linux-kernel - Re: [RFC][PATCH 4/4] ahci: support hot plug when port/controller is runtime suspended

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Date:	Mon, 2 Jan 2012 15:57:11 +0000
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Lin Ming <ming.m.lin@...el.com>, Jeff Garzik <jgarzik@...ox.com>,
	Alan Stern <stern@...land.harvard.edu>,
	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org,
	linux-pm@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	kay.sievers@...y.org
Subject: Re: [RFC][PATCH 4/4] ahci: support hot plug when port/controller
 is runtime suspended

On Thu, Dec 15, 2011 at 11:21:31AM -0800, Tejun Heo wrote:

> SATA link detection requires hot wire and keeping wire hot takes
> power.  I think it's fair tradeoff to not support hotplug while
> powersaving is on.  We have warm plug mechanism (the SCSI rescan
> trigger via sysfs) after all.  It would be nice if things like that is
> somehow exported to userland in easy way tho (cc'ing Kay), but I
> frankly don't have much idea where that would fit.

We expose the ahci_port_cmd field in sysfs, which lets us know whether a 
port is flagged as hotpluggable or external. Userspace could use that to 
identify whether a given port can be safely powered off or not. It's not 
guaranteed - desktop boards with esata ports will typically not have 
this information available (because the esata port could be plugged into 
any on-board port), so I think leaving it up to userspace to set the 
policy makes sense.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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