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 10:47:13 -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 10:33 AM Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
>
> Hi Saravana,
>
> On Tue, Jan 19, 2021 at 7:19 PM Saravana Kannan <saravanak@...gle.com> wrote:
> > On Tue, Jan 19, 2021 at 10:10 AM Geert Uytterhoeven
> > <geert@...ux-m68k.org> wrote:
> > > 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?
>
> Indeed. You can just ignore all nodes with a gpio-hog property, as they're
> always handled by their parent.

Ok, I'll send out an updated patch later (next week probably). Or
maybe we can merge this now and I can fix up gpio-hog handling later
since I'd need to do it anyway for the name-gpios stuff anyway? Or
will those never be combined with gpio-hog?

-Saravana