linux-kernel - Re: [PATCH V2 RFC] acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter

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Date: Tue, 18 Jun 2024 16:24:22 -0500
From: Mario Limonciello <mario.limonciello@....com>
To: Aaron Rainbolt <arainbolt@...cus.org>, linux-acpi@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: rafael@...nel.org, lenb@...nel.org, mmikowski@...cus.org,
 Perry.Yuan@....com
Subject: Re: [PATCH V2 RFC] acpi: Allow ignoring _OSC CPPC v2 bit via kernel
 parameter

On 6/18/2024 15:25, Aaron Rainbolt wrote:
> acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter
> 
> The _OSC is supposed to contain a bit indicating whether the hardware
> supports CPPC v2 or not. This bit is not always set, causing CPPC v2 to
> be considered absent. This results in severe single-core performance
> issues with the EEVDF scheduler on heterogenous-core Intel processors.
> 
> To work around this, provide a new kernel parameter, "ignore_osc_cppc_bit",
> which may be used to ignore the _OSC CPPC v2 bit and act as if the bit was
> enabled. This allows CPPC to be properly detected even if not "enabled" by
> _OSC, allowing users with problematic hardware to obtain decent single-core
> performance.
> 
> Signed-off-by: Aaron Rainbolt <arainbolt@...cus.org>
> 
> ---
> 
> V1 -> V2: Rewrite to work in cpc_supported_by_cpu.
> 
> RFC: I have not yet tested this patch to ensure it functions properly,
>   nor have I attempted to compile it against mainline. My system takes
>   a couple of hours or so to build a kernel, and I'd like to submit this
>   for feedback now and test once it's sent.

Thanks, this matches what I suggested, hopefully it works when you test it.

One comment below though.

> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b600df82669d..af2d8973ba3a 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2063,6 +2063,12 @@
>   			could change it dynamically, usually by
>   			/sys/module/printk/parameters/ignore_loglevel.
>   
> +	ignore_osc_cppc_bit
> +			Assume CPPC is present and ignore the CPPC v2 bit from
> +			the ACPI _OSC method. This is useful for working
> +			around buggy firmware where CPPC is supported, but
> +			_OSC incorrectly reports it as being absent.
> +
>   	ignore_rlimit_data
>   			Ignore RLIMIT_DATA setting for data mappings,
>   			print warning at first misuse.  Can be changed via
> diff --git a/arch/x86/kernel/acpi/cppc.c b/arch/x86/kernel/acpi/cppc.c
> index ff8f25faca3d..7346a25e68ce 100644
> --- a/arch/x86/kernel/acpi/cppc.c
> +++ b/arch/x86/kernel/acpi/cppc.c
> @@ -11,6 +11,14 @@
>   
>   /* Refer to drivers/acpi/cppc_acpi.c for the description of functions */
>   
> +static bool ignore_osc_cppc_bit;
> +static int __init parse_ignore_osc_cppc_bit(char *arg)
> +{
> +	ignore_osc_cppc_bit = true;
> +	return 0;
> +}
> +early_param("ignore_osc_cppc_bit", parse_ignore_osc_cppc_bit);
> +
>   bool cpc_supported_by_cpu(void)
>   {
>   	switch (boot_cpu_data.x86_vendor) {
> @@ -24,6 +32,10 @@ bool cpc_supported_by_cpu(void)
>   			return true;
>   		return boot_cpu_has(X86_FEATURE_CPPC);
>   	}
> +
> +	if (ignore_osc_cppc_bit) {
> +		return true;
> +	}

I think you should move this check before the switch statement.
The reason is that such a workaround could then apply to any CPU
vendors and models that are AMD or Hygon too.

>   	return false;
>   }
>