linux-kernel - Re: [PATCH v7 3/8] pwm: pca9685: Improve runtime PM behavior

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Date:   Fri, 9 Apr 2021 15:03:20 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Clemens Gruber <clemens.gruber@...ruber.com>
Cc:     linux-pwm@...r.kernel.org, Sven Van Asbroeck <TheSven73@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 3/8] pwm: pca9685: Improve runtime PM behavior

On Tue, Apr 06, 2021 at 06:41:35PM +0200, Clemens Gruber wrote:
> The chip does not come out of POR in active state but in sleep state.
> To be sure (in case the bootloader woke it up) we force it to sleep in
> probe.
> 
> On kernels without CONFIG_PM, we wake the chip in .probe and put it to
> sleep in .remove.
> 
> Signed-off-by: Clemens Gruber <clemens.gruber@...ruber.com>
> ---
> Changes since v6:
> - Improved !CONFIG_PM handling (wake it up without putting it to sleep
>   first)
> 
>  drivers/pwm/pwm-pca9685.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> index d4474c5ff96f..0bcec04b138a 100644
> --- a/drivers/pwm/pwm-pca9685.c
> +++ b/drivers/pwm/pwm-pca9685.c
> @@ -474,13 +474,18 @@ static int pca9685_pwm_probe(struct i2c_client *client,
>  		return ret;
>  	}
>  
> -	/* The chip comes out of power-up in the active state */
> -	pm_runtime_set_active(&client->dev);
> -	/*
> -	 * Enable will put the chip into suspend, which is what we
> -	 * want as all outputs are disabled at this point
> -	 */
> -	pm_runtime_enable(&client->dev);
> +	if (IS_ENABLED(CONFIG_PM)) {

This looks odd to me. I've seen similar constructs, but they usually go
something like this (I think):

	pm_runtime_enable(&client->dev);

	if (!pm_runtime_enabled(&client->dev)) {
		/* resume device */
	}

Which I guess in your would be somewhat the opposite and it wouldn't
actually resume the device but rather put it to sleep.

Perhaps something like this:

	pm_runtime_enable(&client->dev);

	if (pm_runtime_enabled(&client->dev)) {
		pca9685_set_sleep_mode(pca, true);
		pm_runtime_set_suspended(&client->dev);
	} else {
		/* wake the chip up on non-PM environments */
		pca9685_set_sleep_mode(pca, false);
	}

? I think that's slightly more correct than your original because it
takes into account things like sysfs power control and such. It also
doesn't rely on the config option alone but instead uses the runtime
PM API to achieve this more transparently.

Thierry

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