linux-kernel - Re: [git pull] drm patches for 2.6.27-rc1

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Date:	Sat, 18 Oct 2008 14:51:30 -0700
From:	Keith Packard <keithp@...thp.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	keithp@...thp.com, Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Dave Airlie <airlied@...ux.ie>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	dri-devel@...ts.sf.net, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [git pull] drm patches for 2.6.27-rc1

On Sat, 2008-10-18 at 22:37 +0200, Ingo Molnar wrote:

> But i think the direction of the new GEM code is subtly wrong here, 
> because it tries to manage memory even on 64-bit systems. IMO it should 
> just map the _whole_ graphics aperture (non-cached) and be done with it. 
> There's no faster method at managing pages than the CPU doing a TLB fill 
> from pagetables.

Yeah, we're stuck thinking that we "can't" map the aperture because it's
too large, but with a 64-bit kernel, we should be able to keep it mapped
permanently.

Of course, the io_reserve_pci_resource and io_map_atomic functions could
do precisely that, as kmap_atomic does on non-HIGHMEM systems today.

> The only real API need i see is on 32-bit: with a 1GB or 2GB graphics 
> aperture we just cannot map that permanently, so kmap_atomic() is a 
> necessity. We can certainly extend that to non-highmem as well.

Yes, this is where exposing an io-specific atomic mapping function will
remain necessary for some time.

> And if i understood your 
> workload correctly you want to do tens of thousand of map/unmap/remap 
> events per frame generated - depending on the type of the 3D app/engine.

Yeah, data transfer from CPU to GPU is through a pwrite interface, and
we perform the transfer within the kernel using map/unmap operations on
the aperture as those are WC and hence do not require clflush.

> Or am i missing something subtle? Why do you want the overhead of kmap 
> on 64-bit?

We don't, but I think it would be nice to have a common API that works
across all 32-bit configurations as well as 64-bit systems.

-- 
keith.packard@...el.com

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