drm/i915/bdw: GEN8 backlight support

Prior to Haswell the CPU control register for backlight
(BLC_PWM_CPU_CTL) toggled the PCH baclight pin for us. This made some
sense as there was no pin on the CPU. With Haswell came the introduction
of a CPU backlight pin, but the interface was still controlled by
software with the same mechnism. Behind the scenes, hardware did all the
dirty work for us.

Broadwell no longer provides this for free. If we want to use the PCH
backlight pin [1] then we have to set the override bit BLC_PWM_PCH_CTL1
and program BLC_PWM_PCH_CTL2 for the PWM values.

This patch implements that. This patch is compile tested only, and given
that I rarely if ever touch this code, careful review is welcome.

[1] According to Art, we know of no devices that exist which use the CPU
pin (and remember it has existed already on HSW). If such a device does
exist, we'll have to handle it properly - this is left as TODO until
then.

v2: Drop the abstraction prep patch, as a bigger backlight overhaul is
    in the works, and do just the mimimal bdw enabling now. (by Jani)

CC: Art Runyan <arthur.j.runyan@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_panel.c

@@ -451,7 +451,9 @@ static u32 intel_panel_get_backlight(struct drm_device *dev,
 
 	spin_lock_irqsave(&dev_priv->backlight.lock, flags);
 
-	if (HAS_PCH_SPLIT(dev)) {
+	if (IS_BROADWELL(dev)) {
+		val = I915_READ(BLC_PWM_PCH_CTL2) & BACKLIGHT_DUTY_CYCLE_MASK;
+	} else if (HAS_PCH_SPLIT(dev)) {
 		val = I915_READ(BLC_PWM_CPU_CTL) & BACKLIGHT_DUTY_CYCLE_MASK;
 	} else {
 		if (IS_VALLEYVIEW(dev))
@@ -479,6 +481,13 @@ static u32 intel_panel_get_backlight(struct drm_device *dev,
 	return val;
 }
 
+static void intel_bdw_panel_set_backlight(struct drm_device *dev, u32 level)
+{
+	struct drm_i915_private *dev_priv = dev->dev_private;
+	u32 val = I915_READ(BLC_PWM_PCH_CTL2) & ~BACKLIGHT_DUTY_CYCLE_MASK;
+	I915_WRITE(BLC_PWM_PCH_CTL2, val | level);
+}
+
 static void intel_pch_panel_set_backlight(struct drm_device *dev, u32 level)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
@@ -496,7 +505,9 @@ static void intel_panel_actually_set_backlight(struct drm_device *dev,
 	DRM_DEBUG_DRIVER("set backlight PWM = %d\n", level);
 	level = intel_panel_compute_brightness(dev, pipe, level);
 
-	if (HAS_PCH_SPLIT(dev))
+	if (IS_BROADWELL(dev))
+		return intel_bdw_panel_set_backlight(dev, level);
+	else if (HAS_PCH_SPLIT(dev))
 		return intel_pch_panel_set_backlight(dev, level);
 
 	if (is_backlight_combination_mode(dev)) {
@@ -666,7 +677,16 @@ void intel_panel_enable_backlight(struct intel_connector *connector)
 		POSTING_READ(reg);
 		I915_WRITE(reg, tmp | BLM_PWM_ENABLE);
 
-		if (HAS_PCH_SPLIT(dev) &&
+		if (IS_BROADWELL(dev)) {
+			/*
+			 * Broadwell requires PCH override to drive the PCH
+			 * backlight pin. The above will configure the CPU
+			 * backlight pin, which we don't plan to use.
+			 */
+			tmp = I915_READ(BLC_PWM_PCH_CTL1);
+			tmp |= BLM_PCH_OVERRIDE_ENABLE | BLM_PCH_PWM_ENABLE;
+			I915_WRITE(BLC_PWM_PCH_CTL1, tmp);
+		} else if (HAS_PCH_SPLIT(dev) &&
 		    !(dev_priv->quirks & QUIRK_NO_PCH_PWM_ENABLE)) {
 			tmp = I915_READ(BLC_PWM_PCH_CTL1);
 			tmp |= BLM_PCH_PWM_ENABLE;