linux-kernel - Re: [PATCH v2 2/4] dt-bindings: add bus number property

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Date:   Wed, 6 Oct 2021 11:29:03 +0200
From:   Paweł Anikiel <pan@...ihalf.com>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Arnd Bergmann <arnd@...db.de>, jarkko.nikula@...ux.intel.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Rob Herring <robh+dt@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Olof Johansson <olof@...om.net>, SoC Team <soc@...nel.org>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Pratyush Yadav <p.yadav@...com>,
        Tudor Ambarus <Tudor.Ambarus@...rochip.com>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sebastian Reichel <sre@...nel.org>,
        "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        DTML <devicetree@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Konrad Adamczyk <ka@...ihalf.com>,
        Tomasz Nowicki <tn@...ihalf.com>,
        Jacek Majkowski <jam@...ihalf.com>,
        Alexandru Stan <amstan@...gle.com>
Subject: Re: [PATCH v2 2/4] dt-bindings: add bus number property

On Tue, Oct 5, 2021 at 6:28 PM Alexandre Belloni
<alexandre.belloni@...tlin.com> wrote:
>
> On 05/10/2021 18:22:12+0200, Arnd Bergmann wrote:
> > On Tue, Oct 5, 2021 at 4:37 PM Paweł Anikiel <pan@...ihalf.com> wrote:
> > >
> > > On SoCFPGA systems, it's desireable to have fixed numbering for
> > > i2c busses, while being able to enable/disable them (e.g. have i2c1
> > > be mapped to /dev/i2c-1, even though i2c0 is disabled). This can also
> > > be achieved using devicetree aliases (see i2c_add_adapter). However,
> > > having the driver be self-contained without relying on aliases is more
> > > robust.
> > >
> > > Signed-off-by: Paweł Anikiel <pan@...ihalf.com>
> >
> > I don't see how adding a nonstandard property in one of the i2c bus
> > drivers helps at all. How do you expect this to work when there are
> > multiple i2c controllers in the system using different drivers? What
> > should happen if both an alias and the busno property are set?
> >
>
> What happens when two nodes have the same busno property because e.g.
> one is in a dtsi and the other one is in a dts?
>

If busno is set, the alias is ignored (the code that checks aliases
is never reached). If two nodes have the same busno property, we get
a WARN in drivers/i2c/i2c-core-base.c:1637, and only on of them
gets attached.

What is a better way of doing this then? Is adding aliases to the
devicetree like this okay?

aliases {
...
i2c0 = &i2c0;
i2c1 = &i2c1;
};