[PATCH 12/12] ARM: tegra: Convert PMC to a driver

[Peter De Schrijver]

On Wed, Jul 16, 2014 at 08:57:16PM +0200, Thierry Reding wrote:
> * PGP Signed by an unknown key
> 
> On Wed, Jul 16, 2014 at 05:22:03PM +0200, Arnd Bergmann wrote:
> > On Wednesday 16 July 2014 17:14:29 Thierry Reding wrote:
> > > > 
> > > > Ok, I'll have a look. I think when this becomes a separate driver, it
> > > > should also have its own header file, so maybe you can in the meantime
> > > > make it a local header file in mach-tegra until we have found a good
> > > > place for it.
> > > 
> > > Why do you think it should be a separate header? We already have a
> > > couple in include/linux and I'm not sure it's useful to add even more.
> > > If anything I would've thought it made sense to move the content of the
> > > other headers into tegra-soc.h.
> > 
> > I very much dislike the idea of having a per-vendor header file that
> > everything gets crammed into. We should try to have proper subsystems
> > and generic interfaces for these wherever possible.
> 
> I completely agree. However spreading the SoC-specific functions across
> multiple header files isn't going to help. If we keep all the per-vendor
> APIs in one file it makes it easier to see what could still be moved off
> into a separate subsystem.
> 
> Now for PMC specifically, we've investigated converting the powergate
> API to power domains. I don't think it will be possible to make that
> work. The issue is that there's a defined sequence that needs to be
> respected to make sure the device is powered up properly. That sequence
> involves the primary clock and reset of the device. It's been proposed
> to make these clocks available to the PMC driver so that it can control
> them, but then we can't make sure that clocks are really off if they
> need to be, since we have two drivers accessing them. The only way I see

resets do not have reference counts, so they can be controlled by a
powerdomain driver without any problems. For clocks, there would only be
a problem for the module clocks if the drivers don't use runtime PM. If
we move all drivers to runtime PM, the clock control can move into the
powerdomain code and runtime PM will ensure domains are not turned off
with active modules.

> to make that work reliably is by moving complete control of the
> powergate into drivers so that they can make sure clocks and resets are
> in the correct states.
> 

Which won't work if you have domains which contain several modules.

Peter.