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

Arnd Bergmann
2016-02-26T16:00:57Z

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?

Arnd