Re: [PATCH 07/14] iio: adc: rzg2l_adc: Enable runtime PM autosuspend support

[Jonathan Cameron <jic23@kernel.org>]

On Tue,  3 Dec 2024 13:13:07 +0200
Claudiu <claudiu.beznea@tuxon.dev> wrote:

> From: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
> 
> Enable runtime PM autosuspend support for the rzg2l_adc driver. With this
> change, consecutive conversion requests will no longer cause the device to
> be runtime-enabled/disabled after each request. Instead, the device will
> transition based on the delay configured by the user.
> 
> This approach reduces the frequency of hardware register access during
> runtime PM suspend/resume cycles, thereby saving CPU cycles. The default
> autosuspend delay is set to zero to maintain the previous driver behavior.

Unless you have a weird user who is polling slow enough to not trigger
autosuspend with a non zero period, but is still saving power I'm not convinced
anyone will notice if you just enable this for a sensible autosuspend delay.
There will of course be a small increase in power usage for each read but
hopefully that is trivial.

So I'd not go with a default of 0, though what value makes sense depends
on the likely usecase + how much power is saved by going to sleep.

If you really want to keep 0 I don't mind that much, just seems odd!

Jonathan

> 
> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
> ---
>  drivers/iio/adc/rzg2l_adc.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iio/adc/rzg2l_adc.c b/drivers/iio/adc/rzg2l_adc.c
> index eed2944bd98d..fda8b42ded81 100644
> --- a/drivers/iio/adc/rzg2l_adc.c
> +++ b/drivers/iio/adc/rzg2l_adc.c
> @@ -207,7 +207,8 @@ static int rzg2l_adc_conversion(struct iio_dev *indio_dev, struct rzg2l_adc *adc
>  	rzg2l_adc_start_stop(adc, false);
>  
>  rpm_put:
> -	pm_runtime_put_sync(dev);
> +	pm_runtime_mark_last_busy(dev);
> +	pm_runtime_put_autosuspend(dev);
>  	return ret;
>  }
>  
> @@ -372,7 +373,8 @@ static int rzg2l_adc_hw_init(struct device *dev, struct rzg2l_adc *adc)
>  	rzg2l_adc_writel(adc, RZG2L_ADM(3), reg);
>  
>  exit_hw_init:
> -	pm_runtime_put_sync(dev);
> +	pm_runtime_mark_last_busy(dev);
> +	pm_runtime_put_autosuspend(dev);
>  	return ret;
>  }
>  
> @@ -412,6 +414,9 @@ static int rzg2l_adc_probe(struct platform_device *pdev)
>  		return PTR_ERR(adc->presetn);
>  	}
>  
> +	/* Default 0 for power saving. Can be overridden via sysfs. */
> +	pm_runtime_set_autosuspend_delay(dev, 0);
> +	pm_runtime_use_autosuspend(dev);
>  	ret = devm_pm_runtime_enable(dev);
>  	if (ret)
>  		return ret;