Re: [PATCH 1/2] thermal: power allocator: change the 'k_i' coefficient estimation

From: Daniel Lezcano <daniel.lezcano@linaro.org>
To: Lukasz Luba <lukasz.luba@arm.com>,
	linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org
Cc: amitk@kernel.org, Dietmar.Eggemann@arm.com
Subject: Re: [PATCH 1/2] thermal: power allocator: change the 'k_i' coefficient estimation
Date: Tue, 13 Oct 2020 12:21:09 +0200	[thread overview]
Message-ID: <cc0e6d85-28ad-3cfc-e5b8-75820552b716@linaro.org> (raw)
In-Reply-To: <20201002122416.13659-2-lukasz.luba@arm.com>

Hi Lukasz,

On 02/10/2020 14:24, Lukasz Luba wrote:
> Intelligent Power Allocation (IPA) is built around the PID controller
> concept. The initialization code tries to setup the environment based on
> the information available in DT or estimate the value based on minimum
> power reported by each of the cooling device. The estimation will have an
> impact on the PID controller behaviour via the related 'k_po', 'k_pu',
> 'k_i' coefficients and also on the power budget calculation.
> 
> This change prevents the situation when 'k_i' is relatively big compared
> to 'k_po' and 'k_pu' values. This might happen when the estimation for
> 'sustainable_power' returned small value, thus 'k_po' and 'k_pu' are
> small.
> 
> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
> ---
>  drivers/thermal/gov_power_allocator.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/thermal/gov_power_allocator.c b/drivers/thermal/gov_power_allocator.c
> index 5cb518d8f156..f69fafe486a5 100644
> --- a/drivers/thermal/gov_power_allocator.c
> +++ b/drivers/thermal/gov_power_allocator.c
> @@ -131,6 +131,7 @@ static void estimate_pid_constants(struct thermal_zone_device *tz,
>  	int ret;
>  	int switch_on_temp;
>  	u32 temperature_threshold;
> +	s32 k_i;
>  
>  	ret = tz->ops->get_trip_temp(tz, trip_switch_on, &switch_on_temp);
>  	if (ret)
> @@ -156,8 +157,11 @@ static void estimate_pid_constants(struct thermal_zone_device *tz,
>  		tz->tzp->k_pu = int_to_frac(2 * sustainable_power) /
>  			temperature_threshold;
>  
> -	if (!tz->tzp->k_i || force)
> -		tz->tzp->k_i = int_to_frac(10) / 1000;
> +	if (!tz->tzp->k_i || force) {
> +		k_i = tz->tzp->k_pu / 10;
> +		tz->tzp->k_i = k_i > 0 ? k_i : 1;
> +	}

I do not understand the rational behind this change.

Do you have some values to share describing what would be the impact of
this change?

Depending on the thermal behavior of a board, these coefficients could
be very different, no ?

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