Re: [RFC PATCH] PM / Runtime: runtime: Add sysfs option for forcing runtime suspend

[Oliver Neukum <oneukum@suse.com>]

On Mon, 2015-09-21 at 16:02 -0400, Alan Stern wrote:
> On Mon, 21 Sep 2015, Dmitry Torokhov wrote:
> 
> > > What happens if the "inhibit" control is turned on and the driver puts 
> > > the device into runtime suspend, but then an I/O request arrives?
> > > 
> > > 	If the I/O request originated from userspace, it means the
> > > 	user is violating the terms of the "inhibit" control.  Should
> > > 	the request simply fail?
> > 
> > What user? User that inhibited it or user that tried to use the device?
> 
> Normally they would be the same.  But even if they aren't, someone has 
> violated the kernel interface: The first user told the kernel a 
> particular device wasn't going to be used, and then the second user 
> tried to use it.

If we assume that user space speaks with a uniform voice on that
issue, it can just as well close the device. It seems to me that
declaring a device idle is a privileged operation.

> Of course, this issue doesn't arise for devices that merely report 
> external events.

Indeed. We can handle output to suspended devices by waking them.
I don't see why this case is different. We are talking about input
only.

> The runtime-PM "usage" value for these devices is a little tricky to 
> calculate.  It should be nonzero if there are any open files _and_ the 
> device isn't "inhibited".  I don't know the best way to represent that 
> kind of condition in the runtime PM framework.

Does that make sense in the generic framework at all? I still
think that drivers should cease IO for input in such cases.
That should involve a common callback, but no counter.

	Regards
		Oliver