Re: REQ_PM vs REQ_TYPE_PM_RESUME

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On Thursday, January 09, 2014 01:17:12 PM Rafael J. Wysocki wrote:
> 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.

That said during system resume, when we are trying to avoid resuming the
device to save time/energy, it makes sense to check its physical state and
do a pm_request_resume() if that is "on" to avoid a situation in which the
device is physically "on", but its runtime PM status is "suspended" and it
never gets powered down because of that.

Thanks!

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