From: Sean Paul seanpaul@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 seanpaul@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,