linux-kernel - Re: [PATCH 1/3] pwm: cros-ec: Don't care about consumers in .get

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Date: Wed, 12 Jun 2024 06:27:14 +0000
From: Tzung-Bi Shih <tzungbi@...nel.org>
To: Uwe Kleine-König <u.kleine-koenig@...libre.com>
Cc: Benson Leung <bleung@...omium.org>, Guenter Roeck <groeck@...omium.org>,
	linux-pwm@...r.kernel.org, chrome-platform@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] pwm: cros-ec: Don't care about consumers in
 .get_state()

On Tue, Jun 11, 2024 at 12:39:44PM +0200, Uwe Kleine-König wrote:
> On Tue, Jun 11, 2024 at 08:50:44AM +0000, Tzung-Bi Shih wrote:
> > On Fri, Jun 07, 2024 at 10:44:15AM +0200, Uwe Kleine-König wrote:
> > > The get_state() callback is never called (in a visible way) after there
> > > is a consumer for a pwm device. The core handles loosing the information
> > > about duty_cycle just fine.
> > 
> > ChromeOS EC has no separated "enabled" state, it sees `duty == 0` as
> > "disabled"[1].  1db37f9561b2 ("pwm: cros-ec: Cache duty cycle value")
> > caches the value in kernel side so that it can retrieve the original duty
> > value even if (struct pwm_state *)->enabled is false.
> 
> There is no need to cache, so the following would work:

Ack.

> > To make sure I understand, did you mean the original duty value could be less
> > important because:
> > - We are less caring as it is in a debug context at [2]?
> > - At [3], the PWM device is still initializing.
> 
> It doesn't really matter that this is about debug or initialisation. The
> key here is that the core can handle the PWM using duty_cycle 0 (or
> anything else) when it was requested to be disabled.
> 
> 
> > [1]: https://crrev.com/0e16954460a08133b2557150e0897014ea2b9672/common/pwm.c#66
> > [2]: https://elixir.bootlin.com/linux/v6.10-rc3/source/drivers/pwm/core.c#L52
> > [3]: https://elixir.bootlin.com/linux/v6.10-rc3/source/drivers/pwm/core.c#L371

I was trying to understand the description in the commit message:
: The get_state() callback is never called (in a visible way) after there
: is a consumer for a pwm device.

I guess I understood; the core reads the duty value via get_state() when:
- Initializing the device for the intial value.
- Debugging for checking if apply() really takes effect.

What 1db37f9561b2 worried about is already addressed by the core[4].

[4]: https://elixir.bootlin.com/linux/v6.10-rc3/source/drivers/pwm/core.c#L495

Reviewed-by: Tzung-Bi Shih <tzungbi@...nel.org>