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    SubjectRe: [rfc patch 2/2] direct-io: remove address alignment check
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    On Thu, 2005-07-14 at 16:39, Andrew Morton wrote:
    > Daniel McNeil <daniel@osdl.org> wrote:
    > >
    > > Do drivers have problems with odd addresses or with
    > > non-512 addresses?
    >
    > I do recall hearing rumours that some bus-masters have fairly strict memory
    > alignment requirements. A cacheline size, perhaps - that would be 32 bytes
    > given the age of the hardware.
    >
    > But yeah, it's v. risky to assume that all bus masters can cope with
    > memory alignments down to two bytes.
    >
    > It would be sane to put the minimum alignment into ->backing_dev_info,
    > default to 512, get the device drivers to override that as they are tested.
    >
    > But this introduces a very very bad problem: people will write applications
    > which work on their hardware, ship the things and then find that the apps
    > break on other people's hardware. So we can't do that.
    >
    > Instead, we need to work out the minimum alignment requirement for all disk
    > controllers and DMA controllers and motherboards in the world. And that
    > includes catering for weird ones which appear to work but which
    > occasionally fail in mysterious ways with finer alignments. That's hard.
    > It's easier to continue to make application developers jump through hoops.

    I was hoping this patch would help turn rumors into real data :)

    If we did put min alignment into backing_dev_info, we could implement
    the equivalent of bounce buffers for direct-io -- or just fall back
    to buffer i/o like it does sometimes anyway. That way application
    would not break, just get worse performance on some hardware.

    Right now I just wanted to get the issues on table, get some test
    results, and see how to proceed from there. Since this patch only
    affects direct i/o, getting test results shouldn't cause too many
    problems.

    Thanks,

    Daniel

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