linux-kernel - Re: [PATCH v5 2/7] mux: Add helper functions for getting optional and selected mux-state

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Message-ID: <CAMuHMdU=wDmDJFYvtzxptAkkOQ8r7sPGUxp9u71e64ikY85+Zw@mail.gmail.com>
Date: Tue, 20 Jan 2026 16:57:21 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Josua Mayer <josua@...id-run.com>
Cc: Marc Kleine-Budde <mkl@...gutronix.de>, Vincent Mailhol <mailhol@...nel.org>, 
	Vinod Koul <vkoul@...nel.org>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Peter Rosin <peda@...ntia.se>, Aaro Koskinen <aaro.koskinen@....fi>, 
	Andreas Kemnade <andreas@...nade.info>, Kevin Hilman <khilman@...libre.com>, 
	Roger Quadros <rogerq@...nel.org>, Tony Lindgren <tony@...mide.com>, 
	Janusz Krzysztofik <jmkrzyszt@...il.com>, Vignesh R <vigneshr@...com>, 
	Andi Shyti <andi.shyti@...nel.org>, Ulf Hansson <ulf.hansson@...aro.org>, 
	Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Geert Uytterhoeven <geert+renesas@...der.be>, Magnus Damm <magnus.damm@...il.com>, 
	Wolfram Sang <wsa+renesas@...g-engineering.com>, 
	Yazan Shhady <yazan.shhady@...id-run.com>, Jon Nettleton <jon@...id-run.com>, 
	Mikhail Anikin <mikhail.anikin@...id-run.com>, linux-can@...r.kernel.org, 
	linux-phy@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-omap@...r.kernel.org, linux-i2c@...r.kernel.org, 
	linux-mmc@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH v5 2/7] mux: Add helper functions for getting optional and
 selected mux-state

Hi Josua,

On Sun, 18 Jan 2026 at 11:29, Josua Mayer <josua@...id-run.com> wrote:
> In-tree phy-can-transceiver driver has already implemented a local
> version of devm_mux_state_get_optional.
>
> The omap-i2c driver gets and selects an optional mux in its probe
> function without using any helper.
>
> Add new helper functions covering both aforementioned use-cases:
>
> - mux_control_get_optional:
>   Get a mux-control if specified in dt, return NULL otherwise.
> - devm_mux_state_get_optional:
>   Get a mux-state if specified in dt, return NULL otherwise.
> - devm_mux_state_get_selected:
>   Get and select a mux-state specified in dt, return error otherwise.
> - devm_mux_state_get_optional_selected:
>   Get and select a mux-state if specified in dt, return error or NULL.
>
> Existing mux_get helper function is changed to take an extra argument
> indicating whether the mux is optional.
> In this case no error is printed, and NULL returned in case of ENOENT.
>
> Calling code is adapted to handle NULL return case, and to pass optional
> argument as required.
>
> To support automatic deselect for _selected helper, a new structure is
> created storing an exit pointer similar to clock core which is called on
> release.
>
> To facilitate code sharing between optional/mandatory/selected helpers,
> a new internal helper function is added to handle quiet (optional) and
> verbose (mandatory) errors, as well as storing the correct callback for
> devm release: __devm_mux_state_get
>
> Due to this structure devm_mux_state_get_*_selected can no longer print
> a useful error message when select fails. Instead callers should print
> errors where needed.
>
> Commit e153fdea9db04 ("phy: can-transceiver: Re-instate "mux-states"
> property presence check") noted that "mux_get() always prints an error
> message in case of an error, including when the property is not present,
> confusing the user."
>
> The first error message covers the case that a mux name is not matched
> in dt. The second error message is based on of_parse_phandle_with_args
> return value.
>
> In optional case no error is printed and NULL is returned.
> This ensures that the new helper functions will not confuse the user
> either.
>
> With the addition of optional helper functions it became clear that
> drivers should compile and link even if CONFIG_MULTIPLEXER was not enabled.
> Add stubs for all symbols exported by mux core.
>
> Signed-off-by: Josua Mayer <josua@...id-run.com>

Thanks for your patch!

> --- a/drivers/mux/core.c
> +++ b/drivers/mux/core.c
> @@ -46,6 +46,16 @@ static const struct class mux_class = {
>         .name = "mux",
>  };
>
> +/*
> + * struct devm_mux_state_state -       Tracks managed resources for mux-state objects.

Please run scripts/checkpatch.pl:

    WARNING: please, no space before tabs
    #80: FILE: drivers/mux/core.c:50:
    + * struct devm_mux_state_state - ^ITracks managed resources for
mux-state objects.$

> + * @mux:                               Pointer to a mux state.
> + * @exit:                              An optional callback to execte before free.
> + */
> +struct devm_mux_state_state {
> +       struct mux_state *mstate;
> +       int (*exit)(struct mux_state *mstate);
> +};
> +
>  static DEFINE_IDA(mux_ida);
>
>  static int __init mux_init(void)

> --- a/include/linux/mux/consumer.h
> +++ b/include/linux/mux/consumer.h

> +static inline void mux_control_put(struct mux_control *mux)
> +{
> +       return;
> +}

    WARNING: void function return statements are not generally useful
    #441: FILE: include/linux/mux/consumer.h:139:
    + return;

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds