linux-kernel - Re: [linux-pm] Power management for SCSI

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Date:	Thu, 14 Aug 2008 16:08:12 +0200
From:	Oliver Neukum <oneukum@...e.de>
To:	Pavel Machek <pavel@...e.cz>
Cc:	linux-pm@...ts.linux-foundation.org,
	Alan Stern <stern@...land.harvard.edu>,
	James.Bottomley@...senpartnership.com,
	"Linux-pm mailing list" <linux-pm@...ts.osdl.org>,
	kernel list <linux-kernel@...r.kernel.org>, teheo@...ell.com
Subject: Re: [linux-pm] Power management for SCSI

Am Donnerstag 14 August 2008 15:50:21 schrieb Pavel Machek:
> On Wed 2008-08-13 18:21:29, Oliver Neukum wrote:
> > Am Mittwoch 13 August 2008 17:44:46 schrieb Alan Stern:
> > > > All children that are USB must be powered down. We know in fact that most
> > > > drives don't care that the device is suspended. The problem was drive
> > > > enclosures that cut power upon suspension losing cached data.
> > > 
> > > You misunderstood my question.  Are there SCSI transports other than
> > > USB sharing the requirement that all child devices must be suspended
> > > before the link can be powered down?
> > 
> > I dispute that USB in general has this property. Some storage devices
> > need their caches flushed. USB itself is perfectly happy with autosuspending
> > the storage device (host) without telling the disks (devices)
> > 
> > You could even argue that these storage devices violate the USB spec.
> 
> Hmm... but suspended devices have very little power budget, right?
> 
> So unless you have external power supply (2.5" frames generally
> don't), you can't really suspend and stay spinned up...
> 

True, but the spec says that no state shall be lost.

I don't really argue against flushing the caches. But I cannot that this would
demand that we should implement autopsuspend for SCSI. It seems like
overengineering to me.

	Regards
		Oliver

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