Re: [RFC Disable suspend on a specific device] This is a little change in linux power scheme

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

On Tuesday 07 April 2009, Alan Stern wrote:
> On Tue, 7 Apr 2009, Rafael J. Wysocki wrote:
> 
> > > >> I works on an hardware that have the gsm connect to wm8753 and the 
> > > >> bluetooth subsystem
> > > >> too direct connected. So I would like that a phone call for example 
> > > >> remains on during
> > > >> system suspend. With this approch I can disable suspend of an entire 
> > > >> subtree of device,
> > > >> that can be used by other hardware component. This change avoid any 
> > > >> specific change to the
> > > >> driver.
> > > >>     
> > > >
> > > > Why do you want to avoid changing the driver, actually?
> > > >
> > > > Rafael
> > > >
> > > >   
> > > Because, the driver may be connected to other device and you must change 
> > > the entire
> > > set. This simple modification can help to share devices on an embedded 
> > > board and control
> > > suspend/resume enable from user space. I don't know if it can be usefull 
> > > on broken board/device too. Do you see any possible risk with this 
> > > change?
> > 
> > Yes, a (rather high) risk of abuse.
> > 
> > > When I write it I try to have a simple modification on linux-pm part.
> > 
> > So, basically, you'd like to introduce an interface allowing the user space to
> > tell the kernel which devices not to suspend, because there may be some
> > dependencies between devices that the kernel presumably doesn't know of.
> > 
> > Well, what about teaching these dependencies to the kernel, so that it
> > can take them into account by itself?
> 
> Isn't that exactly what the patch already does?  It lets the kernel
> know that when the "don't-suspend" flag is set for a device then the
> entire sub-tree rooted at that device should remain unsuspended.

Well, in fact I wanted to know your opinion about this patch. :-)

IMO, there is a danger that suspend will break or even the machine will crash
if this is used for a wrong device.  I don't think it's generally safe to add
switches like this on the core level, because the core doesn't really know
which drivers are prepared for it.

Thanks,
Rafael