On Mon, Mar 31, 2008 at 11:06:16AM +0200, Thomas Hellström wrote:Let me rehprase. Not really time-critical but it is of some importance that CPA is done quickly.
Andi Kleen wrote:
As AGP buffers are moved in- and out of AGP, the caching policy changes, so yes, there may be writes to cache coherent memory that needs to be flushed at some point. Since CPA has been doing that up to now, and the codepaths involved are quite time-critical, a double cache-flush is aAlso I think we need to clarify the semantics of the c_p_a functionality. Right now both AGP and DRM relies on c_p_a doing an explicit cache flush. Otherwise the data won't appear on the device side of the aperture.But surely not in cpa I hope ? Or are you saying you first write data
and then do cpa? If true that would be quite an abuse of CPA I would say and you should fix it ASAP.
That doesn't make sense. You shouldn't be using CPA in any time critical code path. It will always be slow.
For anything time critical you should keep a pool of uncached pages once set up using CPA and reuse them.
CPA really should only be used on initialization or for
larger setup changes which are ok to go somewhat slower.
But then we wouldn't really be discussing SS either? For DRM purposes, the more performance we can squeeze out of CPA, the better.
The cache must be flushed in CPA, there is no way around it.
If you write data into the buffers before you do CPA on them
you could avoid it, but I don't think you should do that anywhere
near time critical code, so it actually shouldn't matter.
-Andi