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@srcf.ucam.org