Re: [PATCH] SCSI midlayer power management

[James Bottomley <James.Bottomley@SteelEye.com>]

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