Re: Suspend without the freezer

[Dmitry Torokhov <dtor@insightbb.com>]

Hi Alan,

On Monday 30 July 2007 16:48, Alan Stern wrote:
> Dmitry:
> 
> I'm trying to help eliminate the need for the freezer during suspend.  
> For it to work, we have to prevent threads which otherwise would have 
> been frozen from trying to bind drivers to suspended devices or trying 
> to register new devices whose parents are already suspended.
> 
> To help accomplish this, the PM core can acquire the device semaphores
> for all existing devices before suspending any of them.  That will
> prevent attempts at binding.  It would also prevent registration of new
> children, _if_ the driver doing the registration had to acquire the
> parent's semaphore first.  But many drivers don't do this.
> 
> One thought was to have the PM core acquire and hold the dpm_list_mutex 
> throughout the suspend.  This would block registration attempts at the 
> point where the new device is added to the PM core's device-list.
>

I think blocking at this point is too late - many drivers muck with
the device in different ways before registering the "result" with
driver core. The device may be half-awaken by then.
  
> Unfortunately it creates several lockdep violations.  For example, the 
> serio core holds serio->drv_mutex while input_register_device is 
> called (which acquires dpm_list_mutex), and it acquires 
> serio->drv_mutex in serio_suspend and serio_resume (which would be 
> called while the PM core holds dpm_list_mutex).
> 
> I'm having trouble coming up with a way to block registrations during 
> suspend that won't create a possibility for deadlock.  Do you have any 
> suggestions?  A scheme that would work for the input layer ought to be 
> generally applicable.
>

One option would be to have a separate thread running registration/binding/
unbinding (like serio core does). You can stop this thead during suspend
and resume so that requests are queued up and you process them later, when
you are ready...

-- 
Dmitry