Re: [PATCH] Hotplug for device power state changes

[Todd Poynor <tpoynor@mvista.com>]

Benjamin Herrenschmidt wrote:

>>Now that you mention it, device power hotplug should be synchronous, to 
>>make sure the power management application has reacted to the changed 
>>state prior to the device going into actual service (in the case of a 
>>resume).
> 
> 
> This is dangerous.
> 
> If the device you are suspending is on the VM path in any way,
> beeing synchronous with a userland call can deadlock you solid.
> 
> This is even more true for system suspend where we are suspending
> all devices including the main swap/storage.

Well, this feature is intended to allow power management of appropriate 
devices; using sysfs or a driver call to individually suspend a device 
required for proper system operation would be a danger, hotplug 
notification or no.  And the individual device notifications provided by 
the patch under discussion are not for use during a system-wide 
suspend/resume sequence.  I would imagine system suspend/resume would be 
separate events that probably would not notify of the individual device 
suspends/resumes performed as a consequence.  At any rate, yes, this 
would occur outside of the code path that freezes processes and such.

-- 
Todd Poynor
MontaVista Software



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