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    SubjectRe: 2.1.111: IDE DMA disabled?
    On Sat, 25 Jul 1998, Linus Torvalds wrote:

    > Which implies that at _least_ we need to disable DMA automatically if it's
    > one of those chipsets.
    >
    > Personally, I never understood why DMA was enabled by default anyway. As
    > far as I can tell the only sane thing is to have it always disabled by
    > default, and enable it with hdparm.
    >
    > Linus

    It is not enabled by default unconditionally; we currently enable
    DMA only for drives which support DMA mode 2 and above (introduced
    along with PIO mode 4), which filters out old (and possibly buggy)
    DMA implementations.

    We might want to switch to a stricter rule, but the above rule was
    used by the kernel ide-dma driver from the very begining, around
    1.3.50 or so, and through all that time we have not received many
    reports about DMA-specific data corruption.

    If the corruption in this case will indeed be fixed by replacing the
    IDE cable or by lowering the chipset timing, I believe that we are
    likely to see data corruption eventually, even without DMA, and the
    real fix is to use a shorter IDE cable or to lower the timings in the
    BIOS -- we can work around chipset bugs and black-list specific drives
    if there is enough evidence which shows such bugs, but I believe that
    there is really not much to be done in software regarding a bad IDE
    cable; even in PIO mode, we have seen data corruption with high PIO
    modes and with long IDE cables in the past.

    Gadi

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