[PATCH 1/2] UBUNTU: [Config] enable MMIOTRACE for graphics debugging

[Andy Whitcroft]

On Wed, Mar 17, 2010 at 03:03:41AM -0700, Bryce Harrington wrote:
> On Wed, Mar 17, 2010 at 09:26:24AM +0000, Andy Whitcroft wrote:
> > Enable MMIOTRACE to allow us to trace low level card operations.  This is
> > commonly used to debug graphics issues particularly under Nouveau.
> > According to the documentation it is a near zero impact when disabled.
> 
> "when enabled" I hope?

The cost is near-zero for having the MMIOTRACE option enabled with the
trace collection turned off (the default).  We take one additional branch
in the mmio map/upmap paths which are non-critical initialisation/teardone
paths in normal usage.  The actual mmio operations are not affected at all.
When we enable trace collection the overhead is high, taking a page
fault for each access to the mmio mapping.

-apw