Messages in this thread | | | Date | Sat, 18 Oct 2008 12:31:33 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [git pull] drm patches for 2.6.27-rc1 |
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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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