[Intel-gfx] [RFC PATCH] drm/i915: avoid round-trip scaling errors in actual_brightness

[Jani Nikula]

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 at intel.com>
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@intel.com
Reference: https://bugzilla.kernel.org/show_bug.cgi?id=85861
Cc: U. Artie Eoff <ullysses.a.eoff at intel.com>
Signed-off-by: Jani Nikula <jani.nikula at intel.com>
---
 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