Messages in this thread | | | Date | Sat, 18 Oct 2008 22:07:55 +0200 | From | Thomas Hellström <> | Subject | Re: [git pull] drm patches for 2.6.27-rc1 |
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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 > > ------------------------------------------------------------------------- > This SF.Net email is sponsored by the Moblin Your Move Developer's challenge > Build the coolest Linux based applications with Moblin SDK & win great prizes > Grand prize is a trip for two to an Open Source event anywhere in the world > http://moblin-contest.org/redirect.php?banner_id=100&url=/ > -- > _______________________________________________ > Dri-devel mailing list > Dri-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/dri-devel >
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