linux-kernel - Re: [PATCH v3] cpufreq/amd-pstate: Refactor max frequency calculation

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Message-ID: <746f627c-49cf-497d-bfc3-646d2668cd51@amd.com>
Date: Tue, 7 Jan 2025 13:02:19 -0600
From: Mario Limonciello <mario.limonciello@....com>
To: Naresh Solanki <naresh.solanki@...ements.com>
Cc: Huang Rui <ray.huang@....com>, "Gautham R. Shenoy"
 <gautham.shenoy@....com>, Perry Yuan <perry.yuan@....com>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>, linux-pm@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] cpufreq/amd-pstate: Refactor max frequency calculation

On 12/19/2024 14:18, Naresh Solanki wrote:
> The previous approach introduced roundoff errors during division when
> calculating the boost ratio. This, in turn, affected the maximum
> frequency calculation, often resulting in reporting lower frequency
> values.
> 
> For example, on the Glinda SoC based board with the following
> parameters:
> 
> max_perf = 208
> nominal_perf = 100
> nominal_freq = 2600 MHz
> 
> The Linux kernel previously calculated the frequency as:
> freq = ((max_perf * 1024 / nominal_perf) * nominal_freq) / 1024
> freq = 5405 MHz  // Integer arithmetic.
> 
> With the updated formula:
> freq = (max_perf * nominal_freq) / nominal_perf
> freq = 5408 MHz
> 
> This change ensures more accurate frequency calculations by eliminating
> unnecessary shifts and divisions, thereby improving precision.
> 
> Signed-off-by: Naresh Solanki <naresh.solanki@...ements.com>
> 
> Changes in V3:
> 1. Also update the same for lowest_nonlinear_freq
> 
> Changes in V2:
> 1. Rebase on superm1.git/linux-next branch
> ---

Reviewed-by: Mario Limonciello <mario.limonciello@....com>

BTW - Over the holiday I added this to my bleeding-edge branch [1] and 
have done testing with it.  I will be including it in my next 6.14 PR, 
thanks!

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/superm1/linux.git/commit/?h=bleeding-edge&id=857a61c2ce74e30fc3b10bc89d68ddd8d05b188c

>   drivers/cpufreq/amd-pstate.c | 13 ++++---------
>   1 file changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
> index d7b1de97727a..6f6f3220ffe4 100644
> --- a/drivers/cpufreq/amd-pstate.c
> +++ b/drivers/cpufreq/amd-pstate.c
> @@ -908,9 +908,8 @@ static int amd_pstate_init_freq(struct amd_cpudata *cpudata)
>   {
>   	int ret;
>   	u32 min_freq, max_freq;
> -	u32 nominal_perf, nominal_freq;
> +	u32 highest_perf, nominal_perf, nominal_freq;
>   	u32 lowest_nonlinear_perf, lowest_nonlinear_freq;
> -	u32 boost_ratio, lowest_nonlinear_ratio;
>   	struct cppc_perf_caps cppc_perf;
>   
>   	ret = cppc_get_perf_caps(cpudata->cpu, &cppc_perf);
> @@ -927,16 +926,12 @@ static int amd_pstate_init_freq(struct amd_cpudata *cpudata)
>   	else
>   		nominal_freq = cppc_perf.nominal_freq;
>   
> +	highest_perf = READ_ONCE(cpudata->highest_perf);
>   	nominal_perf = READ_ONCE(cpudata->nominal_perf);
> -
> -	boost_ratio = div_u64(cpudata->highest_perf << SCHED_CAPACITY_SHIFT, nominal_perf);
> -	max_freq = (nominal_freq * boost_ratio >> SCHED_CAPACITY_SHIFT);
> +	max_freq = div_u64((u64)highest_perf * nominal_freq, nominal_perf);
>   
>   	lowest_nonlinear_perf = READ_ONCE(cpudata->lowest_nonlinear_perf);
> -	lowest_nonlinear_ratio = div_u64(lowest_nonlinear_perf << SCHED_CAPACITY_SHIFT,
> -					 nominal_perf);
> -	lowest_nonlinear_freq = (nominal_freq * lowest_nonlinear_ratio >> SCHED_CAPACITY_SHIFT);
> -
> +	lowest_nonlinear_freq = div_u64((u64)nominal_freq * lowest_nonlinear_perf, nominal_perf);
>   	WRITE_ONCE(cpudata->min_freq, min_freq * 1000);
>   	WRITE_ONCE(cpudata->lowest_nonlinear_freq, lowest_nonlinear_freq * 1000);
>   	WRITE_ONCE(cpudata->nominal_freq, nominal_freq * 1000);