linux-kernel - Re: [PATCH v3 3/6] [SCSI] runtime resume device before system suspend

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Date:	Thu, 24 Nov 2011 19:17:08 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tejun Heo <tj@...nel.org>
cc:	Lin Ming <ming.m.lin@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Jeff Garzik <jgarzik@...ox.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	James Bottomley <JBottomley@...allels.com>,
	"Huang, Ying" <ying.huang@...el.com>,
	"Zhang, Rui" <rui.zhang@...el.com>
Subject: Re: [PATCH v3 3/6] [SCSI] runtime resume device before system suspend

On Thu, 24 Nov 2011, Tejun Heo wrote:

> Hello,
> 
> On Thu, Nov 24, 2011 at 2:54 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> > Therefore, when entering the SUSPEND or HIBERNATE stages of system
> > sleep, we can skip the callback to the driver if the device is already
> > in runtime suspend.  When entering the FREEZE stage, however, we should
> > first issue a runtime resume.  The overhead of doing this is
> > negligible, because a suspended drive would be spun up during the THAW
> > stage of hibernation anyway.
> 
> My brain is half fried at the moment so maybe you're saying the same
> thing, but I hope this is something simple. Like...
> 
>     /* for libata runtime suspend is equivalent to suspend */
>     if (runtime suspended && target == SUSPEND) {
>         yeah! skip drive pm ops;
>     } else {
>         end runtime pm;
>         ask EH to enter target PM state;
>     }

Almost.  More like this:

	/*
	 * sd is the only high-level SCSI driver to implement runtime 
	 * PM, and sd treats runtime suspend, system suspend, and 
	 * system hibernate identically (but not system freeze).
	 */
	if (runtime-suspended) {
		if (target == SUSPEND || target == HIBERNATE)
			return 0;	/* already suspended */
		/* wake up device so that FREEZE will succeed */
		pm_runtime_resume();
	}
	tell device driver to enter target PM state

libata does not contain the SCSI device driver; it contains the host
driver.

> I really don't think we need to optimize this further than this.

The above has two lines (plus comments) of optimization.  Okay?

Alan Stern

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