linux-kernel - Re: [PATCH 2/5] thermal: devfreq_cooling: get a copy of device status

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Message-ID: <20201007161120.GC15063@arm.com>
Date:   Wed, 7 Oct 2020 17:11:20 +0100
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, amit.kucheria@...durent.com,
        airlied@...ux.ie, daniel.lezcano@...aro.org, steven.price@....com,
        alyssa.rosenzweig@...labora.com, rui.zhang@...el.com,
        orjan.eide@....com
Subject: Re: [PATCH 2/5] thermal: devfreq_cooling: get a copy of device status

On Monday 21 Sep 2020 at 13:20:04 (+0100), Lukasz Luba wrote:
> Devfreq cooling needs to now the correct status of the device in order
> to operate. Do not rely on Devfreq last_status which might be a stale data
> and get more up-to-date values of the load.
> 
> Devfreq framework can change the device status in the background. To
> mitigate this situation make a copy of the status structure and use it
> for internal calculations.
> 
> In addition this patch adds normalization function, which also makes sure
> that whatever data comes from the device, it is in a sane range.
> 
> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> ---
>  drivers/thermal/devfreq_cooling.c | 52 +++++++++++++++++++++++++------
>  1 file changed, 43 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/thermal/devfreq_cooling.c b/drivers/thermal/devfreq_cooling.c
> index 7063ccb7b86d..cf045bd4d16b 100644
> --- a/drivers/thermal/devfreq_cooling.c
> +++ b/drivers/thermal/devfreq_cooling.c
> @@ -227,6 +227,24 @@ static inline unsigned long get_total_power(struct devfreq_cooling_device *dfc,
>  							       voltage);
>  }
>  
> +static void _normalize_load(struct devfreq_dev_status *status)

Is there a reason for the leading "_" ?
AFAIK, "__name()" is meant to suggest a "worker" function for another
"name()" function, but that would not apply here.

> +{
> +	/* Make some space if needed */
> +	if (status->busy_time > 0xffff) {
> +		status->busy_time >>= 10;
> +		status->total_time >>= 10;
> +	}

How about removing the above code and adding here:

status->busy_time = status->busy_time ? : 1;

> +
> +	if (status->busy_time > status->total_time)

This check would then cover the possibility that total_time is 0.

> +		status->busy_time = status->total_time;

But a reversal is needed here:
		status->total_time = status->busy_time;

> +
> +	status->busy_time *= 100;
> +	status->busy_time /= status->total_time ? : 1;
> +
> +	/* Avoid division by 0 */
> +	status->busy_time = status->busy_time ? : 1;
> +	status->total_time = 100;

Then all of this code can be replaced by:

status->busy_time = (unsigned long)div64_u64((u64)status->busy_time << 10,
					     status->total_time);
status->total_time = 1 << 10;

This way you gain some resolution to busy_time and the divisions in the
callers would just become shifts by 10.

Hope it helps,
Ionela.