[RFC PATCH 1/1] pinctrl: rockchip: add support for per-pinmux io-domain dependency

[Sascha Hauer]

Hi Quentin,

On Tue, Aug 02, 2022 at 11:52:52AM +0200, Quentin Schulz wrote:
> From: Quentin Schulz <quentin.schulz at theobroma-systems.com>
> 
> On some Rockchip SoCs, some SoC pins are split in what are called IO
> domains.
> 
> An IO domain is supplied power externally, by regulators from a PMIC for
> example. This external power supply is then used by the IO domain as
> "supply" for the IO pins if they are outputs.
> 
> Each IO domain can configure which voltage the IO pins will be operating
> on (1.8V or 3.3V).
> 
> There already exists an IO domain driver for Rockchip SoCs[1]. This
> driver allows to explicit the relationship between the external power
> supplies and IO domains[2]. This makes sure the regulators are enabled
> by the Linux kernel so the IO domains are supplied with power and
> correctly configured as per the supplied voltage.
> This driver is a regulator consumer and does not offer any other
> interface for device dependency.
> 
> However, IO pins belonging to an IO domain need to have this IO domain
> correctly configured before they are being used otherwise they do not
> operate correctly (in our case, a pin configured as output clock was
> oscillating between 0 and 150mV instead of the expected 1V8).
> 
> In order to make this dependency transparent to the consumer of those
> pins and not add Rockchip-specific code to third party drivers (a camera
> driver in our case), it is hooked into the pinctrl driver which is
> Rockchip-specific obviously.

I don't know the status of this patch, but I haven't found anything
newer, so please point me to newer patches if the discussion has
continued somewhere else. Anyway, here are some thoughts about this
patch

I think the general approach is fine but could be improved. Right now we
have one io-domain device with several supplies. That means once one
consumer needs an io-domain, the supplies for all domains need to be
probed beforehand.  We could relax this requirement by adding a subnode
for each domain, so instead of doing this:

pmu_io_domains: io-domains {
	compatible = "rockchip,rk3568-pmu-io-voltage-domain";
	pmuio1-supply = <&vcc3v3_pmu>;
	pmuio2-supply = <&vcc3v3_pmu>;
	vccio1-supply = <&vccio_acodec>;
	vccio2-supply = <&vcc_1v8>;
	vccio3-supply = <&vccio_sd>;
	vccio4-supply = <&vcc_1v8>;
	vccio5-supply = <&vcc_3v3>;
	vccio6-supply = <&vcc_1v8>;
	vccio7-supply = <&vcc_3v3>;
};

We could do this:

pmu_io_domains: io-domains {
	compatible = "rockchip,rk3568-pmu-io-voltage-domain";

	io_domain_pmuio1: io-domain@ {
		reg = <0>;
		supply = <&vcc3v3_pmu>;
	};

	io_domain_pmuio2: io-domain at 1 {
		reg = <1>;
		supply = <&vcc3v3_pmu>;
	};

	...
};

This way we could put a driver on each io-domain. When another device
needs an io-domain we no longer have to wait for all regulators to
appear, but only for the regulator that actually supplies that domain.

With that we could specify the io-domain dependencies at dtsi or core
level. A board would only have to make sure that the io-domain that is
needed to access the PMIC does not itself need a supply from the very
same PMIC to not get into circular dependencies. The supplies for the
io-domains are specified at board level anyway, so all that a board
would have to do is to skip (or replace with a fixed-regulator) the
supply for the io-domain that provides access to the I2C port the PMIC
is on. That is not too bad I guess as the regulator that supplies the
io-domain to access the PMIC needs to be always-on anyway. In the end if
we would turn that regulator off, we would no longer be able to turn it
on again.

One thing about putting the "rockchip,io-domains" property into the
pingroups. We would have to put this property into each and every
existing pingroup in all dts[i] files and new files would have to be
reviewed in this regard as well. The pinctrl driver already has
knowledge about all pins, so I think that would be the natural place to
also add the knowledge about which io-domain a pin is in. With that in
place we would get the knowledge if a io-domain is in use and could
disable unused io-domains. I am afraid that the "rockchip,io-domains"
property would only be added in places where it actually hurts someone.

Sascha

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