Re: [RFC 10/11] iio: Add OF support

[Lars-Peter Clausen <lars-Qo5EllUWu/uELgA04lAiVw@public.gmane.org>]

On 02/03/2013 12:47 PM, Lars-Peter Clausen wrote:
> On 02/03/2013 12:39 PM, Jonathan Cameron wrote:
>> On 02/02/2013 04:10 PM, Guenter Roeck wrote:
>>> On Sat, Feb 02, 2013 at 10:29:02AM +0000, Jonathan Cameron wrote:
>>>> On 01/31/2013 09:43 PM, Guenter Roeck wrote:
>>>>> Provide bindings, new API access functions, and parse OF data
>>>>> during initialization.
>>>>>
>>>> Firstly thanks for working on this Guenter, it's been a big hole
>>>> for a while largely because non of our largest developers were
>>>> actually using development platforms with device tree support.
>>>>
>>>> Given my knowledge of device tree is based on the odd article
>>>> and looking at similar sets of bindings this morning, my comments
>>>> are likely to be somewhat superficial and uninformed ;)
>>>>
>>>> Mostly on this one I'll take a back seat and let those who
>>>> know this stuff better come to a consensus.
>>>>
>>>> Jonathan
>>>>
>>>>> Signed-off-by: Guenter Roeck <linux-0h96xk9xTtrk1uMJSBkQmQ@public.gmane.org>
>>>>> ---
>>>>>  .../devicetree/bindings/iio/iio-bindings.txt       |   97 ++++++++
>>>>>  drivers/iio/inkern.c                               |  241 ++++++++++++++++----
>>>>>  include/linux/iio/consumer.h                       |    8 +
>>>>>  3 files changed, 299 insertions(+), 47 deletions(-)
>>>>>  create mode 100644 Documentation/devicetree/bindings/iio/iio-bindings.txt
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/iio/iio-bindings.txt b/Documentation/devicetree/bindings/iio/iio-bindings.txt
>>>>> new file mode 100644
>>>>> index 0000000..0f51c95
>>>>> --- /dev/null
>>>>> +++ b/Documentation/devicetree/bindings/iio/iio-bindings.txt
>>>>> @@ -0,0 +1,97 @@
>>>>> +This binding is a work-in-progress, and are based on clock bindings and
>>>>> +suggestions from Lars-Peter Clausen [1].
>>>>> +
>>>>> +Sources of IIO channels can be represented by any node in the device
>>>>> +tree.  Those nodes are designated as IIO providers.  IIO consumer
>>>>> +nodes use a phandle and IIO specifier pair to connect IIO provider
>>>>> +outputs to IIO inputs.  Similar to the gpio specifiers, an IIO
>>>>> +specifier is an array of one more more cells identifying the IIO
>>>>> +output on a device.  The length of an IIO specifier is defined by the
>>>>> +value of a #io-channel-cells property in the clock provider node.
>>>>> +
>>>>> +[1] http://marc.info/?l=linux-iio&m=135902119507483&w=2
>>>>> +
>>>>> +==IIO providers==
>>>>> +
>>>>> +Required properties:
>>>>> +#io-channel-cells: Number of cells in an IIO specifier; Typically 0 for nodes
>>>>> +		   with a single IIO output and 1 for nodes with multiple
>>>>> +		   IIO outputs.
>>>>> +
>>>>> +Optional properties:
>>>>> +io-channel-output-names:
>>>>> +		    Recommended to be a list of strings of IIO output signal
>>>>> +		    names indexed by the first cell in the IIO specifier.
>>>>> +		    However, the meaning of io-channel-output-names is domain
>>>>> +		    specific to the IIO provider, and is only provided to
>>>>> +		    encourage using the same meaning for the majority of IIO
>>>>> +		    providers.  This format may not work for IIO providers
>>>>> +		    using a complex IIO specifier format.  In those cases it
>>>>> +		    is recommended to omit this property and create a binding
>>>>> +		    specific names property.
>>>>> +
>>>>> +		    IIO consumer nodes must never directly reference
>>>>> +		    the provider's io-channel-output-names property.
>>>>> +
>>>>> +For example:
>>>>> +
>>>>> +    adc: adc@35 {
>>>>> +	compatible = "maxim,max1139";
>>>>> +	reg = <0x35>;
>>>>> +        #io-channel-cells = <1>;
>>>>> +        io-channel-output-names = "adc1", "adc2";
>>>>> +    };
>>>>> +
>>>>> +- this node defines a device with two named IIO outputs, the first named
>>>>> +  "adc1" and the second named "adc2".  Consumer nodes always reference
>>>>> +  IIO channels by index. The names should reflect the IIO output signal
>>>>> +  names for the device.
>>>>> +
>>>>> +==IIO consumers==
>>>>> +
>>>>> +Required properties:
>>>>> +io-channels:	List of phandle and IIO specifier pairs, one pair
>>>>> +		for each IIO input to the device.  Note: if the
>>>>> +		IIO provider specifies '0' for #clock-cells, then
>>>>> +		only the phandle portion of the pair will appear.
>>>>> +
>>>>> +Optional properties:
>>>>> +io-channel-names:
>>>>> +		List of IIO input name strings sorted in the same
>>>>> +		order as the io-channels property.  Consumers drivers
>>>>> +		will use io-channel-names to match IIO input names
>>>>> +		with IIO specifiers.
>>>>> +io-channel-ranges:
>>>>> +		Empty property indicating that child nodes can inherit named
>>>>> +		IIO channels from this node. Useful for bus nodes to provide
>>>>> +		and IIO channel to their children.
>>>>> +
>>>>> +For example:
>>>>> +
>>>>> +    device {
>>>>> +        io-channels = <&adc 1>, <&ref 0>;
>>>>> +        io-channel-names = "vcc", "vdd";
>>>>> +    };
>>>>> +
>>>>> +This represents a device with two IIO inputs, named "vcc" and "vdd".
>>>>> +The vcc channel is connected to output 1 of the &adc device, and the
