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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Fri, 22 Jan 2021 19:11:56 +0100
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Subject: Re: [net-next PATCH v4 09/15] device property: Introduce fwnode_get_id()
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Calvin Johnson <calvin.johnson@oss.nxp.com>,
        Grant Likely <grant.likely@arm.com>,
        Jeremy Linton <jeremy.linton@arm.com>,
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        Russell King - ARM Linux admin <linux@armlinux.org.uk>,
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On Fri, Jan 22, 2021 at 6:12 PM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Fri, Jan 22, 2021 at 05:40:41PM +0100, Rafael J. Wysocki wrote:
> > On Fri, Jan 22, 2021 at 4:46 PM Calvin Johnson
> > <calvin.johnson@oss.nxp.com> wrote:
> > >
> > > Using fwnode_get_id(), get the reg property value for DT node
> > > or get the _ADR object value for ACPI node.
> >
> > So I'm not really sure if this is going to be generically useful.
> >
> > First of all, the meaning of the _ADR return value is specific to a
> > given bus type (e.g. the PCI encoding of it is different from the I2C
> > encoding of it) and it just happens to be matching the definition of
> > the "reg" property for this particular binding.
>
> > IOW, not everyone may expect the "reg" property and the _ADR return
> > value to have the same encoding and belong to the same set of values,
>
> I have counted three or even four attempts to open code exact this scenario
> in the past couple of years. And I have no idea where to put a common base for
> them so they will not duplicate this in each case.

In that case it makes sense to have it in the core, but calling the
_ADR return value an "id" generically is a stretch to put it lightly.

It may be better to call the function something like
fwnode_get_local_bus_id() end explain in the kerneldoc comment that
the return value provides a way to distinguish the given device from
the other devices on the same bus segment.

Otherwise it may cause people to expect that the "reg" property and
_ADR are generally equivalent, which is not the case AFAICS.

At least the kerneldoc should say something like "use only if it is
known for a fact that the _ADR return value can be treated as a
fallback replacement for the "reg" property that is missing in the
given use case".

> > so maybe put this function somewhere closer to the code that's going
> > to use it, because it seems to be kind of specific to this particular
> > use case?