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

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Date:	Sat, 18 Oct 2008 22:07:55 +0200
From:	Thomas Hellström <thomas@...gstengraphics.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Keith Packard <keithp@...thp.com>, Dave Airlie <airlied@...ux.ie>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	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

Linus Torvalds wrote:
> 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);"
>   
Actually, a "kmap_atomic_prot_pfn()" has been lurking in the drm repos 
for some time now, but hasn't been suggested for upstream. It was 
intended for drivers that require quick in-kernel patching of 
write-combined io and highmem pages. The latter is a common situation 
for PCIE graphics devices with their own MMU, so IMHO an exported 
kmap_atomic_pfn_prot() would be a big help in such cases.

/Thomas

> 		Linus
>
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