linux-kernel - Re: [PATCH v3 2/2] i2c: i2c-mux-gpio: Enable this driver in ACPI land

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Date:   Tue, 27 Oct 2020 16:00:37 -0700
From:   Evan Green <evgreen@...omium.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Peter Rosin <peda@...ntia.se>, Wolfram Sang <wsa@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Peter Korsgaard <peter.korsgaard@...co.com>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] i2c: i2c-mux-gpio: Enable this driver in ACPI land

On Mon, Oct 19, 2020 at 9:53 AM Evan Green <evgreen@...omium.org> wrote:
>
> On Sun, Oct 18, 2020 at 11:58 AM Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
> >
> > On Sat, Oct 17, 2020 at 8:30 AM Evan Green <evgreen@...omium.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.

Andy or Peter, Any other thoughts on this series?
-Evan