linux-kernel - Re: [linux-pm] calling runtime PM from system PM methods

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Date:	Sun, 19 Jun 2011 11:01:06 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	linux-pm@...ts.linux-foundation.org, <linux-omap@...r.kernel.org>,
	Kevin Hilman <khilman@...com>, Paul Walmsley <paul@...an.com>,
	Magnus Damm <magnus.damm@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [linux-pm] calling runtime PM from system PM methods

On Sun, 19 Jun 2011, Rafael J. Wysocki wrote:

> Well, that was kind of difficult to debug, but not impossible. :-)
> 
> The problem here turns out to be related to the SCSI subsystem.
> Namely, when the AHCI controller is suspended, it uses the SCSI error
> handling mechanism for scheduling the suspend operation (I'm still at a little
> loss why that is necessary, but Tejun says it is :-)).  This (after several
> convoluted operations) causes scsi_error_handler() to be woken up and
> it calls scsi_autopm_get_host(shost), which returns error code (-EAGAIN),
> because the runtime PM has been disabled at the host level.

Oh no.  I was afraid something like this was going to happen 
eventually.

It's clear that we don't want runtime PM kicking in while the SCSI 
error handler is running.  That's why I added the 
scsi_autopm_get_host().  But this also means we will run into trouble 
if the error handler needs to be used during a power transition.

> This happens because scsi_autopm_get_host() uses
> pm_runtime_get_sync(&shost->shost_gendev) and returns error code when
> shost_gendev.power.disable_depth is nonzero.

Maybe get_sync doesn't need to return an error if the runtime status is 
already ACTIVE.  I'm not sure about this; it's just an idea...

> So, the problem is either in scsi_autopm_get_host() that should check the
> error code returned by pm_runtime_get_sync(), or in rpm_suspend() that should
> return 0 if RPM_GET_PUT is set in flags.  I'm inclined to say that the
> problem should be fixed in rpm_suspend() and hence the appended patch that
> works (well, it probably should be split into three separate patches).

Maybe it would be good enough if the error handler ended up doing a 
get_noresume instead of get_sync?  Although I could be wrong, I don't 
think scsi_error_handler() will ever run in a situation where the host 
adapter is not runtime-active.

Tejun, does that sound right to you?

Alan Stern

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