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

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Message-ID: <20241203200046.0dfb784a@jic23-huawei>
Date: Tue, 3 Dec 2024 20:00:46 +0000
From: Jonathan Cameron <jic23@...nel.org>
To: Claudiu <claudiu.beznea@...on.dev>
Cc: prabhakar.mahadev-lad.rj@...renesas.com, lars@...afoo.de,
 robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org,
 geert+renesas@...der.be, magnus.damm@...il.com, mturquette@...libre.com,
 sboyd@...nel.org, p.zabel@...gutronix.de, linux-iio@...r.kernel.org,
 linux-renesas-soc@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org, Claudiu Beznea
 <claudiu.beznea.uj@...renesas.com>
Subject: Re: [PATCH 07/14] iio: adc: rzg2l_adc: Enable runtime PM
 autosuspend support

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

> From: Claudiu Beznea <claudiu.beznea.uj@...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@...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;