linux-kernel - Re: Power management for SCSI

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Message-ID: <20080814130812.GC2262@elf.ucw.cz>
Date:	Thu, 14 Aug 2008 15:08:12 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	kernel list <linux-kernel@...r.kernel.org>,
	Linux-pm mailing list <linux-pm@...ts.osdl.org>,
	James.Bottomley@...senPartnership.com, teheo@...ell.com,
	oneukum@...e.de
Subject: Re: Power management for SCSI

Hi!

> > Add support for autosuspend/autoresume. Lowlevel driver can use it to
> > spin the disk down and power down its SATA link, to turn off the USB
> > interface, etc.
> > 
> > Spinning down the disk is useful - saves ~0.5W here. Powering down
> > SATA controller is even better -- should save ~1W.
> > 
> > Now, I guess the patch will need to be split to small pieces for
> > merge... I tried to rearrange it so that the documentation and hooks
> > go before stuff that needs the hooks, and before Kconfig enabler. If
> > it looks reasonably good, I'll split it into smaller pieces.
> 
> James had a number of objections to my original patch; you can read 
> them here:
> 
> https://lists.linux-foundation.org/pipermail/linux-pm/2008-March/016849.html
> 
> I haven't had time yet to work on an improved version.

Ok, I see, "its done at the wrong level" sounds pretty serious.

First the general comments/questions:

#
#1. It's done at the wrong level: suspend "device" is actually a target
#function.  There's no way on a multi-lun device we want to keep the
#flags and last_busy anywhere but in the target

So... if there's one device with Lun0==cdrom1 and Lun1==cdrom2, it is a
single target, and we want to keep flags/last busy common to all that?

What is good data structure to add? I see scsi_tgt*.h, but it is very
short, and there does not seem to be good structure to hook into.

#2. As you say in the comment, the thing we're trying to power down is
#the link.  In most SCSI implementations, the link has a rather complex
#relationship to the target, what we want to do in
#periodic_autosuspend_scan() is run over the devices on each link, and
#if
#they're not busy suspend the link?  What's probably needed is a set of
#adjunct helpers for the transport classes to do this.

So the host suspend/resume stuff should go into struct
scsi_transport_template?

#3. The link power down is much faster than device spin down ... in
#your
#patch these two things seem to be coupled ... we really need to keep
#them separate.
#

ACK.

#4. The entanglement with error handling is incredibly problematic
#(since
#eh is a nastily complex state machine in its own right).  What do
#transports that use eh_strategy_handler do about all of this?

/me scared...
								Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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