Re: [PATCH RFC v4 05/15] spi: dt-bindings: add PWM SPI offload trigger

[David Lechner <dlechner@baylibre.com>]

On 10/26/24 10:18 AM, Jonathan Cameron wrote:
> On Wed, 23 Oct 2024 15:59:12 -0500
> David Lechner <dlechner@baylibre.com> wrote:
> 
>> Add a new binding for using a PWM signal as a trigger for SPI offloads.
> 
> I don't have a better suggestion for this, but it does smell rather like
> other bridge binding (iio-hwmon for example) where we have had push back on
> representing something that doesn't really exist but is just a way to
> tie two bits of hardware together. Those kind of exist because we snuck
> them in a long time back when no one was paying attention.
> 
> So this one may need more explanation and justification and I'd definitely
> like some DT maintainer review on this at a fairly early stage!
> (might have happened in earlier reviews but it has been a while so I've
> forgotten if it did)
> 
> Jonathan
> 
We could probably make it work like the leds version of this
binding where the trigger-sources property can have phandles
to anything, not just a dedicated class of device. It just
gets messy to implement because every subsystem needs to have
core code modified to be able to handle using a device or
one channel/gpio/etc. of a device as a trigger instead of
whatever it normally is.

> 
>>
>> Signed-off-by: David Lechner <dlechner@baylibre.com>
>> ---
>>
>> v4 changes: new patch in v4
>> ---
>>  .../devicetree/bindings/spi/trigger-pwm.yaml       | 39 ++++++++++++++++++++++
>>  1 file changed, 39 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/spi/trigger-pwm.yaml b/Documentation/devicetree/bindings/spi/trigger-pwm.yaml
>> new file mode 100644
>> index 000000000000..987638aa4732
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/spi/trigger-pwm.yaml
>> @@ -0,0 +1,39 @@
>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
>> +%YAML 1.2
>> +---
>> +$id: http://devicetree.org/schemas/spi/trigger-pwm.yaml#
>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>> +
>> +title: Generic SPI offload trigger using PWM
>> +
>> +description: Remaps a PWM channel as a trigger source.
>> +
>> +maintainers:
>> +  - David Lechner <dlechner@baylibre.com>
>> +
>> +$ref: /schemas/spi/trigger-source.yaml#
>> +
>> +properties:
>> +  compatible:
>> +    const: trigger-pwm
>> +
>> +  '#trigger-source-cells':
>> +    const: 0
>> +
>> +  pwms:
>> +    maxItems: 1
>> +
>> +required:
>> +  - compatible
>> +  - '#trigger-source-cells'
>> +  - pwms
>> +
>> +additionalProperties: false
>> +
>> +examples:
>> +  - |
>> +    trigger {
>> +        compatible = "trigger-pwm";
>> +        #trigger-source-cells = <0>;
>> +        pwms = <&pwm 0 1000000 0>;
>> +    };
>>
>