linux-kernel - Re: [net-next PATCH v4 09/15] device property: Introduce fwnode_get

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Message-ID: <CAJZ5v0izwiuD+gRmbw=i=DojDMwqOevDQwXArcmq4WyPVrEDfQ@mail.gmail.com>
Date:   Fri, 22 Jan 2021 19:11:56 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Calvin Johnson <calvin.johnson@....nxp.com>,
        Grant Likely <grant.likely@....com>,
        Jeremy Linton <jeremy.linton@....com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Cristi Sovaiala <cristian.sovaiala@....com>,
        Florin Laurentiu Chiculita <florinlaurentiu.chiculita@....com>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Madalin Bucur <madalin.bucur@....nxp.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        Pieter Jansen Van Vuuren <pieter.jansenvv@...boosystems.io>,
        Jon <jon@...id-run.com>, Saravana Kannan <saravanak@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        "linux.cj" <linux.cj@...il.com>,
        Diana Madalina Craciun <diana.craciun@....com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>
Subject: Re: [net-next PATCH v4 09/15] device property: Introduce fwnode_get_id()

On Fri, Jan 22, 2021 at 6:12 PM Andy Shevchenko
<andy.shevchenko@...il.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@....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?