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    SubjectRe: kmap_atomic_pfn for PCI BAR access?
    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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