Re: OMAP serial runtime PM and autosuspend (was: Re: [PATCH 4/7] dt-bindings: gnss: add u-blox binding))

[Johan Hovold <johan@kernel.org>]

On Thu, May 24, 2018 at 06:32:37AM -0700, Tony Lindgren wrote:
> * Johan Hovold <johan@kernel.org> [180524 09:20]:
> > On Mon, May 21, 2018 at 08:48:32AM -0700, Tony Lindgren wrote:

> > > Well if you have some better mechanism in mind let's try it out. Short of
> > > sprinkling pm_runtime_force_suspend/resume calls all over, I'm out of ideas
> > > right now.
> > 
> > Yeah, that would be too much of a hack and likely wouldn't work either
> > (and we really should do away with those _force calls altogether).
> > 
> > I've been thinking a bit too much about this already, but it may be
> > possible to use the pm QoS framework for this. A resume latency can be
> > set through sysfs where "n/a" is defined to mean "no latency accepted"
> > (i.e. controller remains always-on while port is open) and "0" means
> > "any latency accepted" (i.e. omap aggressive serial RPM is allowed).
> 
> Oh yeah, PM QoS might work here!

Actually, after reading a recent QoS related bug report, I realised that
a resume latency request of "n/a" is actually a third way of disabling
runtime PM, which similarly to the negative autosuspend would prevent
also a closed port from suspending.

Using a small positive resume latency for this feels like too much of a
hack, but defining a new QoS flag might still work.

Johan