Subject: Re: [PATCH] iio: inv_mpu6050: Add support for auxiliary I2C master
To: Jonathan Cameron <jic23@kernel.org>,
        Crestez Dan Leonard <leonard.crestez@intel.com>,
        <linux-iio@vger.kernel.org>
References: <5b1937140558514b3e43ed2c21ee860ecbac61ec.1461172603.git.leonard.crestez@intel.com>
 <a978ad85-8191-7506-8090-809f89d0b2e2@kernel.org>
CC: <linux-kernel@vger.kernel.org>, Hartmut Knaack <knaack.h@gmx.de>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Peter Meerwald-Stadler <pmeerw@pmeerw.net>,
        Daniel Baluta <daniel.baluta@intel.com>, <linux-i2c@vger.kernel.org>,
        Wolfram Sang <wsa@the-dreams.de>, <devicetree@vger.kernel.org>,
        Rob Herring <robh+dt@kernel.org>, Pawel Moll <pawel.moll@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Ian Campbell <ijc+devicetree@hellion.org.uk>,
        Kumar Gala <galak@codeaurora.org>
From: Peter Rosin <peda@axentia.se>
Message-ID: <6ab228db-9695-ac22-9c65-fd15c4837c45@axentia.se>
Date: Wed, 27 Apr 2016 10:39:17 +0200
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Hi!

On 2016-04-23 23:32, Jonathan Cameron wrote:
> On 20/04/16 18:17, Crestez Dan Leonard wrote:
>> The MPU has an auxiliary I2C bus for connecting external
>> sensors. This bus has two operating modes:
>> * pass-through, which connects the primary and auxiliary busses
>> together. This is already supported via an i2c mux.
>> * I2C master mode, where the mpu60x0 acts as a master to any external
>> connected sensors. This is implemented by this patch.
>>
>> This I2C master mode also works when the MPU itself is connected via
>> SPI.
>>
>> I2C master supports up to 5 slaves. Slaves 0-3 have a common operating
>> mode while slave 4 is different. This patch implements an i2c adapter
>> using slave 4 because it has a cleaner interface and it has an
>> interrupt that signals when data from slave to master arrived.
>>
>> Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
> This one needs acks from:
>
> Device tree maintainer (odd binding ;)
> Peter Rosin (odd binding interacting with the mux support)
> Wolfram (it has a whole i2c master driver in here).
>
> (just thought I'd list these for the avoidance of doubt).

I spot some overlap with the questions in "[RFC] i2c: device-tree:
Handling child nodes which are not i2c devices"
http://marc.info/?l=linux-i2c&m=146073452819116&w=2

And I think I agree with Stephen Warren that an intermediate placeholder
node would make sense. I.e.

    mpu6050@68 {
        compatible = "...";
        reg = <0x68>;
        ...
        i2c-aux-mux {
            i2c@0 {
                #address-cells = <1>;
                #size-cells = <0>;
                reg = <0>;

                foo@44 {
                    compatible = "bar";
                    reg = <0x44>;
                    ...
                }
            }
        }
    }

Or

    mpu6050@68 {
        compatible = "...";
        reg = <0x68>;
        ...
        i2c-aux-master {
            #address-cells = <1>;
            #size-cells = <0>;

            gazonk@44 {
                compatible = "baz";
                reg = <0x44>;
                ...
            }
        }
    }

depending on if you want an aux-mux or an aux-master.

But I don't know if that intermediate i2c-aux-mux node causes any
problems?

Cheers,
Peter