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

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Date:   Fri, 20 Nov 2020 10:59:12 -0800
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: [RESEND PATCH v3 2/2] i2c: i2c-mux-gpio: Enable this driver in
 ACPI land

On Thu, Nov 19, 2020 at 7:24 AM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
>
> On Thu, Nov 19, 2020 at 1:40 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.
>
> ...
>
> > +#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.
-Evan

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