linux-kernel - Re: [PATCH v3 1/6] dt-bindings: iio: introduce trigger providers, consumers

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Message-ID: <948e5b96-7a8f-406d-6d03-d129d1b2ce54@kernel.org>
Date:   Sun, 5 Mar 2017 11:43:02 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Rob Herring <robh@...nel.org>,
        Fabrice Gasnier <fabrice.gasnier@...com>
Cc:     linux@...linux.org.uk, linux-arm-kernel@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-iio@...r.kernel.org, mark.rutland@....com,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com,
        lars@...afoo.de, knaack.h@....de, pmeerw@...erw.net,
        benjamin.gaignard@...aro.org, benjamin.gaignard@...com,
        linus.walleij@...aro.org
Subject: Re: [PATCH v3 1/6] dt-bindings: iio: introduce trigger providers,
 consumers

On 03/03/17 06:21, Rob Herring wrote:
> On Tue, Feb 28, 2017 at 05:51:14PM +0100, Fabrice Gasnier wrote:
>> Document iio provider and consumer bindings.
>>
>> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@...com>
>> ---
>>  .../devicetree/bindings/iio/iio-bindings.txt       | 38 ++++++++++++++++++++++
>>  1 file changed, 38 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/iio/iio-bindings.txt b/Documentation/devicetree/bindings/iio/iio-bindings.txt
>> index 68d6f8c..01765e9 100644
>> --- a/Documentation/devicetree/bindings/iio/iio-bindings.txt
>> +++ b/Documentation/devicetree/bindings/iio/iio-bindings.txt
>> @@ -95,3 +95,41 @@ vdd channel is connected to output 0 of the &ref device.
>>  		io-channels = <&adc 10>, <&adc 11>;
>>  		io-channel-names = "adc1", "adc2";
>>  	};
>> +
>> +==IIO trigger providers==
>> +Sources of IIO triggers can be represented by any node in the device
>> +tree. Those nodes are designated as IIO trigger providers. IIO trigger
>> +consumer uses a phandle and an IIO trigger specifier to connect to an
>> +IIO trigger provider.
>> +An IIO trigger specifier is an array of one or more cells identifying
>> +the IIO trigger output on a device. The length of an IIO trigger
>> +specifier is defined by the value of a #io-trigger-cells property in
>> +the IIO trigger provider node.
>> +
>> +Required properties:
>> +#io-trigger-cells:
>> +		Number of cells in an IIO trigger specifier; Typically
>> +		0 for nodes with a simple IIO trigger output.
>> +
>> +Example:
>> +	trig0: interrupt-trigger0 {
>> +		#io-trigger-cells = <0>;
>> +		compatible = "interrupt-trigger";
>> +		interrupts = <11 0>;
>> +		interrupt-parent = <&gpioa>;
>> +	}
>> +
>> +==IIO trigger consumers==
>> +Required properties:
>> +- io-triggers:	List of phandle representing the IIO trigger specifier.
>> +
>> +Optional properties:
>> +- io-trigger-names :
>> +		List of IIO trigger name strings that matches elements
>> +		in 'io-triggers' list property.
>> +
>> +Example:
>> +	some_trigger_consumer {
>> +		io-triggers = <&trig0>;
>> +		io-trigger-names = "mytrig";
>> +	}
> 
> I have some reservations about this. We could just as easily add the 
> interrupt directly to the consumer node and use "trigger" for a standard 
> interrupt name. So the question is whether this extra level of 
> indirection is needed? 

First thing to note here, is that Fabrice's use of the generic interrupt
trigger is an extremely 'unusual' one! Normal use case is that we have
a random gpio pin providing interrupts to driver triggering on random
devices - there need be no association between the two whatsoever.
So what we are doing here is 'allowing' an interrupt to provide a trigger.
It's not necessarily the one going to be used by any particular device
driver.  The decision of which trigger to use is definitely one for
userspace, not something that should be configured in to the device tree.

For this particular case you could in theory just do it by using an interrupt
as you describe.  Ultimately though we should be able to play more complex
games with this device and having it able to handle any trigger - which 
includes those not using the direct hardware route.  It'll be up to the
driver to figure out when it can use the fast method and when it can't.

Conversely, even when we are using this hardware route to drive the
triggering it should be possible to hang off a device to be triggered
by the interrupt via the kernel rather than directly. 

So from a device tree point of view we are just describing the fact that
there is a pin, which may be used to trigger something.  What that something
is, is a question for userspace not the device tree.

Jonathan
> 
> Rob
> --
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