linux-kernel - Re: [PATCH] cpufreq/amd-pstate: Fix the clamping of perf values

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Message-ID: <6feda9d4-aa51-4903-8d6e-9444ad423fd2@amd.com>
Date: Mon, 24 Feb 2025 18:09:38 -0600
From: Mario Limonciello <mario.limonciello@....com>
To: Dhananjay Ugwekar <dhananjay.ugwekar@....com>, gautham.shenoy@....com
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Viresh Kumar <viresh.kumar@...aro.org>, rafael@...nel.org, perry.yuan@....com
Subject: Re: [PATCH] cpufreq/amd-pstate: Fix the clamping of perf values

On 2/21/2025 21:32, Dhananjay Ugwekar wrote:
> The clamping in freq_to_perf() is broken right now, as we first typecast
> (read wraparound) the overflowing value into a u8 and then clamp it down.
> So, use a u32 to store the >255 value in certain edge cases and then clamp
> it down into a u8.
> 
> Also, use a "explicit typecast + clamp" instead of just a "clamp_t" as the
> latter typecasts first and then clamps between the limits, which defeats
> our purpose.
> 
> Fixes: 305621eb6a8b ("cpufreq/amd-pstate: Modularize perf<->freq conversion")
> Signed-off-by: Dhananjay Ugwekar <dhananjay.ugwekar@....com>

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

I'll rebase my series on this for v5.

> ---
>   drivers/cpufreq/amd-pstate.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
> index 9ab95ec1f828..4705a644db6d 100644
> --- a/drivers/cpufreq/amd-pstate.c
> +++ b/drivers/cpufreq/amd-pstate.c
> @@ -144,10 +144,10 @@ static struct quirk_entry quirk_amd_7k62 = {
>   
>   static inline u8 freq_to_perf(struct amd_cpudata *cpudata, unsigned int freq_val)
>   {
> -	u8 perf_val = DIV_ROUND_UP_ULL((u64)freq_val * cpudata->nominal_perf,
> +	u32 perf_val = DIV_ROUND_UP_ULL((u64)freq_val * cpudata->nominal_perf,
>   					cpudata->nominal_freq);
>   
> -	return clamp_t(u8, perf_val, cpudata->lowest_perf, cpudata->highest_perf);
> +	return (u8)clamp(perf_val, cpudata->lowest_perf, cpudata->highest_perf);
>   }
>   
>   static inline u32 perf_to_freq(struct amd_cpudata *cpudata, u8 perf_val)