Backlight brightness curves can have different shapes. The two main
types are linear and non-linear curves. The human eye doesn't
perceive linearly increasing/decreasing brightness as linear (see
also 88ba95bedb79 "backlight: pwm_bl: Compute brightness of LED
linearly to human eye"), hence many backlights use non-linear (often
logarithmic) brightness curves. The type of curve currently is opaque
to userspace, so userspace often relies on more or less reliable
heuristics (like the number of brightness levels) to decide whether
to treat a backlight device as linear or non-linear.
Export the type of the brightness curve via the new sysfs attribute
'scale'. The value of the attribute may be a simple string like
'linear' or 'non-linear', or a composite string similar to
'compatible' strings of the device tree. A composite string consists
of different elements separated by commas, starting with the
most-detailed description and ending with the least-detailed one. An
example for a composite string is "cie-1931,perceptual,non-linear"
This brightness curve was generated with the CIE 1931 algorithm, it
is perceptually linear, but not actually linear in terms of the
emitted light. If userspace doesn't know about 'cie-1931' or
'perceptual' it should at least be able to interpret the 'non-linear'
part.
For devices that don't provide information about the scale of their
brightness curve the value of the 'scale' attribute is 'unknown'.
Signed-off-by: Matthias Kaehlcke
---
.../ABI/testing/sysfs-class-backlight | 32 +++
MAINTAINERS | 1 +
drivers/video/backlight/backlight.c | 22 +
include/linux/backlight.h | 10 ++
4 files changed, 65 insertions(+)
create mode 100644 Documentation/ABI/testing/sysfs-class-backlight
diff --git a/Documentation/ABI/testing/sysfs-class-backlight
b/Documentation/ABI/testing/sysfs-class-backlight
new file mode 100644
index ..924fb68940e6
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-backlight
@@ -0,0 +1,32 @@
+What: /sys/class/backlight//scale
+Date: June 2019
+KernelVersion: 5.4
+Contact: Daniel Thompson
+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 curves 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.
diff --git a/MAINTAINERS b/MAINTAINERS
index d51e74340870..c46812510ba5 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -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
diff --git a/drivers/video/backlight/backlight.c
b/drivers/video/backlight/backlight.c
index 1ef8b6fd62ac..c6e6ea6f2688 100644
--- a/drivers/video/backlight/backlight.c
+++ b/drivers/video/backlight/backlight.c
@@ -32,6 +32,15 @@ static const char *const backlight_types[] = {
[BACKLIGHT_FIRMWARE] = "firmware",
};
+static const char *const backlight_scale_types[] = {
+ [BACKLIGHT_SCALE_UNKNOWN] = "unknown",
+ [BACKLIGHT_SCALE_CIE1931] = "cie-1931,perceptual,non-linear",
+ [BACKLIGHT_SCALE_PERCEPTUAL]= "perceptual,non-linear",
+ [BACKLIGHT_SCALE_LINEAR]= "linear",
+ [BACKLIGHT_SCALE_NON_LINEAR]= "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 +255,18 @@ static ssize_t actual_brightness_show(struct device *dev,
}
static DEVICE_ATTR_RO(actual_brightness);
+static ssize_t scale_show(struct device *dev,
+ struct
