linux-kernel - Re: [PATCH v4 2/2] gpio: add gpio-line-mux driver

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Message-ID: <b3a4e2b3-f360-4dee-815b-cee3b9095074@bootlin.com>
Date: Wed, 5 Nov 2025 14:26:42 +0100
From: Thomas Richard <thomas.richard@...tlin.com>
To: Jonas Jelonek <jelonek.jonas@...il.com>,
 Linus Walleij <linus.walleij@...aro.org>, Bartosz Golaszewski
 <brgl@...ev.pl>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Peter Rosin <peda@...ntia.se>,
 Geert Uytterhoeven <geert+renesas@...der.be>
Cc: linux-gpio@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] gpio: add gpio-line-mux driver

On 11/5/25 11:36 AM, Jonas Jelonek wrote:
> Add a new driver which provides a 1-to-many mapping for a single real
> GPIO using a multiplexer. Each virtual GPIO corresponds to a multiplexer
> state which, if set for the multiplexer, connects the real GPIO to the
> corresponding virtual GPIO.
> 
> 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 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.
> 
> Due to several practical issues, this is input-only and doesn't support
> IRQs.
> 
> Signed-off-by: Jonas Jelonek <jelonek.jonas@...il.com>
> ---
>  MAINTAINERS                  |   6 ++
>  drivers/gpio/Kconfig         |   9 +++
>  drivers/gpio/Makefile        |   1 +
>  drivers/gpio/gpio-line-mux.c | 129 +++++++++++++++++++++++++++++++++++
>  4 files changed, 145 insertions(+)
>  create mode 100644 drivers/gpio/gpio-line-mux.c
> 
Reviewed-by: Thomas Richard <thomas.richard@...tlin.com>

Best Regards,
Thomas