linux-kernel - Re: [PATCH 2/3] pinctrl: msm: Mux out gpio function with gpio

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Message-ID: <CACRpkdZYyOT-Ua9BN=wbzmCDog3Hh0Un73-_f-fozZghrmO0=A@mail.gmail.com>
Date:   Fri, 13 Jul 2018 08:59:11 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Doug Anderson <dianders@...omium.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH 2/3] pinctrl: msm: Mux out gpio function with gpio_request()

On Mon, Jul 9, 2018 at 5:37 PM Stephen Boyd <swboyd@...omium.org> wrote:
> Quoting Linus Walleij (2018-07-09 06:54:01)
> > On Mon, Jul 2, 2018 at 7:56 PM Stephen Boyd <swboyd@...omium.org> wrote:
> >
> > > I could do with some more clarity from Linus in the "Drivers needing
> > > both pin control and GPIOs" section of
> > > Documentation/driver-api/pinctl.rst but I read that section as stating
> > > that the GPIO driver needs to mux the pin as a GPIO by requesting the
> > > pinctrl backend to do so, unless the hardware overrides the muxed
> > > function selection when the GPIO is used, without involving pinctrl
> > > software.
> >
> > Yeah that text is especially terse :/
> >
> > What it says (or what I meant to say) is that there is a choice
> > between letting the pin control and GPIO functionality on the
> > same pin be handled orthogonally or implementing these
> > gpio_*() callbacks into the pin control backend, but in either case
> > the two APIs must be used in sequence:
> > pin control setting comes first, second the GPIO subsystem can
> > request the GPIO line.
> >
> > I'll see if I can clarify.
>
> Ok. Is my interpretation correct though? The fundamental question here
> is if gpio_request() should remux the GPIO for the GPIO function or if
> drivers are expected to have pinmux settings to use their pin as a GPIO.

It's an either/or situation.

So there are two ways to do it, as the gpio_request() callback to
pinctrl_gpio_request() etc are not compulsory to implement.

For any one specific system, it is either done such that gpio_request()
does it by calling down to pinctrl_gpio_request() and talking to the
pinctrl back-end, OR the pin muxing is done as a side dish
without any interaction with the GPIO subsystem.

So pick one...

I know this is not very consistent. Sorry for the inconvenience :(

Yours,
Linus Walleij