Re: [RFC 1/4] ARM: tegra: Move SoC drivers to drivers/soc/tegra

[Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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On Mon, Jun 30, 2014 at 09:20:30AM +0200, Arnd Bergmann wrote:
> On Saturday 28 June 2014 22:40:26 Thierry Reding wrote:
> > > > pmc.c implements various routines to access the power management
> > > > controller, some of which is needed by suspend/resume, some of it is
> > > > needed by SMP. powergate.c implements a subset of the PMC that needs to
> > > > be exported to drivers to enable power partitions on the SoC. I'm not
> > > > aware of subsystems that deal with this kind of driver.
> > > >
> > > Please see above.
> > 
> > Like I said, I don't see what business suspend/resume related code has
> > in drivers/power. What we're talking about here really is functionality
> > very specific to Tegra. Also some of that code needs to be run at very
> > early points in the boot process, so we can't reasonably turn it into a
> > proper driver anyway.
> 
> I believe the powergate.c stuff can be changed into pm_domain code, but
> we don't have a good subsystem with generic DT bindings yet, so that
> may need some more groundwork first. drivers/power or a subdirectory
> of that may end up being the right place though.

Last time I checked the PM domain code didn't seem like a good fit. I'll
go recheck and see if I can make it work.

One minor issue with putting the driver in drivers/power is that that
directory is only built when POWER_SUPPLY is selected. And since this
isn't really a power supply driver at all, either we need to change the
drivers/Makefile to descend unconditionally or we need to find another
home.

I'll see if I can come up with yet another generic binding for this in
an attempt to move this out of arch/arm/mach-tegra...

> > Also, the real win we get from moving code out into drivers/ is if we
> > can provide a common framework for them. I don't see how we can possibly
> > do that for this code since there simply isn't enough commonality
> > between SoCs. At the same time we now have a situation where both 32-bit
> > and 64-bit variants of some SoCs share some of the same hardware at the
> > very low level and since we don't have mach-* directories for 64-bit ARM
> > and shouldn't be duplicating code either, we need to find a new home for
> > this type of code. drivers/soc seemed to fit perfectly well.
> 
> For the low-level stuff yes, but I agree with Santosh there needs to be
> some more work trying to split out individual high-level drivers.
> 
> There are two common patterns for the interface between the low-level
> register access and the more high-level stuff:  you can either use
> a "syscon" driver that just exports a struct regmap and that gets referenced
> from other drivers using a phandle in their device nodes, or you have
> a driver in drivers/soc that exports a somewhat higher-level interface
> and comes with its own header file that gets included by other drivers.
> At the moment, the syscon/regmap variant can only be used once device
> drivers are loaded, but there is some broad agreement that it should
> be changed to allow calling syscon_regmap_lookup_by_phandle() at
> early boot using just DT accessors.

Note that we already have all these drivers and they do work for earlier
Tegra generations. My attempt to move these out of arch/arm/mach-tegra
is merely about being able to share them with upcoming 64-bit variants.
So it's not about adding new functionality but rather about keeping the
existing level of functionality on 64-bit.

I'm not a fan of the syscon/regmap approach at all and the existing
drivers use a higher-level API already, so I think we're going to stick
with that.

Thierry

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