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From:   Evan Green <evgreen@chromium.org>
Date:   Mon, 19 Oct 2020 09:53:59 -0700
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Subject: Re: [PATCH v3 2/2] i2c: i2c-mux-gpio: Enable this driver in ACPI land
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     Peter Rosin <peda@axentia.se>, Wolfram Sang <wsa@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Peter Korsgaard <peter.korsgaard@barco.com>,
        linux-i2c <linux-i2c@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Sun, Oct 18, 2020 at 11:58 AM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Sat, Oct 17, 2020 at 8:30 AM Evan Green <evgreen@chromium.org> wrote:
> >
> > Enable i2c-mux-gpio devices to be defined via ACPI. The idle-state
> > property translates directly to a fwnode_property_*() call. The child
> > reg property translates naturally into _ADR in ACPI.
> >
> > The i2c-parent binding is a relic from the days when the bindings
> > dictated that all direct children of an I2C controller had to be I2C
> > devices. These days that's no longer required. The i2c-mux can sit as a
> > direct child of its parent controller, which is where it makes the most
> > sense from a hardware description perspective. For the ACPI
> > implementation we'll assume that's always how the i2c-mux-gpio is
> > instantiated.
>
> Can you tell me if the following is relevant to what you are looking for?
> https://elixir.bootlin.com/linux/latest/source/drivers/i2c/i2c-mux.c#L393

I don't think so, but let me know if I'm reading between the lines incorrectly.

The code you pointed to links the newly-minted fake i2c controller
back together with its ACPI node. This is important, since I think
that's how child I2C devices underneath the fake busses get populated
in ACPI land. But the paragraph above is discussing how to identify
the parent adapter (ie the real hardware) for an i2c-mux-gpio device.

In DT-land, the i2c-mux-gpio floats at the top of the tree directly
under /, and then uses a phandle to point to where transactions should
be forwarded. I'm told the reason for this is historical limitations
with the DT bindings. Rather than trying to translate the phandle over
1:1 into ACPI-land, I'm asserting that the mux device should live
underneath the adapter it wants to forward traffic to.

-Evan

>
> --
> With Best Regards,
> Andy Shevchenko