linux-kernel - Re: [RFC 0/5] scsi, sd, pm, request based runtime PM for scsi disk

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Date:	Tue, 07 Feb 2012 12:59:33 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	ming.m.lin@...el.com, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-pm@...r.kernel.org,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	James Bottomley <JBottomley@...allels.com>
Subject: Re: [RFC 0/5] scsi, sd, pm, request based runtime PM for scsi disk

Hi, Alan,

On Mon, 2012-02-06 at 10:13 -0500, Alan Stern wrote:
> On Mon, 6 Feb 2012, Huang Ying wrote:
> 
> > SSD becomes more and more popular, this makes it possible to put disk into
> > low power state more often.  And request based runtime PM for scsi disk is
> > more useful than open/close based one because disk is normally mounted at
> > most time.
> > 
> > One known issue, because SCSI TEST_UNIT_READY will be put into request
> > queue every 2 seconds by default, this makes it hard for disk to sleep.
> > Maybe we can implement check_events callback in some other way?
> > 
> > [RFC 1/5] pm, runtime, Add resume notifier
> > [RFC 2/5] scsi, pm, rename scsi_autopm_get/put_xxx to
> > [RFC 3/5] scsi, pm, add pm_runtime_get/put in scsi request
> > [RFC 4/5] scsi, pm, use autosuspend for scsi runtime PM
> > [RFC 5/5] scsi, sd, pm, request based runtime PM support
> 
> Your whole approach is at the wrong level.  Runtime PM between I/O 
> requests for block devices should be implemented in the block layer, 
> not in the SCSI layer.
> 
> It also is much more difficult than your patches would indicate.  For 
> example, some USB card readers indicate a media change every time they 
> resume; therefore they must not be suspended while the device file is 
> open.
> 
> Another difficulty arises because some drivers need to send SCSI 
> commands (such as SYNCHRONIZE CACHE) _while_ suspending or resuming.

Thank you very much for your valuable response!  We will do more
research work on this topic, including your previous effort. :)

Best Regards,
Huang Ying


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