Add a new binding for using a PWM signal as a trigger for SPI offloads.

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>;
+    };

-- 
2.43.0

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


> 
> 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>;
> +    };
>

On Sat, Oct 26, 2024 at 04:18:37PM +0100, 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.

I dunno. iio-hwmon to me is a particularly strange one, because it is
the exact same device being used in different subsystems. Like that
voltage monitoring device with 10000 compatibles that I CCed you and
Peter on the other day feels like it should really in your subsytem. A
"hwmon" isn't a class of device at all.

This however, I think is more like pwm-clock (or clk-pwm, they both
exist and are opposites) where the node is used to change the type of
device rather than the subsystem using it.

> 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!

Ye, /shrug. Maybe the others have dissenting opinions. I'd like to hear
from them, but I don't personally have a problem with this.

On 10/31/24 1:16 PM, Conor Dooley wrote:
> On Sat, Oct 26, 2024 at 04:18:37PM +0100, 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.
> 
> I dunno. iio-hwmon to me is a particularly strange one, because it is
> the exact same device being used in different subsystems. Like that
> voltage monitoring device with 10000 compatibles that I CCed you and
> Peter on the other day feels like it should really in your subsytem. A
> "hwmon" isn't a class of device at all.
> 
> This however, I think is more like pwm-clock (or clk-pwm, they both
> exist and are opposites) where the node is used to change the type of
> device rather than the subsystem using it.

Yes, this is the key reason for the binding. When I was looking at
the trigger bindings in the leds subsystem, I came to the realization
that we need some way to get the underlying type of the trigger. In the
leds bindings, I don't think this was intentional, but effectively this
is done with the linux,default-trigger property.

So unless there is a reason why copying the clk-pwm/pwm-clock style
bindings is not a good idea, that seems the preferable way to do it
to me and I'll stick with that.

> 
>> 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!
> 
> Ye, /shrug. Maybe the others have dissenting opinions. I'd like to hear
> from them, but I don't personally have a problem with this.

On Sat, Oct 26, 2024 at 04:18:37PM +0100, Jonathan Cameron wrote:

> 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)

I intend looking at this, but it'll be Wednesday before I do.

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>;
>> +    };
>>
>