Date:	Fri, 22 Apr 2016 13:15:16 +0300
From:	Crestez Dan Leonard <leonard.crestez@...el.com>
To:	Peter Rosin <peda@...ntia.se>, Jonathan Cameron <jic23@...nel.org>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Hartmut Knaack <knaack.h@....de>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Peter Meerwald-Stadler <pmeerw@...erw.net>,
	Daniel Baluta <daniel.baluta@...el.com>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
	Wolfram Sang <wsa@...-dreams.de>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>
Subject: Re: [PATCH] iio: inv_mpu6050: Add support for auxiliary I2C master

On 04/21/2016 04:56 PM, Peter Rosin wrote:
> Crestez Dan Leonard wrote:
>> On 04/20/2016 11:31 PM, Peter Rosin wrote:
>>> Crestez Dan Leonard wrote:
>>>> Changes since that version:
>>>> * Nest the adapter in inv_mpu6050_state instead of making it static
>>>> * Explicitly forward of_node "i2c-aux-master" to allow describing aux devices
>>>> via devicetree.
>>>>
>>>> For bypass/mux mode devicetree works automatically. The forwarding is based on
>>>> the "chan_id" parameter to i2c_add_mux_adapter and is implemented here:
>>>>
>>>> http://lxr.free-electrons.com/source/drivers/i2c/i2c-mux.c?v=4.5#L158
>>>>
>>>> Perhaps it might be better for devices handled via master mode to be described
>>>> via i2c@1? This would work by scanning the mpu node's children for something
>>>> with reg == 1.
>>>
>>> The 0 in i2c@0 (which is used by the mux mode) is the index of the mux slave
>>> meaning that i2c@1 would be a second mux slave on the same mux, but this is
>>> not a real mux as such, it is a gate which is piggybacking on the i2c mux infra.
>>> So, this "mux" can't have a second slave which is why only 0 is valid.
>>
>> This behavior is automatic in i2c mux code and seems to assume that all
>> the children of mux_dev are i2c muxes. This might be obviously correct
>> and useful for dedicated i2c mux devices but in my case mux_dev is just
>> the i2c_client for a sensor.
>>
>> From Documentation/devicetree/bindings/i2c/i2c-mux.txt:
>>
>>> An i2c bus multiplexer/switch will have several child busses that are
>>> numbered uniquely in a device dependent manner.  The nodes for an i2c
>>> bus multiplexer/switch will have one child node for each child bus.
>>
>> This seems to be written in a way that would allow me to define the
>> "auxiliary i2c master" as bus "1". After all, the numbering is device
>> dependent and it's not clear that all the child busses need to be
>> accessible through muxing rather than indirect access through device
>> registers.
> 
> You are correct that if you have devicetree children where reg does
> not match the chan_id given to i2c_add_mux_adapter() those children
> will be ignored by the i2c-mux code. So, the code would be happy with
> a devicetree such as:
> 
> 	mpu6050@68 {
> 		compatible = "...";
> 		reg = <0x68>;
> 		...
> 		i2c-aux-mux@0 {
> 			#address-cells = <1>;
> 			#size-cells = <0>;
> 			reg = <0>;
> 
> 			foo@44 {
> 				compatible = "bar";
> 				reg = <0x44>;
> 				...
> 			}
> 		}
> 		i2c-aux-master@1 {
> 			#address-cells = <1>;
> 			#size-cells = <0>;
> 			reg = <1>;
> 
> 			gazonk@44 {
> 				compatible = "baz";
> 				reg = <0x44>;
> 				...
> 			}
> 		}
> 	}
> 
> as long as you do only call i2c_add_mux_adapter() with chan_id = 0. And that
> is what you are doing. But I think it is a bit subtle...

This kind of stuff needs to be written up in
Documentation/devicetree/bindings anyway.

>>> I think the naming could be i2c-master0, i2c-master1 etc if it, with
>>> future work, would be possible to add more than one master (you talked about
>>> 5 i2c slaves..).
>>
>> The device has 5 sets of registers for controlling i2c slaves but only
>> one physical auxiliary i2c bus. As far as I can tell slaves 0-3 are
>> intended to be used for gathering readings for slaved sensors
>> periodically without external intervention. Slave 4 can generate an
>> interrupt on completion and is more suitable for general-purpose
>> communication with any number of devices.
> 
> Ah, ok, so all 5 sets of slave registers are about the same physical i2c
> bus. So, you basically cannot sanely use this physical aux i2c bus as an
> i2c-mux and an extra i2c adapter in the same hw design. Correct?

You can access devices on the auxiliary i2c bus either through mux-ing
or the adapter added by this patch. I think mux mode works better (lower
latency) but is not available when the primary connection is via SPI.
You can use both but it doesn't particularly make sense.

> In that case, couldn't you look at the names of any devicetree children
> and use that to decide if you should even attempt to call
> i2c_add_mux_adapter or i2c_add_adapter?

But the adapter should be added even if nothing is defined for it.
Registering i2c clients by echoing in new_device is a valid usecase.

What could be done is only register the i2c mux in i2c mode and the i2c
master in spi mode and make the bindings identical.

> (But please don't clobber stuff for my i2c-mux rework, or you will
> have to wait even longer for that deadlock to be resolved)

I won't. I should have sent this as an [RFC] anyway, it will take a
while to get it.

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