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

[Seth Forshee]

On Thu, Dec 08, 2011 at 10:36:37AM +0000, Andy Whitcroft wrote:
> 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.

I was pretty sure that wasn't the case, but I went back and
double-checked to be sure. read_brightness is in fact being called for
each loop iteration, but the value it returns doesn't always reflect
the actual brightness (sometimes the loop works, but rarely).