linux-kernel - Re: [PATCH 2/2 V8] intel_pstate: add kernel parameter to force loading.

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Date:	Fri, 05 Dec 2014 11:09:29 -0500
From:	Linda Knippers <linda.knippers@...com>
To:	Ethan Zhao <ethan.zhao@...cle.com>, dirk.j.brandewie@...el.com,
	kristen@...ux.intel.com, viresh.kumar@...aro.org,
	rjw@...ysocki.net, corbet@....net
CC:	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, ethan.kernel@...il.com,
	alexey.kodanev@...cle.com
Subject: Re: [PATCH 2/2 V8] intel_pstate: add kernel parameter to force loading.

On 12/5/2014 4:40 AM, Ethan Zhao wrote:
> To force loading on Oracle Sun X86 servers, provide one kernel command line
> parameter
> 
>   intel_pstate = force
> 
> For those who be aware of the risk of no power capping capabily working and
> try to get better performance with this driver.
> 
> Signed-off-by: Ethan Zhao <ethan.zhao@...cle.com>
> Tested-by: Alexey Kodanev <alexey.kodanev@...cle.com>
> ---
>  v2: change to hardware vendor specific naming parameter.
>  v4: refine code and doc.
>  v5&v6: fix a typo in doc.
>  v7: change enum PCC to PPC.
>  v8: change the name of kernel command line parameter to generic one.
> 
>  Documentation/kernel-parameters.txt | 5 +++++
>  drivers/cpufreq/intel_pstate.c      | 6 +++++-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 479f332..7d0983e 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -1446,6 +1446,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  		       disable
>  		         Do not enable intel_pstate as the default
>  		         scaling driver for the supported processors
> +		       force
> +			 Enable intel_pstate on systems where it may cause problems to
> +			 happen due to conflicts with platform firmware attempting to
> +			 drive P-states by itself in certain situations (for thermal 
> +			 control or power capping in general or other purposes).

I suggest something like:
			Enable intel_pstate on systems that prohibit it by
			default in favor of acpi-cpufreq.  Forcing the
			intel_pstate driver instead of acpi-cpufreq may disable
			platform features, such as thermal controls and power
			capping, that rely on ACPI p-state information being
			used by the OS and therefore should be used with care.
			This option does not work with processors that aren't
			supported by the intel_pstate driver or on platforms
			that use pcc-cpufreq instead of acpi-cpufreq.

Maybe this is too specific but I believe it is accurate.  Comments?

-- ljk

>  
>  	intremap=	[X86-64, Intel-IOMMU]
>  			on	enable Interrupt Remapping (default)
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index 1bb62ca..2654e13 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -866,6 +866,7 @@ static struct cpufreq_driver intel_pstate_driver = {
>  };
>  
>  static int __initdata no_load;
> +static unsigned int  force_load;
>  
>  static int intel_pstate_msrs_not_valid(void)
>  {
> @@ -1003,7 +1004,8 @@ static bool intel_pstate_platform_pwr_mgmt_exists(void)
>  			case PSS:
>  				return intel_pstate_no_acpi_pss();
>  			case PPC:
> -				return intel_pstate_has_acpi_ppc();
> +				return intel_pstate_has_acpi_ppc() &&
> +					(!force_load);
>  			}
>  	}
>  
> @@ -1078,6 +1080,8 @@ static int __init intel_pstate_setup(char *str)
>  
>  	if (!strcmp(str, "disable"))
>  		no_load = 1;
> +	if (!strcmp(str, "force"))
> +		force_load = 1;
>  	return 0;
>  }
>  early_param("intel_pstate", intel_pstate_setup);
> 

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