linux-kernel - Re: [PATCH v3 04/10] cpufreq: amd-pstate: add debug message while CPPC is supported and disabled by SBIOS

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Date: Tue, 11 Jun 2024 14:00:39 -0500
From: Mario Limonciello <mario.limonciello@....com>
To: Perry Yuan <perry.yuan@....com>, gautham.shenoy@....com
Cc: rafael.j.wysocki@...el.com, viresh.kumar@...aro.org, Ray.Huang@....com,
 Borislav.Petkov@....com, Alexander.Deucher@....com, Xinmei.Huang@....com,
 Xiaojian.Du@....com, Li.Meng@....com, linux-pm@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/10] cpufreq: amd-pstate: add debug message while
 CPPC is supported and disabled by SBIOS

On 6/11/2024 03:52, Perry Yuan wrote:
> If CPPC feature is supported by the CPU however the CPUID flag bit is not
> set by SBIOS, the `amd_pstate` will be failed to load while system
> booting.
> So adding one more debug message to inform user to check the SBIOS setting,
> The change also can help maintainers to debug why amd_pstate driver failed
> to be loaded at system booting if the processor support CPPC.
> 
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218686
> Signed-off-by: Perry Yuan <perry.yuan@....com>
> ---
>   drivers/cpufreq/amd-pstate.c | 25 +++++++++++++++++++++++++
>   1 file changed, 25 insertions(+)
> 
> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
> index f166b3b94091..6b9fc24001f2 100644
> --- a/drivers/cpufreq/amd-pstate.c
> +++ b/drivers/cpufreq/amd-pstate.c
> @@ -1746,11 +1746,36 @@ static int __init amd_pstate_set_driver(int mode_idx)
>    */
>   static bool amd_cppc_supported(void)
>   {
> +	struct cpuinfo_x86 *c = &cpu_data(0);
> +
>   	if ((boot_cpu_data.x86 == 0x17) && (boot_cpu_data.x86_model < 0x30)) {
>   		pr_debug_once("CPPC feature is not supported by the processor\n");
>   		return false;
>   	}
>   
> +	/*
> +	 * If the CPPC feature is disabled in the BIOS for processors that support MSR-based CPPC,
> +	 * the AMD Pstate driver may not function correctly.
> +	 * Check the CPPC flag and display a warning message if the platform supports CPPC.
> +	 * Notice: below checking code will not abort the driver registeration process.
> +	 */
> +	if (!cpu_feature_enabled(X86_FEATURE_CPPC)) {
> +		if (cpu_feature_enabled(X86_FEATURE_ZEN1) || cpu_feature_enabled(X86_FEATURE_ZEN2)) {
> +			if (c->x86_model > 0x60 && c->x86_model < 0xaf)
> +				goto warn;
> +		} else if (cpu_feature_enabled(X86_FEATURE_ZEN3) || cpu_feature_enabled(X86_FEATURE_ZEN4)) {
> +			if ((c->x86_model > 0x10 && c->x86_model < 0x1F) || (c->x86_model > 0x40 && c->x86_model < 0xaf))
> +				goto warn;
> +		} else if (cpu_feature_enabled(X86_FEATURE_ZEN5)) {
> +			goto warn;
> +		}
> +	}
> +
> +	return true;
> +
> +warn:
> +	pr_debug_once("The CPPC feature is supported but currently disabled by the BIOS.\n"
> +					"Please enable it if your BIOS has the CPPC option.\n");

This should be pr_warn_once() or pr_notice_once() IMO otherwise people 
won't really be able to know there is a problem.

>   	return true;

I don't really like the goto and warn label with two identical return 
statements.  Instead how about something like this?

    static bool amd_cppc_supported(void)
    {
	struct cpuinfo_x86 *c = &cpu_data(0);
	bool warn = false;

	.
	.
	.

	if (checks) {
		.
		.
		warn = TRUE;
	}

	if (warn)
		pr_warn_once();

	return TRUE;

That would be a better design pattern, especially if we end up having 
different classes of warnings to show to users in this function in the 
future.