On Sun, 24 Nov 2013, Matthew Garrett wrote:

On Sat, Nov 23, 2013 at 10:40:15PM -0200, Henrique de Moraes Holschuh wrote:

quoted

On Fri, 22 Nov 2013, Matthew Garrett wrote:

quoted

We have userspace that relies on uevents of type 
BACKLIGHT_UPDATE_HOTKEY. I don't know that we have userspace that relies 
on uevents of type BACKLIGHT_UPDATE_SYSFS.

Any OSD application would have to rely on both uevent types, or it is broken
(and to test that, just write a level to sysfs and watch the OSD app fail to
tell you about the backlight level change...)

Right, OSDs are supposed to respond to keypresses, not arbitrary changes 
of backlight. If the user's just echoed 8 into brightness, they know 
they set the brightness to 8 - they don't need an OSD to tell them that. 

It is not just the user that sets the brightness.

Still, if you're sure that all userspace users react only to the hotkey type
of event, removing the sysfs one won't break anything any further.

But it will be *really* annoying the day we revisit this because someone
started abusing the hotkey uevent and we have to deploy a proper fix (rate
limiting or switching to a proper event report interface that doesn't use
uevents).

BACKLIGHT_UPDATE_HOTKEY is when the firmware itself has changed the 
brightness in response to a keypress, and so reporting the keypress 
would result in additional backlight changes.

Yeah, I know that bug quite well, thinkpads were the first victims of
idiotic feedback event loops caused by braindead userspace.

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh