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

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Date:	Sat, 18 Oct 2008 12:31:33 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Keith Packard <keithp@...thp.com>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Dave Airlie <airlied@...ux.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	dri-devel@...ts.sf.net
Subject: Re: [git pull] drm patches for 2.6.27-rc1

On Sat, 18 Oct 2008, Keith Packard wrote:
>
> The basic plan is to have four new functions (yes, I'm making up names 
> here):
> 
> struct io_mapping *io_reserve_pci_resource(struct pci_dev *dev, 
>                                            int bar, 
>                                            int prot);
> void io_mapping_free(struct io_mapping *mapping);
> 
> void *io_map_atomic(struct io_mapping *mapping, unsigned long pfn);
> void io_unmap_atomic(struct io_mapping *mapping, unsigned long pfn);

The important thing is that mappings need to be per-CPU, so the above may 
work, but only if it's designed so that "io_reserve_pci_resource()" will 
actually reserve space for 'nr_possible_cpu' page mappings, and then the 
"io_[un]map_atomic()" functions do per-CPU mappings.

Anything else is a disaster, because anything else implies TLB shootdown.

And quite frankly, even so, we'd possibly still be _better_ off with just 
exposing the "kmap_atomic_pfn()" functionality even so. Because quite 
frankly, your "io_reserve_pci_resource()" infrastructure is going to 
inevitably be more complex and slower than the rather efficient 
kmap_atomic_pfn() thing we have.

[ The *non-atomic* kmap() functions are fairly high-overhead, in that they 
  want to keep track of cached mappings and remember page addresses etc. 
  So those are the ones we don't want to support for non-HIGHMEM setups. 

  But the atomic kmaps are pretty simple, and really only need some 
  trivial FIXMAP support. We could easily extend it for x86-64, methinks, 
  and do it for x86-32 even when we don't do HIGHMEM.

  Ingo? ]

One small detail: our we currently have "kmap_atomic_pfn()" and 
"kmap_atomic_prot()", and we really should maek the fundamental core 
operation be "kmap_atomic_pfn_prot()", and have everything be done in 
terms of that. Looking at it, it also looks like kmap_atomic_prot() is 
actually incorrect right now, and doesn't do a "prot" thing for 
non-highmem pages, but just returns "page_address(page);"

		Linus
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