[PATCH 1/1][Oneiric SRU] Platform: Brightness quirk for samsung laptop driver

Andy Whitcroft apw at canonical.com
Thu Dec 8 10:36:37 UTC 2011


On Wed, Dec 07, 2011 at 08:29:07AM -0600, Seth Forshee wrote:

> I actually tried this as on the NF310 as I occasionally see the first
> part (setting the brightness to zero) succeed and the second part
> (setting it to the desired value) fail, so I end up with a brightness of
> 0. These are pretty rare though; I've only seen them when I'm pounding
> continuously on the backlight controls.
> 
> When using the loop you suggested, for whatever reason the value the
> BIOS returns for the current brightness never changes even though the
> brightness itself is changing, so the loop never terminates. If you
> read the brightness later on the value is correct, but as long as you're
> reading it in that loop the value doesn't change. Bouncing through 0
> seems to be the most reliable method on these models.

That almost sounds like gcc is not realising the function is volatile
and optimising it away.  You might want to look at the loop and see if
the function is being inlined, and whether the optimiser has neutered
it.  Stuffing in a full memory barrier in the loop may get gcc's head in
the game.

-apw



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