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    SubjectRe: [PATCH] SCSI midlayer power management
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    On Wed, 2004-08-11 at 08:13, Nathan Bryant wrote:
    > No. Remember that DMA works differently under SCSI than it does under
    > IDE. SCSI DMA is a host controller feature, whereas under IDE it is
    > enabled/disabled at the drive level and the drives have special
    > knowledge of DMA. Since generic_scsi_suspend() is the device level
    > suspend routine, it is called before the host controller's suspend
    > routine, (due to depth first traversal of device tree), which is
    > responsible for disabling the PCI slot. Only after the host controller
    > is suspended will there be no DMA, but if your real question is "can I
    > generically control a SCSI disk with PIO for software suspend" then the
    > answer is NO. For purposes of not suspending the drivers, I haven't
    > looked into how swsusp would see which host adapter owns which drive,
    > but some of the required information seems to be present in sysfs.

    Actually, the answer is to most intents and purposes "yes". You are
    technically correct: there's no way to disable DMA in SCSI. However,
    once a device is quiesced, it has no outstanding commands, so there will
    be no outstanding DMA to that device. When all devices on a host have
    been quiesced, then there will be no DMA at all going on *except* if the
    user initiates any via another interface (like sending a device probe or
    doing a unit scan). The guarantee should be strong enough for swsusp to
    proceed, but we can look at quiescing a host properly (however, we'd
    need to move to a better host state model than we currently possess).

    James


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