Pipe vs. plane fixes (hopefully third time's the charm)

[Jesse Barnes]

On Thursday, September 13, 2007, Roland Dreier wrote:
> > We already have to read a few registers on this path though, so any
> > added cost should be hard to measure...
>
> I would assume that register reads are not posted, so every one ends
> up getting serialized and every additional read would hurt a lot,
> unless you're on a path where performance doesn't matter anyway.

Right, but my point was that this path already has a fairly high fixed 
overhead--adding a single additional register read shouldn't double the 
cost or anything.  Moreover, it's not a performance critical path for 
the most part:  vblank swaps just have to get done before the next 
frame is displayed, which is a fairly long time in terms of CPU 
instructions.  But again, if it turns out to be a problem we can always 
use another scheme.

Jesse