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    SubjectRe: can device drivers return non-ram via vm_ops->nopage?
    On Sat, Mar 20, 2004 at 04:06:21PM +0100, Andrea Arcangeli wrote:
    > On Sat, Mar 20, 2004 at 06:40:22AM -0800, William Lee Irwin III wrote:
    > > On Sat, Mar 20, 2004 at 02:30:25PM +0100, Andrea Arcangeli wrote:
    > > > Anyways returning to the non-ram returned by ->nopage see the below
    > > > email exchange with Jens. the bug triggering of course is the
    > > > BUG_ON(!pfn_valid(page_to_pfn(new_page))).
    > > > If we want to return non-ram, we could, but I believe we should change
    > > > the API to return a pfn not a page_t * if we want to.
    > >
    > > This would be very helpful for other reasons also. There's a general
    > > API issue with drivers that want or need to do this. The one I've
    >
    > I'm afraid I'll have to teach ->nopage how to deal with non-ram with
    > this page_t API too (changing it to pfn sounds too intrusive in the
    > short term), it seems to me that alsa can return non-ram (in the nopage
    > callback there's a virt_to_page on some iomm region), and changing alsa
    > to use remap_file_pages sounds too intrusive too.

    Actually, ALSA is broken in that respect - it isn't portable as it
    stands. It isn't the API which is broken - it's ALSA which is broken.
    Performing virt_to_page() on any non-direct mapped RAM page (which
    means the value returned from dma_alloc_coherent or pci_alloc_consistent)
    is undefined.

    One of my current projects is fixing this crap in ALSA.

    Besides, returning an invalid struct page will lead to Bad Things(tm)
    in set_pte().

    --
    Russell King
    Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
    maintainer of: 2.6 PCMCIA - http://pcmcia.arm.linux.org.uk/
    2.6 Serial core
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