Re: [RFC PATCH v1 2/2] gpio: add gpio-split driver

[Linus Walleij <linus.walleij@linaro.org>]

Hi Jonas,

thanks for your patch!

overall I like the idea and I think we definitely need
something like this.

On Fri, Oct 10, 2025 at 12:35 AM Jonas Jelonek <jelonek.jonas@gmail.com> wrote:

> Add a new driver which allows to split a physical GPIO into multiple
> virtual GPIOs by using a multiplexer.
>
> For now, this doesn't support advanced features like IRQs, just normal
> IN and OUT functionality of GPIOs.
>
> This can help in various usecases. One practical case is the special
> hardware design of the Realtek-based XS1930-10 switch from Zyxel. It
> features two SFP+ ports/cages whose signals are wired to directly to the
> switch SoC. Although Realtek SoCs are short on GPIOs, there are usually
> enough the fit the SFP signals without any hacks.
>
> However, Zyxel did some weird design and connected RX_LOS, MOD_ABS and
> TX_FAULT of one SFP cage onto a single GPIO line controlled by a
> multiplexer (the same for the other SFP cage). The single multiplexer
> controls the lines for both SFP and depending on the state, the
> designated 'signal GPIO lines' are connected to one of the three SFP
> signals.
>
> Because the SFP core/driver doesn't support multiplexer but needs single
> GPIOs for each of the signals, this driver fills the gap between both.
> It registers a gpio_chip, provides multiple virtual GPIOs and sets the
> backing multiplexer accordingly.
>
> Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>

This can be made easier these days, reusing the
forwarder library. I think! Check if I'm right.
Make sure you use kernel v6.18-rc1 as a
baseline for your next patch iteration.

select GPIO_AGGREGATOR

#include <linux/gpio/forwarder.h>

Look into this driver for an example of forwarding
GPIO lines:
drivers/pinctrl/pinctrl-upboard.c

See
commit dca2f73cf19fedd7bc38fa6a0eb50fea63cd0214

Now that is a pin controller so it contains a lot of
irrelevant stuff. Focus on the forwarding part.

This part is maybe the most interesting:

        fwd = devm_gpiochip_fwd_alloc(dev, pctrl->pctrl_data->ngpio);

Here ngpio will be 1 for your usecase.

        if (IS_ERR(fwd))
                return dev_err_probe(dev, PTR_ERR(fwd), "Failed to
allocate the gpiochip forwarder\n");

        chip = gpiochip_fwd_get_gpiochip(fwd);
        chip->request = upboard_gpio_request;
        chip->free = upboard_gpio_free;
        chip->get_direction = upboard_gpio_get_direction;
        chip->direction_output = upboard_gpio_direction_output;
        chip->direction_input = upboard_gpio_direction_input;

        ret = gpiochip_fwd_register(fwd, pctrl);
        if (ret)
                return dev_err_probe(dev, ret, "Failed to register the
gpiochip forwarder\n");

As you can see you can override request/free/get_direction etc.

In this case you probably don't want to override these functions,
but instead override chip->get and chip->set so that these set
the mux (and delay a bit?) before reading/writing the line.

->get_multiple and ->set_multiple seems hard to implement
and should probably be overridden with functions returning
an error.

> +++ b/drivers/gpio/gpio-split.c

As mentioned I would call this gpio-line-mux.c

Yours,
Linus Walleij