From: Sean Paul <seanp...@chromium.org>
If the backlight is updated while the panel is being enabled, the value
from userspace (which is stored in panel->backlight.device->props.brightness)
can be replaced by the hardware's minimum level. There's really no good
way to tell if this is happening in enable_backlight() since
props.brightness can be initialized to the same value as is being set by
userspace. So we'll try to reduce the race window as much as possible.
Signed-off-by: Sean Paul <seanp...@chromium.org>
---
I don't think there's any way to eliminate this race since grabbing
bd->op_lock in panel_enable would cause a lock inversion deadlock with
the connection_mutex lock in backlight_device_update_status
Suggestions very much welcome!
drivers/gpu/drm/i915/display/intel_panel.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/drivers/gpu/drm/i915/display/intel_panel.c
b/drivers/gpu/drm/i915/display/intel_panel.c
index 3c5056dbf607..abdfb9cc281b 100644
--- a/drivers/gpu/drm/i915/display/intel_panel.c
+++ b/drivers/gpu/drm/i915/display/intel_panel.c
@@ -1285,8 +1285,22 @@ static int intel_backlight_device_update_status(struct
backlight_device *bd)
struct intel_connector *connector = bl_get_data(bd);
struct intel_panel *panel = &connector->panel;
struct drm_device *dev = connector->base.dev;
+ int value;
+
+ /*
+ * Before we attempt to grab the connection mutex, cache the incoming
+ * brightness value. If we're in the middle of a modeset,
+ * intel_panel_enable_backlight will be called and could pave over
+ * props.brightness. This is still racey, but the race window should be
+ * significantly smaller and reflects the inherent raceyness of the
+ * updating props.brightness outside of bd->op_lock.
+ */
+ value = bd->props.brightness;
drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
+
+ bd->props.brightness = value;
+
DRM_DEBUG_KMS("updating intel_backlight, brightness=%d/%d\n",
bd->props.brightness, bd->props.max_brightness);
intel_panel_set_backlight(connector->base.state, bd->props.brightness,
--
Sean Paul, Software Engineer, Google / Chromium OS
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