Re: REQ_PM vs REQ_TYPE_PM_RESUME

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On Thursday, January 09, 2014 09:29:59 AM Aaron Lu wrote:
> On 01/09/2014 05:21 AM, Alan Stern wrote:
> > On Wed, 8 Jan 2014, Phillip Susi wrote:
> >> You issue a REQUEST SENSE command and that returns status indicating
> >> whether the drive is stopped, or in standby.  See my patches.  One of
> > 
> > I never saw your patches.  Where were they posted?
> > 
> > If you issue the REQUEST SENSE command in the usual way (going through
> > the SCSI and block layers), and the disk is already in runtime suspend,
> > it won't do what you want.  The block layer won't deliver requests
> > until the device leaves the RPM_SUSPENDED state.  In addition, when it
> > receives the command the block layer will submit a deferred
> > runtime-resume request, which rather defeats the whole purpose.
> > 
> > (I guess you actually saw some of this happen, and that's what led to
> > this email thread in the first place.)
> > 
> > It's a knotty situation.  The only way to find out whether the disk is
> > spinning is to ask it, which requires doing I/O, which requires
> > spinning up the disk.  Maybe we need to add a mechanism to the block
> > layer's runtime PM implementation for addressing just this situation.
> 
> I think it's knotty because the runtime PM status is a view from the
> kernel/host side, i.e. it is runtime suspended if it is not being used,
> no matter which power state it is in. The trigger for the PM state
> transition are all based on this, if we want to do it the other way
> around(update device's runtime PM status based on device's actual power
> state), we are in a knotty situation.

Agreed.

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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