[PATCH v6] drm/i915: respect the VBT minimum backlight brightness again

["Shih-Yuan Lee (FourDollars)" <sylee@canonical.com>]

There was a wonderful period after

commit 6dda730e55f412a6dfb181cae6784822ba463847
Author: Jani Nikula <jani.nikula at intel.com>
Date:   Tue Jun 24 18:27:40 2014 +0300

    drm/i915: respect the VBT minimum backlight brightness

The backlight class 0 brightness means the PWM min and it does not turn
off the backlight. After kernel 3.18, the backlight class 0 brightness
is used to turn off the backlight and the PWM min is missing.

Because of

commit e6755fb78e8f20ecadf2a4080084121336624ad9
Author: Jani Nikula <jani.nikula at intel.com>
Date:   Tue Aug 12 17:11:42 2014 +0300

    drm/i915: switch off backlight for backlight class 0 brightness

Use "VBT backlight PWM modulation frequency 200 Hz, active high, min
brightness 10, level 255" as an example. It means the VBT min is 10
out of [0..255] and the PWM max is 937 from other place so the
corresponding PWM min should be 37 (10 * 937 / 255).

When we set backlight class 0 brightness, the backlight is turned off.
Althought the PWM value is 37 when the backlight is off but it is
useless. When we set set backlight class 1 brightness, the backlight is
on but the PWM value is 38 and it doesn't match the corresponding PWM
min of the VBT minimum backlight.

And it has another minor issue that there are some backlight class
brightness values are mapped to the same PWM value because the backlight
class brightness range is larger than the valid PWM brightness range.

The backlight class brightness range [0..937]
The valid PWM brightness range [37..937]

This commit makes that backlight class 1 brightness means the PWM min
and backlight class max_brightness means the PWM max, and the ranges
become

The backlight class brightness range [0..901]
The valid PWM brightness range [36..937]

That's no backlight class brightness value mapped to the same PWM value.

Signed-off-by: Shih-Yuan Lee (FourDollars) <sylee@canonical.com>
---
 drivers/gpu/drm/i915/intel_panel.c | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index b05c6d9..8efa199 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -1194,10 +1194,9 @@ static int intel_backlight_device_register(struct intel_connector *connector)
 	props.type = BACKLIGHT_RAW;
 
 	/*
-	 * Note: Everything should work even if the backlight device max
-	 * presented to the userspace is arbitrarily chosen.
+	 * Expose the whole valid PWM brightness range to the backlight class.
 	 */
-	props.max_brightness = panel->backlight.max;
+	props.max_brightness = panel->backlight.max - panel->backlight.min;
 	props.brightness = scale_hw_to_user(connector,
 					    panel->backlight.level,
 					    props.max_brightness);
@@ -1400,7 +1399,8 @@ static u32 get_backlight_min_vbt(struct intel_connector *connector)
 	struct drm_device *dev = connector->base.dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct intel_panel *panel = &connector->panel;
-	int min;
+	int vbt_min;
+	u32 pwm_min;
 
 	WARN_ON(panel->backlight.max == 0);
 
@@ -1411,14 +1411,24 @@ static u32 get_backlight_min_vbt(struct intel_connector *connector)
 	 * against this by letting the minimum be at most (arbitrarily chosen)
 	 * 25% of the max.
 	 */
-	min = clamp_t(int, dev_priv->vbt.backlight.min_brightness, 0, 64);
-	if (min != dev_priv->vbt.backlight.min_brightness) {
+	vbt_min = clamp_t(int, dev_priv->vbt.backlight.min_brightness, 0, 64);
+	if (vbt_min != dev_priv->vbt.backlight.min_brightness) {
 		DRM_DEBUG_KMS("clamping VBT min backlight %d/255 to %d/255\n",
-			      dev_priv->vbt.backlight.min_brightness, min);
+			      dev_priv->vbt.backlight.min_brightness, vbt_min);
 	}
 
 	/* vbt value is a coefficient in range [0..255] */
-	return scale(min, 0, 255, 0, panel->backlight.max);
+	pwm_min = scale(vbt_min, 0, 255, 0, panel->backlight.max);
+
+	/*
+	 * Because backlight class brightness 0 is used to turn off the backlight, we
+	 * need to step down a little bit here to make backlight class brightness 1
+	 * match the real PWM min.
+	 */
+	if (pwm_min > 0)
+		return pwm_min - 1;
+	else
+		return 0;
 }
 
 static int lpt_setup_backlight(struct intel_connector *connector, enum pipe unused)
-- 
1.9.1

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