Re: pinctrl-scmi: Support for pin-only mode when groups are unavailable

[Cristian Marussi <cristian.marussi@arm.com>]

On Thu, Oct 16, 2025 at 07:05:21PM +0300, Ciprian Marian Costea wrote:
> Hello,
> 

[CC: <linus.walleij@linaro.org>]

Hi Ciprian,

I will have a better look at this in the coming days ...

...but I think, in the meantime, better if we CC also the PINCTRL
Maintainer Linus Walleij that helped us reviewing also this driver from the
PINCTRL subsystem perspective....

> 
> I’m currently exploring a solution to improve the 'pinctrl-scmi' [1] driver
> so it can better support SCMI platforms that only provide individual pin
> control. At the moment, the driver handles only group-based operations,
> which means platforms without pin groups run into limitations.

...ok

> 
> According to the SCMI v4.0 specification (section 3.11.2.8), both
> group-based and per-pin control modes should be supported. However, on
> pin-only platforms, the current implementation cannot enumerate groups,
> configure pinmux, or assign functions per pin. This results in probe
> failures and '-EINVAL' errors when trying to configure non-existent groups.
> 

...indeed

> I haven’t seen any work in this area yet, but if nothing is in progress, I’d
> like to propose an approach that keeps compatibility with the existing Linux
> pinctrl framework while adding per-pin support. The idea is to detect

...nothing planned or requested as of now that I know...so feel free to propose
your improvements...becasue it is easier to discuss in front of a series :P

Cheers,
Cristian

> whether the platform supports groups or pins-only during probe, and when
> groups are missing, create virtual groups from the DT pin specifications.
> This would allow us to reuse the existing group-based logic while enabling
> per-pin muxing.
> 
> For example, a DTS node could look like this:
> 
> uart_pins: uart-pins {
>     pins = <10 11>;      /* SCMI pin IDs */
>     functions = <2 3>;   /* Per-pin function IDs: UART_TX, UART_RX */
> };
> 
> or:
> 
> uart_pins: uart-pins {
>     pins = <10 11>;      /* SCMI pin IDs */
>     function = <2>;      /* Same function for all pins */
> };
> 
> To make this work, the driver would need to:
>   - Detect capabilities during probe (groups vs pins-only). Can be
> implemented via 'pinctrl_ops->count_get()'.
>   - Create virtual groups when groups are unsupported.
>   - Add per-pin mux/config operations by introducing a 'pin_mux_set()'
> callback in the SCMI core, since currently only group-based 'mux_set' is
> implemented.
> 
> I’d really appreciate your thoughts on whether this direction makes sense
> and if there’s anything I should consider before starting.
> Thanks a lot for your time and guidance!
> 
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pinctrl/pinctrl-scmi.c
> 
> Best Regards,
> Ciprian Costea