 		   GEN8_HDCUNIT_CLOCK_GATE_DISABLE_HDCREQ);
+
+	/*
+	 * Wa: Backlight PWM may stop in the asserted state, causing backlight
+	 * to stay fully on.
+	 */
+	if (IS_BXT_REVID(dev_priv, BXT_REVID_B0, REVID_FOREVER))
+		I915_WRITE(GEN9_CLKGATE_DIS_0, I915_READ(GEN9_CLKGATE_DIS_0) |
+			   PWM1_GATING_DIS | PWM2_GATING_DIS);
 }
 
 static void i915_pineview_get_mem_freq(struct drm_device *dev)