Re: Question about hibernation

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Thursday, October 14, 2010, Raj Kumar wrote:
> 
> Hi Rafael,
>  
> 1) I have question on system resume that e.g. in device driver let us say for example structure is defined global,
> e.g.
>  
> struct dev
> {
> int a;
>  
> } dev_t;
>  
>  
> this structure is global. Now during system suspend, suppose the value of 
>  
> dev_t.a =2;
>  
> So when the resume happens, will the dev_t.a will remain the same i.e. 2?

Yes, it will.  For hibernation, the contents of all memory from before the image
was created is restored during resume.

> 2) During the system suspend, if the system hibernates, then the message sent from power management core to all drivers is
> PMSG_HIBERNATE and if system suspends to ram, then message sent from power management core to all drivers is PMSG_SUSPEND.

That was in the old model, which isn't recommended any more.

Currently, it is recommended that drivers implement callbacks in struct dev_pm_ops,
as described in Documentation/power/devices.txt and Documentation/power/pci.txt.

Thanks,
Rafael

  
> > From: rjw@sisk.pl
> > To: rajkumar278@hotmail.com
> > Subject: Re: Question about hibernation
> > Date: Fri, 8 Oct 2010 21:31:48 +0200
> > CC: linux-pm@lists.linux-foundation.org
> > 
> > On Friday, October 08, 2010, you wrote:
> > > 
> > > Hi Rafael,
> > 
> > Hi,
> > 
> > > I am working on device driver whose some part is user space and some part is kernel space. So the user space part
> > > of the driver provides interface to user space applications. 
> > > 1) Now, Suppose some user space is application is going on which is accessing the user space driver and user space driver
> > > scheduled some jobs to kernel space, Now suppose system hibernation start, since it freezes all the user space processes
> > > so suppose jobs are not completed yet which are scheduled to kernel space by user space.
> > > 
> > > when the kernel space device driver completes the jobs and gives the notification to user space driver, since user space
> > > process is already frozen, so there will be problem if we suspend the driver using suspend and resume.
> > > 
> > > right?
> > 
> > Yes, there's going to be a problem if the driver is not prepared to handle such situations.
> > 
> > Depending of the particular case, one possible approach is to use a PM notifier
> > as described in Documentation/power/notifiers.txt.
> > 
> > > 2) The other point I was seeing, when the power management core freezes all the processes before suspending and resuming
> > > devices, it will take some time for freezing and after that if some device driver refuses to suspend, then it will again resume
> > > the whole processes, so latency time be more. right?
> > 
> > Yes, it works as described.
> > 
> > > 3) Next question is regarding the kernel space threads, when these are frozen during suspend/hibernation?
> > 
> > They are frozen immediately after user space has been frozen, but only a
> > handful of kernel threads are frozen.
> > 
> > Thanks,
> > Rafael
>