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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