linux-kernel - Re: [PATCH v1] of: property: Add fw_devlink support for "gpio" and "gpios" binding

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Date:   Tue, 19 Jan 2021 19:10:43 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        linux-tegra <linux-tegra@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Jon Hunter <jonathanh@...dia.com>,
        Android Kernel Team <kernel-team@...roid.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] of: property: Add fw_devlink support for "gpio" and
 "gpios" binding

Hi Saravana,

On Tue, Jan 19, 2021 at 6:54 PM Saravana Kannan <saravanak@...gle.com> wrote:
> On Tue, Jan 19, 2021 at 2:20 AM Linus Walleij <linus.walleij@...aro.org> wrote:
> > On Tue, Jan 19, 2021 at 9:50 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > > > Can we pull this into driver-core-next please? It fixes issues on some
> > > > boards with fw_devlink=on.
> > >
> > > On r8a77951-salvator-xs.dts, it introduces one more failure:
> > >
> > >     OF: /soc/i2c@...d8000/gpio@...pcie-sata-switch-hog: could not get
> > > #gpio-cells for /cpus/cpu@102
>
> Geert,
>
> One good thing is that it's noticing this being weird and ignoring it
> in your particular board. I *think* it interprets the "7" as a phandle
> and that's cpu@102 and realizes it's not a gpio-controller. For at
> least in your case, it's a safe failure.

While 7 is the GPIO index, relative to the current GPIO controller,
represented by the parent device node.

> > > Seems like it doesn't parse gpios properties in GPIO hogs correctly.
> >
> > Could it be that the code assumes no self-referencing phandles?
> > (Just guessing...)
>
> Ok I tried to understand what gpio-hogs means. It's not fully clear to
> me. But it looks like if a gpio-controller has a gpio-hog, then it
> doesn't have/need gpio-cells? Is that right?

A GPIO hog is a way to fix (strap) a GPIO line to a specific value.
Usually this is done to enable a piece of hardware on a board, or
control a mux.

The controller still needs gpio-cells.

> So if a gpio-controller has a gpio-hog, can it ever be referred to by
> another consumer in DT using blah-gpios = ...? If so, I don't see any
> obvious code that's handling the missing gpio-cells in this case.

Yes it can.

> Long story short, please help me understand gpio-hog in the context of
> finding dependencies in DT.

The hog references a GPIO on the current controller.  As this is always
the parent device node, the hog's gpios properties lack the phandle.

E.g. a normal reference to the first GPIO of gpio5 looks like:

    gpios = <&gpio5 0 GPIO_ACTIVE_LOW>;

A hog on the first GPIO of gpio5 would be a subnode of gpio5,
and would just use:

    gpios = <0 GPIO_ACTIVE_LOW>;

instead.

Hope this helps.

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