Messages in this thread | | | Date | Wed, 25 Jun 2008 20:11:05 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: kmap_atomic_pfn for PCI BAR access? |
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Dave Airlie wrote: > On Thu, Jun 26, 2008 at 11:18 AM, Jeremy Fitzhardinge <jeremy@goop.org> wrote: > >> Keith Packard wrote: >> >>> The graphics memory BAR is generally fairly good sized; on Intel chips, >>> it's between 256M and 1G (and growing). I want to write data into this >>> region from kernel space, but it's really too big to map the whole thing >>> into kernel address space, especially on 32-bit systems. ioremap is not >>> a good option here -- it's way too slow. >>> >>> With CONFIG_HIGHMEM enabled, I can use kmap_atomic_pfn (well, actually >>> the kmap_atomic_proc_pfn included in the DRM tree) and things work quite >>> well -- performance is good, with barely any measurable time spent in >>> the PTE whacking (~1%). >>> >>> However, with CONFIG_HIGHMEM disabled, there aren't any PTEs reserved >>> for this kind of mapping fun. This makes me suspect that abusing >>> kmap_atomic for this operation would not be appreciated. >>> Should I use kmap_atomic_pfn to reach my PCI BAR like this? >>> >>> Would it be reasonable to supply a patch that made this work even >>> without CONFIG_HIGHMEM? >>> >>> >> Usually people use ioremap to map device memory. Wouldn't that work in this >> case? >> >> > > "but it's really too big to map the whole thing > into kernel address space, especially on 32-bit systems. ioremap is not > a good option here -- it's way too slow." > > From the original mail. >
Uh, yep.
> doing tlb flush for iounmap is slow as all hell if you do it a lot, > and we can't afford to mmap the whole aperture it can 1GB. >
Maybe Nick's vmap reimplementation would help here. It effectively allows you to map stuff into the vmalloc space, and do lazy tlb flushes to mitigate the cost of map/unmap. He posted the patches week or so ago.
J
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