Re: [PATCH 2/2] ARM: dts: meson: Adding hwrev syscon node

[Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>]

On Friday 26 February 2016 17:43:54 Carlo Caione wrote:
> On Fri, Feb 26, 2016 at 5:00 PM, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> > On Friday 26 February 2016 16:34:59 Carlo Caione wrote:
> >> On Fri, Feb 19, 2016 at 1:54 PM, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> >>
> >> <cut>
> >>
> >> > This still really sounds like a mixed bag to me, which should better get represented
> >> > as a syscon node, except that there are also some more structured areas in
> >> > CBUS.
> >> >
> >> > Having just the registers between METAL_REVISION and HW_REV in a syscon
> >> > is clearly wrong, as that would include the pinctrl area that already has
> >> > a driver, but would not include some other parts that want the syscon.
> >> >
> >> > Maybe the best way is to make it compatible with both syscon and
> >> > simple-bus and put the other nodes underneath. That is still rather
> >> > ugly, but at least it works and can be extended.
> >>
> >> More on this topic.
> >>
> >> On the meson platforms (at least on the meson8 / meson8b) we have two
> >> buses: cbus and aobus. Since in cbus we have a bunch of scattered
> >> registers, Arnd suggested to make it compatible with both syscon and
> >> simple-bus. So my idea was actually to update the meson8b DTSI file
> >> adding the two buses to make it closer to the actual hardware.
> >>
> >> In the most simple cases moving the devices under the correct bus is a
> >> trivial operation since (of course) the same driver can be used when
> >> the device is attached to a different bus, like in the uart_AO case
> >> (http://lxr.free-electrons.com/source/arch/arm/boot/dts/meson8b.dtsi#L114).
> >>
> >> Unfortunately pinctrl is a different beast since it requires
> >> (http://lxr.free-electrons.com/source/drivers/pinctrl/meson/pinctrl-meson.c#L659)
> >> at least two subnodes: one accessing registers from aobus, and the
> >> other one from cbus.
> >>
> >> I know this is quite a peculiar case but I'm wondering what is the
> >> best way to approach this issue.
> >>
> >> 1) We could move only the pinctrl device outside both aobus and cbus
> >> but IMO this is ugly (at this point is probably better not having the
> >> two buses at all described in the DTS).
> >> 2) The second option is just to fix the driver so that the two
> >> subnodes are not strictly required. The problem with this second
> >> solution is that in the driver we still need to access some data
> >> (http://lxr.free-electrons.com/source/drivers/pinctrl/meson/pinctrl-meson8b.c#L873)
> >> that is specific to each bus. So we will end up having two different
> >> compatibles for the two buses (meson8b-pinctrl-aobus using data from
> >> 'ao-bank', and meson8b-pinctrl-cbus using data from 'banks').
> >> 3) Another option is just to have the driver with a unique compatible
> >> poking the parent node (or just to another property) to determine on
> >> which bus it is so that it can use the correct bus-specific data.
> >>
> >> Any idea / feedback?
> >
> > Would it be possible to split the pin controller driver into two drivers
> > that each only access registers on one of the buses? Is that a split
> > that makes sense from the point of view of that driver?
> 
> AFAICT (I'm not the driver author) there is no a really strict reason
> to have one single driver accessing registers on both buses. Of course
> the driver has to be changed a bit.
> Are you suggesting to have two different drivers with two different compatibles?

Just an idea, but yes: if the register layout is different, then they
would also need different compatible strings.

This is mostly a question of how the hardware design really looks:
are these two separate pin controllers that each are responsible for
a clear subset of the pins?
	
	Arnd
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