linux-kernel - Re: [PATCH 3/4] doc: dt: pwm: add binding for ChromeOS EC PWM

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Date:	Sat, 28 May 2016 22:00:45 -0700
From:	Gwendal Grignou <gwendal@...omium.org>
To:	Brian Norris <briannorris@...omium.org>
Cc:	Lee Jones <lee.jones@...aro.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Olof Johansson <olof@...om.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Doug Anderson <dianders@...omium.org>,
	Brian Norris <computersforpeace@...il.com>,
	linux-pwm@...r.kernel.org, devicetree@...r.kernel.org,
	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Stephen Barber <smbarber@...omium.org>,
	Sameer Nanda <snanda@...omium.org>,
	Javier Martinez Canillas <javier@....samsung.com>,
	Benson Leung <bleung@...omium.org>,
	Enric Balletbo <enric.balletbo@...labora.co.uk>,
	Randall Spangler <rspangler@...omium.org>,
	Shawn Nematbakhsh <shawnn@...omium.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Todd Broch <tbroch@...omium.org>,
	Gwendal Grignou <gwendal@...omium.org>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>
Subject: Re: [PATCH 3/4] doc: dt: pwm: add binding for ChromeOS EC PWM

Instead of using device tree, assuming you have firmware control,
another way could be to add a firmware feature:
for instance, there is one EC_FEATURE_PWM_FAN, the fan PWM, one for
the keyboard lightning as well. (see num ec_feature_code)
By adding one more, you let cros_ec_dev load the platform driver for
you, it works even if the machine does not use device tree.

Gwendal.

On Fri, May 27, 2016 at 6:39 PM, Brian Norris <briannorris@...omium.org> wrote:
> The ChromeOS Embedded Controller can support controlling its attached
> PWMs via its host-command interface. The number of supported PWMs varies
> on a per-board basis, so we define a "google,max-pwms" property to
> handle this. And because the EC only allows specifying the duty cycle
> and not the period, we don't specify the period via pwm-cells, and
> instead have only support 1 cell -- to specify the index.
>
> Signed-off-by: Brian Norris <briannorris@...omium.org>
> ---
>  .../devicetree/bindings/pwm/google,cros-ec-pwm.txt | 25 ++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/pwm/google,cros-ec-pwm.txt
>
> diff --git a/Documentation/devicetree/bindings/pwm/google,cros-ec-pwm.txt b/Documentation/devicetree/bindings/pwm/google,cros-ec-pwm.txt
> new file mode 100644
> index 000000000000..f1c9540fc23f
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/pwm/google,cros-ec-pwm.txt
> @@ -0,0 +1,25 @@
> +* PWM controlled by ChromeOS EC
> +
> +Google's ChromeOS EC PWM is a simple PWM attached to the Embedded Controller
> +(EC) and controlled via a host-command interface.
> +
> +An EC PWM node should be only found as a sub-node of the EC node (see
> +Documentation/devicetree/bindings/mfd/cros-ec.txt).
> +
> +Required properties:
> +- compatible: Must contain "google,cros-ec-pwm"
> +- #pwm-cells: Should be 1. The cell specifies the PWM index.
> +- google,max-pwms: Specifies the number of PWMs supported by the EC.
> +
> +Example:
> +       cros-ec@0 {
> +               compatible = "google,cros-ec-spi";
> +
> +               ...
> +
> +               cros_ec_pwm: ec-pwm {
> +                       compatible = "google,cros-ec-pwm";
> +                       #pwm-cells = <1>;
> +                       google,max-pwms = <4>;
> +               };
> +       };
> --
> 2.8.0.rc3.226.g39d4020
>