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SubjectRe: [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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