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From:   Evan Green <evgreen@chromium.org>
Date:   Mon, 30 Nov 2020 11:11:39 -0800
Message-ID: <CAE=gft4LDh7TmvMgL4_eov5=1Nh0PYVRDxn4eCLsDd5JJyCwAw@mail.gmail.com>
Subject: Re: [RESEND 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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Hi Andy, Peter,

On Fri, Nov 20, 2020 at 10:59 AM Evan Green <evgreen@chromium.org> wrote:
>
> On Thu, Nov 19, 2020 at 7:24 AM Andy Shevchenko
> <andy.shevchenko@gmail.com> wrote:
> >
> > On Thu, Nov 19, 2020 at 1:40 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.
> >
> > ...
> >
> > > +#ifdef CONFIG_ACPI
> > > +
> > > +static int i2c_mux_gpio_get_acpi_adr(struct device *dev,
> > > +                                    struct fwnode_handle *fwdev,
> > > +                                    unsigned int *adr)
> > > +
> > > +{
> > > +       unsigned long long adr64;
> > > +       acpi_status status;
> > > +
> > > +       status = acpi_evaluate_integer(ACPI_HANDLE_FWNODE(fwdev),
> > > +                                      METHOD_NAME__ADR,
> > > +                                      NULL, &adr64);
> > > +
> > > +       if (!ACPI_SUCCESS(status)) {
> > > +               dev_err(dev, "Cannot get address\n");
> > > +               return -EINVAL;
> > > +       }
> > > +
> > > +       *adr = adr64;
> > > +       if (*adr != adr64) {
> > > +               dev_err(dev, "Address out of range\n");
> > > +               return -ERANGE;
> > > +       }
> > > +
> > > +       return 0;
> > > +}
> > > +
> > > +#else
> > > +
> > > +static int i2c_mux_gpio_get_acpi_adr(struct device *dev,
> > > +                                    struct fwnode_handle *fwdev,
> > > +                                    unsigned int *adr)
> > > +{
> > > +       return -EINVAL;
> > > +}
> > > +
> > > +#endif
> >
> > I'm wondering if you may use acpi_find_child_device() here.
> > Or is it a complementary function?
>
> I think it's complementary. The code above is "I have a device, I want
> its _ADR". whereas acpi_find_child_device() is "I have an _ADR, I want
> its device". I could flip things around to use this, but it would turn
> the code from linear into quadratic. I'd have to scan each possible
> address and call acpi_find_child_device() with that _ADR to see if
> there's a child device there.
>
> >
> > ...
> >
> > > +       device_for_each_child_node(dev, child) {
> > > +               if (is_of_node(child)) {
> > > +                       fwnode_property_read_u32(child, "reg", values + i);
> > > +
> > > +               } else if (is_acpi_node(child)) {
> > > +                       rc = i2c_mux_gpio_get_acpi_adr(dev, child, values + i);
> > > +                       if (rc)
> > > +                               return rc;
> > > +               }
> > > +
> > >                 i++;
> > >         }
> >
> > And for this I already told in two different threads with similar code
> > that perhaps we need common helper that will check reg followed by
> > _ADR.
>
> Oh, I'm not aware of those threads. I'd need some advice: I guess a
> new fwnode_* API would make sense for this, but I had trouble coming
> up with a generic interface. _ADR is just a blobbo 64 bit int, but
> DT's "reg" is a little more flexible, having a length, and potentially
> being an array. I suppose it would have to be something like:
>
> int fwnode_property_read_reg(const struct fwnode_handle *fwnode,
>                                  size_t index, uint64_t *addr, uint64_t *len);
>
> But then ACPI would always return 0 for length, and only index 0 would
> ever work? I'm worried I'm designing an API that's only useful to me.
>
> I tried to look around for other examples of this specific pattern of
> _ADR then "reg", but struggled to turn up much.

Any thoughts on this?

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