[PATCH v4] drm/i915: Set brightness maximum to a fixed value 100.

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

Take Dell XPS 13 (2015) as an example. The vbt min 10 out of [0..255].
The PWM max is 937 so the corresponding PWM min is 37 (10*937/256).
This commit makes the sysfs brightness 1 map to the PWM brightness 37
and 100 map to the PWM brightness 937.

Signed-off-by: Shih-Yuan Lee (FourDollars) <sylee@canonical.com>
---
 drivers/gpu/drm/i915/intel_panel.c | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index a24df35..8652ce2 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -1207,11 +1207,12 @@ static int intel_backlight_device_register(struct intel_connector *connector)
 	memset(&props, 0, sizeof(props));
 	props.type = BACKLIGHT_RAW;
 
-	/*
-	 * Note: Everything should work even if the backlight device max
-	 * presented to the userspace is arbitrarily chosen.
-	 */
-	props.max_brightness = panel->backlight.max;
+	/* Setting max to fixed 100 if the range is large enough. */
+	if (panel->backlight.max > panel->backlight.min + 99)
+		props.max_brightness = 100;
+	else
+		props.max_brightness = panel->backlight.max - panel->backlight.min;
+
 	props.brightness = scale_hw_to_user(connector,
 					    panel->backlight.level,
 					    props.max_brightness);
@@ -1414,6 +1415,8 @@ 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;
+	u32 pwm_min;
+	u32 pwm_step;
 	int min;
 
 	WARN_ON(panel->backlight.max == 0);
@@ -1432,7 +1435,22 @@ static u32 get_backlight_min_vbt(struct intel_connector *connector)
 	}
 
 	/* vbt value is a coefficient in range [0..255] */
-	return scale(min, 0, 255, 0, panel->backlight.max);
+	pwm_min = scale(min, 0, 255, 0, panel->backlight.max);
+
+	/* Calculate the PWM step */
+	if (panel->backlight.max > pwm_min + 99)
+		pwm_step = scale(1, 0, 99, 0, panel->backlight.max - pwm_min);
+	else
+		pwm_step = 1;
+
+	/*
+	 * Because sysfs brightness 0 is used to turn off the backlight, we need step
+	 * down a little bit to make sysfs brightness 1 match the lowest brightness.
+	 */
+	if (pwm_min >= pwm_step)
+		pwm_min -= pwm_step;
+
+	return pwm_min;
 }
 
 static int lpt_setup_backlight(struct intel_connector *connector, enum pipe unused)
-- 
1.9.1
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