EXA for radeon experimental patch

Benjamin Herrenschmidt benh at kernel.crashing.org
Thu Sep 1 04:15:45 PDT 2005

> > They do, though they require some kernel support to get to the physical
> > address of pages and need some proper scatter/gather support on the card
> > side.
> The all you need is a drm module for your card. Even if the card doesn't have 
> scatter/gather support, drm allows you to allocate a piece of consistent 
> physical ram, and mmap it in the server. The handle you get is the physical 
> address, so you should be able to use that to implement PCI dma transfers.

Yup, but it's very likely that allocating physically contiguous memory
will fail. The kernel isn't that good as keeping physical memory non
fragmented, and thus, physical allocations above PAGE_SIZE are quite
likely fail after boot.

> > Nope. When writing/reading vram via MMIO, you go through the swappers on
> > the PCI->VRAM path which are set for the bpp of the front buffer. If you
> > picture you are up/downloading has a different bpp, you need to change
> > the swapper during the access. Same problem with the fb* fallbacks in
> > EXA, which is why I'm adding the Prepare/Finish hooks.
> Hmmm... but fbComposite can access up to three pixmaps, all of which can have 
> different bit depth. I don't really see how you could make that work (as you 
> need different swapping behavior for all three pixmaps. 

See my post "EXA prepare/finish hooks & random X questions". On radeon,
for example, I can setup up to 8 "surfaces" that have different swapper
setup on the PCI -> VRAM path.

On cards that don't have such capability, I intend to have the
PrepareAccess() hook fail, causing EXA to DownloadFromScreen() the
pixmap to RAM before the composite operation.

> No idea. I am no expert here. As far as I can see in the code there is some 
> byteswapping happening in some places, but not in others. So the exa code I 
> have might very well be broken on big-endian machines (though I can't test).

It is, I verified it :) That's why I'm adding these hooks.


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