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

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Message-ID: <CAGETcx_Ud+UJcdWkCH-WzEGjH4voRBoO++np7ARSi5Gnh51jYg@mail.gmail.com>
Date:   Tue, 19 Jan 2021 10:18:38 -0800
From:   Saravana Kannan <saravanak@...gle.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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

On Tue, Jan 19, 2021 at 10:10 AM Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
>
> 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.

I'm still not sure if I've understood this fully, but does this just
boil down to:
Don't parse [name-]gpio[s] to find dependencies if the node has
gpio-hog property?

-Saravana