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    SubjectRe: splice methods in character device driver
    On Wed, Jun 03 2009, Leon Woestenberg wrote:
    > Hello all,
    >
    > On Wed, May 13, 2009 at 6:59 PM, Steve Rottinger <steve@pentek.com> wrote:
    > > is passing in the pages into splice_to_pipe.  The pages are associated
    > > with a PCI BAR, not main memory.  I'm wondering if this could be a problem?
    > >
    > Good question; my newbie answer would be the pages need to be mapped
    > in kernel space.

    That is what the ->map() hook is for.

    > I have a similar use case but with memory being DMA'd to host main
    > memory (instead of the data sitting in your PCI device) in a character
    > device driver. The driver is a complete rewrite from scratch from
    > what's currently sitting-butt-ugly in staging/altpcichdma.c
    > so-please-don't-look-there.
    >
    > I have already implemented zero-latency overlapping transfers in the
    > DMA engine (i.e. it never sits idle if async I/O is performed through
    > threads), now it would be really cool to add zero-copy.
    >
    > What is it my driver is expected to do?
    >
    > .splice_read:
    >
    > - Allocate a bunch of single pages
    > - Create a scatter-gather list
    > - "stuff the data pages in question into a struct page *pages[]." a la
    > "fs/splice.c:vmsplice_to_pipe()"
    > - Start the DMA from the device to the pages (i.e. the transfer)
    > - Return.
    >
    > .splice_write:
    >
    > - Create a scatter-gather list
    >
    > interrupt handler / DMA service routine:
    > - device book keeping
    > - wake_up_interruptible(transfer_queue)
    >
    > .confirm():
    >
    > "then you need to provide a suitable ->confirm() hook that can wait on
    > this IO to complete if needed."
    > - wait_on_event_interruptibe(transfer_queue)
    >
    > .release():
    >
    > - release the pages
    >
    > .steal():
    >
    > unsure

    This is what allows zero copy throughout the pipe line. ->steal(), if
    sucesful, should pass ownership of that page to the caller. The previous
    owner must no longer modify it.

    > .map
    >
    > unsure

    See above :-)

    --
    Jens Axboe

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