Re: DRM memory manager on cards with hardware contexts

Tue, 19 Sep 2006 03:40:23 -0700

Benjamin Herrenschmidt wrote: 
On Tue, 2006-09-19 at 11:27 +0200, Thomas Hellström wrote:

  
But this should be the same problem encountered by the agpgart driver?
x86 and x86-64 calls change_page_attr() to take care of this.
On powerpc it is simply a noop. (<asm/agp.h>)
    

Possibly. We sort-of ignore the issue for now on PowerPC and happen to
be lucky most of the time because 32 bits PowerPC aren't that agressive
at prefetching...

I haven't looked at change_page_attr() implementation on x86 but I know
they also map the linear mapping with large pages. I don't know what
happens if you start trying to change a single page attribute. x86 can
breakup that large page into 4k pages, so maybe that's what happens.
Yes, I think that's what happens. I know some Athlon chips had a big issue with this some time ago.

I notice there are special functions in <agp.h> to alloc / free GATT pages, so the general idea might be to have a pool of uncached pages in the future for powerpc? Even better would perhaps be to have pages that aren't mapped for the kernel. (like highmem pages on x86).

/Thomas

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