backlight: Drop support for actual_brightness

Some AMD systems have support for features like custom brightness
curve or adaptive backlight management.  These features allow the
display driver to adjust the brightness based upon other factors
than just the user brightness request.

The user's brightness request is indicated in the 'brightness' file
but the effective result of the logic in the display driver is stored
in the 'actual_brightness' file.

This leads to problems when shutting the system down because the value
of 'actual_brightness' may be lower than 'brightness' and the wrong value
gets stored for the next boot.

For example if the brightness a user requested was 150, the actual_brightness
might be 130. So the next boot the brightness will be "set" to 130, but the
actual brightness might be 115. If the user reboots again it will be set to 115
for the next boot but the actual brightness might be 100. That is this gets worse
and worse each reboot cycle until the system eventually boots up at minimum
brightness.

Furthermore the kernel documentation indicates that the brightness and
actual_brightness files are not guaranteed to be the same values.

Due to this; drop the use of 'actual_brightness' when saving/restoring brightness
and instead rely only upon 'brightness'.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
(cherry picked from commit 9a224c307b36610e3675390eb2620e74d0f4efb0)
(cherry picked from commit 8f93a8c5282397feb30a54687dce6921faa90d84)

diff --git a/src/backlight/backlight.c b/src/backlight/backlight.c
index 2de6c2008af0d6e01f9c74c94f7250b4fbcc7ff9..018cd3532132a38b20907b86cc193c73b9bb0c63 100644 (file)
--- a/src/backlight/backlight.c
+++ b/src/backlight/backlight.c
@@ -408,35 +408,6 @@ static int read_brightness(sd_device *device, unsigned max_brightness, unsigned
         assert(device);
         assert(ret_brightness);
 
-        if (device_in_subsystem(device, "backlight")) {
-                r = sd_device_get_sysattr_value(device, "actual_brightness", &value);
-                if (r == -ENOENT) {
-                        log_device_debug_errno(device, r, "Failed to read 'actual_brightness' attribute, "
-                                               "fall back to use 'brightness' attribute: %m");
-                        goto use_brightness;
-                }
-                if (r < 0)
-                        return log_device_debug_errno(device, r, "Failed to read 'actual_brightness' attribute: %m");
-
-                r = safe_atou(value, &brightness);
-                if (r < 0) {
-                        log_device_debug_errno(device, r, "Failed to parse 'actual_brightness' attribute, "
-                                               "fall back to use 'brightness' attribute: %s", value);
-                        goto use_brightness;
-                }
-
-                if (brightness > max_brightness) {
-                        log_device_debug(device, "actual_brightness=%u is larger than max_brightness=%u, "
-                                         "fall back to use 'brightness' attribute", brightness, max_brightness);
-                        goto use_brightness;
-                }
-
-                log_device_debug(device, "Current actual_brightness is %u", brightness);
-                *ret_brightness = brightness;
-                return 0;
-        }
-
-use_brightness:
         r = sd_device_get_sysattr_value(device, "brightness", &value);
         if (r < 0)
                 return log_device_debug_errno(device, r, "Failed to read 'brightness' attribute: %m");