>>>>> +vdd channel is connected to output 0 of the &ref device.
>>>>> +
>>>>> +==Example==
>>>>> +
>>>>> +	adc: max1139@35 {
>>>>> +		compatible = "maxim,max1139";
>>>>> +		reg = <0x35>;
>>>>> +		#io-channel-cells = <1>;
>>>>> +	};
>>>>> +
>>>>> +	...
>>>>> +
>>>>> +	iio_hwmon {
>>>>> +		compatible = "iio-hwmon";
>>>>> +		io-channels = <&adc 0>, <&adc 1>, <&adc 2>,
>>>>> +			<&adc 3>, <&adc 4>, <&adc 5>,
>>>>> +			<&adc 6>, <&adc 7>, <&adc 8>,
>>>>> +			<&adc 9>, <&adc 10>, <&adc 11>;
>>>>> +		io-channel-names = "vcc", "vdd", "vref", "1.2V";
>>>> Having different numbers of channels and channel names seems
>>>> unusual... Deliberate or you got bored making up channel names?
>>>>
>>>> Why use indexed values for <&adc 0> etc rather than the output
>>>> channel names on adc?  For the iio_map stuff we initialy used
>>>> indexes but got a lot of responses that it was a silly idea and
>>>> naming was much more consistent and easy to follow.
>>>>
>>>> Is there a fundamental reason for it here?
>>>>
>>>> (note I don't mind either way as this seems more compact and cleaner
>>>> in some ways)
>>>>
>>>
>>> It follows the structure used by clocks, which uses the provided name(s) to
>>> calculate an index into io-channels. This way, the provider does not have to
>>> provide the mapping, the consumer does not have to know the io-channel index,
>>> and the consumer code can call something like
>>>
>>> 	channel = iio_get_channel(dev, "vcc");
>>>
>>> In the above example, "vcc" will map to "<&adc, 0>", and "vref" to "<&adc, 2>".
>>>
>>> This works for both platform data and OF data (though platform data will
>>> still need provider-based mapping, at least for now).
>>>
>>> This lets the code use a static name (eg "vcc"), and the mapping to the actual
>>> provider happens through devicetree. Since the name is only used locally and
>>> consumer driver specific, there is no need to define globally unique names.
>>>
>>> With this approach, the io channel map is not needed at all for the OF case.
>>> I had used it in this version of the patch set, but got rid of it now.
>>>
>>> Actually, provider based mapping doesn't even work. If the consumer is
>>> instantiated before the provider, the mapping doesn't exist yet, and the
>>> call to iio_channel_get_all will fail. There is no way to prevent this,
>>> as providers can come online at any time and there is no means to enforce that
>>> all providers are already active by the time the consumers are instantiated.
>>> Even if a mapping exists, there is no way to know if it is complete, if a
>>> consumer is mapped to multiple providers.
>>>
>>> With the consumer based mapping, iio_channel_get_all 'knows' that not all
>>> requested providers are available and can return -EPROBEDEFER in that case.
>> Thanks. That makes sense.  At the moment iio_hwmon is the only case that
>> does a 'get all'. Clearly things are easier when the driver is requesting a
>> specific set and we can do the back off much more easily.
>>
>>>
>>> As a side effect, we can also use the names - if provided - as channel
>>> labels in iio_hwmon.
>>>
>>> Note this will require the iio_get_channel API to change from taking the
>>> consumer device name to taking the consumer device pointer as argument.
>>> This will enable it to work for both OF and non-OF cases, should address Lars'
>>> concerns about duplicate API functions, and synchronize the code to match how
>>> the clock framework works.
>>
>> Agreed, doing this gives us a cleaner syntax as well.  Note there are other
>> users of that function in tree so be sure to get them all!
>>
>>>
>>> Thanks,
>>> Guenter
>> Thanks for the explanation.  What I was actually suggesting was something
>> like:
>>
>> adc: max1139@35 {
>> 		compatible = "maxim,max1139";
>> 		reg = <0x35>;
>> 		#io-channel-cells = <1>;
>> 		io-channel-output-names = "adc1", "adc2", "adc3"				
>> 	};
>>
>> iio_hwmon {
>> 	compatible = "iio-hwmon";
>> 	io-channels = <&adc "adc1">, <&adc "adc2">, <&adc "adc3">,
>> 	io-channel-names = "vcc", "vdd", "vref";
>> }
>>
>> Having taken a look at the available syntax, those <> pairs have
>> to be unsigned integers?  Hence the additional level of indirection?
> 
> Yea, I think mixing phandles and strings simply doesn't work, due how
> devicetree stores things.
> 

Another possibility would beto do things the way the regulator framework does
an have each channel as a subnode to the converter devices eg.


adc: max1139@35 {
 		compatible = "maxim,max1139";
 		reg = <0x35>;
		adc0: adc@0 {
			reg = <0>;
		};
		adc1: adc@1 {
			reg = <1>;
		};
		adc2: adc@2 {
			reg = <2>;
		};

 	};


iio_hwmon {
 	compatible = "iio-hwmon";
 	io-channels = <&adc0>, <&adc1>, <&adc2>;
 	io-channel-names = "vcc", "vdd", "vref";
};

But I'm not sure how much sense this makes for IIO.

- Lars

>>
>> (sorry, I'm getting you to give me a tutorial on device tree syntax rather
>> than the actual issue here!)
>>
>> I guess it was desirable to keep the syntax relatively simple but that occasionally
>> adds the requirement for a bit of indirection.
>>
>>
>>
>> Jonathan
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