Re: [PATCH v2 1/2] dt-bindings: pinctrl: airoha: Add EN7581 pinctrl controller

[Conor Dooley <conor@kernel.org>]

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On Fri, Aug 23, 2024 at 11:17:34PM +0200, Lorenzo Bianconi wrote:
> > On Fri, Aug 23, 2024 at 05:14:30PM +0100, Conor Dooley wrote:
> > > On Thu, Aug 22, 2024 at 10:50:52PM +0200, Benjamin Larsson wrote:
> > > > On 22/08/2024 18:06, Conor Dooley wrote:
> > > > 
> > > > 
> > > > Hi.
> > > > 
> > > > > before looking at v1:
> > > > > I would really like to see an explanation for why this is a correct
> > > > > model of the hardware as part of the commit message. To me this screams
> > > > > syscon/MFD and instead of describing this as a child of a syscon and
> > > > > using regmap to access it you're doing whatever this is...
> > > > 
> > > > Can you post a link to a good example dts that uses syscon/MFD ?
> > > > 
> > > > It is not only pinctrl, pwm and gpio that are entangled in each other. A
> > > > good example would help with developing a proper implementation.
> > > 
> > > Off the top of my head, no unfortunately. Maybe Rob or Krzk have a good
> > > example. I would suggest to start by looking at drivers within gpio or
> > > pinctrl that use syscon_to_regmap() where the argument is sourced from
> > > either of_node->parent or dev.parent->of_node (which you use depends on
> > > whether or not you have a child node or not).
> > > 
> > > I recently had some questions myself for Rob about child nodes for mfd
> > > devices and when they were suitable to use:
> > > https://lore.kernel.org/all/20240815200003.GA2956351-robh@kernel.org/
> > > 
> > > Following Rob's line of thought, I'd kinda expect an mfd driver to create
> > > the devices for gpio and pwm using devm_mfd_add_devices() and the
> > > pinctrl to have a child node.
> > 
> > Just to not get confused and staring to focus on the wrong kind of
> > API/too complex solution, I would suggest to check the example from
> > Lorenzo.
> > 
> > The pinctrl/gpio is an entire separate block and is mapped separately.
> > What is problematic is that chip SCU is a mix and address are not in
> > order and is required by many devices. (clock, pinctrl, gpio...)
> > 
> > IMHO a mfd is overkill and wouldn't suite the task. MDF still support a
> > single big region and in our case we need to map 2 different one (gpio
> > AND chip SCU) (or for clock SCU and chip SCU)
> > 
> > Similar problem is present in many other place and syscon is just for
> > the task.
> > 
> > Simple proposed solution is:
> > - chip SCU entirely mapped and we use syscon

That seems reasonable.

> > - pinctrl mapped and reference chip SCU by phandle

ref by phandle shouldn't be needed here, looking up by compatible should
suffice, no?

> > - pwm a child of pinctrl as it's scrambled in the pinctrl mapped regs

The pwm is not a child of the pinctrl though, they're both subfunctions of
the register region they happen to both be in. I don't agree with that
appraisal, sounds like an MFD to me.

> > Hope this can clear any confusion.
> 
> To clarify the hw architecture we are discussing about 3 memory regions:
> - chip_scu: <0x1fa20000 0x384>
> - scu: <0x1fb00020 0x94c>
                  ^
I'm highly suspicious of a register region that begins at 0x20. What is
at 0x1fb00000?

> - gpio: <0x1fbf0200 0xbc>

Do you have a link to the register map documentation for this hardware?

> The memory regions above are used by the following IC blocks:
> - clock: chip_scu and scu

What is the differentiation between these two different regions? Do they
provide different clocks? Are registers from both of them required in
order to provide particular clocks?

> - pinctrl (io-muxing/gpio_chip/irq_chip): chip_scu and gpio

Ditto here. Are these actually two different sets of iomuxes, or are
registers from both required to mux a particular pin?

> - pwm: gpio
> 
> clock and pinctrl devices share the chip_scu memory region but they need to
> access even other separated memory areas (scu and gpio respectively).
> pwm needs to just read/write few gpio registers.
> As pointed out in my previous email, we can define the chip_scu block as
> syscon node that can be accessed via phandle by clock and pinctrl drivers.
> clock driver will map scu area while pinctrl one will map gpio memory block.
> pwm can be just a child of pinctrl node.

As I mentioned above, the last statement here I disagree with. As
someone that's currently in the process of fixing making a mess of this
exact kind of thing, I'm going to strongly advocate not taking shortcuts
like this.

IMO, all three of these regions need to be described as syscons in some
form, how exactly it's hard to say without a better understanding of the
breakdown between regions.

If, for example, the chip_scu only provides a few "helper" bits, I'd
expect something like

syscon@1fa20000 {
	compatible = "chip-scu", "syscon";
	reg = <0x1fa2000 0x384>;
};

syscon@1fb00000 {
	compatible = "scu", "syscon", "simple-mfd";
	#clock-cells = 1;
};

syscon@1fbf0200 {
	compatible = "gpio-scu", "syscon", "simple-mfd";
	#pwm-cells = 1;

	pinctrl@x {
		compatible = "pinctrl";
		reg = x;
		#pinctrl-cells = 1;
		#gpio-cells = 1;
	};
};

And you could look up the chip-scu by its compatible from the clock or
pinctrl drivers. Perhaps the "helper" bits assumption is incorrect
however, and both the scu and chip scu provide independent clocks?

> What do you think about this approach? Can we address the requirements above
> via classic mfd driver?



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