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    SubjectRe: kmap_atomic_pfn for PCI BAR access?
    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?

    J



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