Due to scaling, the userspace and hardware brightness ranges might not
have a 1:1 mapping, causing the backlight class sysfs actual_brightness
not match the brightness attribute just written. While this is not a
strict requirement per Documentation/ABI/stable/sysfs-class-backlight,
try the userspace->hardware scaling for a match first and return the
cached value to not confuse userspace.
The problem was already mitigated by
commit 673e7bbdb3920b62cfc6c710bea626b0a9b0f43a
Author: U. Artie Eoff <ullysses.a.eoff@xxxxxxxxx>
Date: Mon Sep 29 15:49:32 2014 -0700
drm/i915: intel_backlight scale() math WA
but this should be more robust for cases where the userspace expects
actual_brightness to match the just written brightness.
Reference: http://mid.gmane.org/1415737838-9640-1-git-send-email-ullysses.a.eoff@xxxxxxxxx
Reference: https://bugzilla.kernel.org/show_bug.cgi?id=85861
Cc: U. Artie Eoff <ullysses.a.eoff@xxxxxxxxx>
Signed-off-by: Jani Nikula <jani.nikula@xxxxxxxxx>
---
drivers/gpu/drm/i915/intel_panel.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index 4d63839bd9b4..675b56e105e7 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -1024,7 +1024,17 @@ static int intel_backlight_device_get_brightness(struct backlight_device *bd)
drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
hw_level = intel_panel_get_backlight(connector);
- ret = scale_hw_to_user(connector, hw_level, bd->props.max_brightness);
+
+ /*
+ * Check the userspace->hardware scaling for a match first to avoid
+ * scaling errors in the userspace->hardware->userspace round-trip.
+ */
+ if (hw_level == scale_user_to_hw(connector, bd->props.brightness,
+ bd->props.max_brightness))
+ ret = bd->props.brightness;
+ else
+ ret = scale_hw_to_user(connector, hw_level,
+ bd->props.max_brightness);
drm_modeset_unlock(&dev->mode_config.connection_mutex);
intel_runtime_pm_put(dev_priv);
--
2.1.3
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