[PATCH v2 2/4] backlight: Expose brightness curve type through sysfs

@@ -0,0 +1,32 @@
+What:		/sys/class/backlight/<backlight>/scale
+Date:		June 2019
+KernelVersion:	5.4
+Contact:	Daniel Thompson <daniel.thompson@linaro.org>
+Description:
+		Description of the scale of the brightness curve. The
+		description consists of one or more elements separated by
+		commas, from the most detailed to the least detailed
+		description.
+
+		Possible values are:
+
+		unknown
+		  The scale of the brightness curve is unknown.
+
+		linear
+		  The brightness changes linearly in terms of the emitted
+		  light, changes are perceived as non-linear by the human eye.
+
+		non-linear
+		  The brightness changes non-linearly in terms of the emitted
+		  light, changes might be perceived as linear by the human eye.
+
+		perceptual,non-linear
+		  The brightness changes non-linearly in terms of the emitted
+		  light, changes should be perceived as linear by the human eye.
+
+		cie-1931,perceptual,non-linear
+		  The brightness curve was calculated with the CIE 1931
+		  algorithm. Brightness changes non-linearly in terms of the
+		  emitted light, changes should be perceived as linear by the
+		  human eye.

@@ -2858,6 +2858,7 @@ F:	include/linux/backlight.h
 F:	include/linux/pwm_backlight.h
 F:	Documentation/devicetree/bindings/leds/backlight
 F:	Documentation/ABI/stable/sysfs-class-backlight
+F:	Documentation/ABI/testing/sysfs-class-backlight
 
 BATMAN ADVANCED
 M:	Marek Lindner <mareklindner@neomailbox.ch>

@@ -32,6 +32,14 @@ static const char *const backlight_types[] = {
 	[BACKLIGHT_FIRMWARE] = "firmware",
 };
 
+static const char *const backlight_scale_types[] = {
+	[BACKLIGHT_SCALE_UNKNOWN]	= "unknown",
+	[BACKLIGHT_SCALE_LINEAR]	= "linear",
+	[BACKLIGHT_SCALE_NON_LINEAR]	= "non-linear",
+	[BACKLIGHT_SCALE_PERCEPTUAL]	= "perceptual,non-linear",
+	[BACKLIGHT_SCALE_CIE1931]	= "cie-1931,perceptual,non-linear",
+};
+
 #if defined(CONFIG_FB) || (defined(CONFIG_FB_MODULE) && \
 			   defined(CONFIG_BACKLIGHT_CLASS_DEVICE_MODULE))
 /* This callback gets called when something important happens inside a

@@ -246,6 +254,18 @@ static ssize_t actual_brightness_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(actual_brightness);
 
+static ssize_t scale_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct backlight_device *bd = to_backlight_device(dev);
+
+	if (WARN_ON(bd->props.scale > BACKLIGHT_SCALE_NON_LINEAR))
+		return sprintf(buf, "unknown\n");
+
+	return sprintf(buf, "%s\n", backlight_scale_types[bd->props.scale]);
+}
+static DEVICE_ATTR_RO(scale);
+
 static struct class *backlight_class;
 
 #ifdef CONFIG_PM_SLEEP

@@ -292,6 +312,7 @@ static struct attribute *bl_device_attrs[] = {
 	&dev_attr_brightness.attr,
 	&dev_attr_actual_brightness.attr,
 	&dev_attr_max_brightness.attr,
+	&dev_attr_scale.attr,
 	&dev_attr_type.attr,
 	NULL,
 };

@@ -46,6 +46,14 @@ enum backlight_notification {
 	BACKLIGHT_UNREGISTERED,
 };
 
+enum backlight_scale {
+	BACKLIGHT_SCALE_UNKNOWN = 0,
+	BACKLIGHT_SCALE_LINEAR,
+	BACKLIGHT_SCALE_NON_LINEAR,	/* needed for backwards compatibility */
+	BACKLIGHT_SCALE_PERCEPTUAL,
+	BACKLIGHT_SCALE_CIE1931,
+};
+
 struct backlight_device;
 struct fb_info;
 

@@ -80,6 +88,8 @@ struct backlight_properties {
 	enum backlight_type type;
 	/* Flags used to signal drivers of state changes */
 	unsigned int state;
+	/* Type of the brightness scale (linear, non-linear, ...) */
+	enum backlight_scale scale;
 
 #define BL_CORE_SUSPENDED	(1 << 0)	/* backlight is suspended */
 #define BL_CORE_FBBLANK		(1 << 1)	/* backlight is under an fb blank event */