linux-kernel - Re: [PATCH v2 2/4] x86, mwaitt: make delay method configurable

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Message-ID: <20150609090543.GA22105@pd.tnic>
Date:	Tue, 9 Jun 2015 11:05:43 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Huang Rui <ray.huang@....com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	Fengguang Wu <fengguang.wu@...el.com>,
	Aaron Lu <aaron.lu@...el.com>,
	Suravee Suthikulanit <suravee.suthikulpanit@....com>,
	Tony Li <tony.li@....com>, Ken Xue <ken.xue@....com>
Subject: Re: [PATCH v2 2/4] x86, mwaitt: make delay method configurable

On Tue, Jun 09, 2015 at 11:13:39AM +0800, Huang Rui wrote:
> This patch introduces a kernel parameter (delay), user is able to configure
> it at boot loader to choose different types of delay method.
> 
> Default schema is to use TSC delay. This update can be more flexiable to
> add new delay method.
> 
> Suggested-by: Suravee Suthikulanit <suravee.suthikulpanit@....com>
> Signed-off-by: Huang Rui <ray.huang@....com>
> ---
>  arch/x86/include/asm/delay.h |    7 +++++++
>  arch/x86/kernel/setup.c      |   19 +++++++++++++++++++
>  arch/x86/lib/delay.c         |   12 +++++++++++-
>  3 files changed, 37 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/delay.h b/arch/x86/include/asm/delay.h
> index 9b3b4f2..99873ec 100644
> --- a/arch/x86/include/asm/delay.h
> +++ b/arch/x86/include/asm/delay.h
> @@ -5,4 +5,11 @@
>  
>  void use_tsc_delay(void);
>  
> +extern unsigned long boot_option_delay;
> +
> +enum delay_type {
> +	DELAY_LOOP=0,
> +	DELAY_TSC,
> +};
> +
>  #endif /* _ASM_X86_DELAY_H */
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 0b10698..cc2886d 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -111,6 +111,7 @@
>  #include <asm/mce.h>
>  #include <asm/alternative.h>
>  #include <asm/prom.h>
> +#include <asm/delay.h>
>  
>  /*
>   * max_low_pfn_mapped: highest direct mapped pfn under 4GB
> @@ -844,6 +845,24 @@ dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
>  	return 0;
>  }
>  
> +static int __init delay_setup(char *str)
> +{
> +	if (!str)
> +		return -EINVAL;
> +
> +	if (!strcmp(str, "tsc")) {
> +		pr_info("using tsc delay\n");
> +		boot_option_delay = DELAY_TSC;
> +	} else if (!strcmp(str, "loop")) {
> +		pr_info("using loop delay\n");
> +		boot_option_delay = DELAY_LOOP;
> +	} else
> +		return -1;

What Peter said.

And we did talk about this already in the last review. You guys want to
drop all those kernel parameters and use MWAITX delay by default when:

1. HW supports MWAITX

*and*

2. delay fits in u32.

Kernel parameters is a bad bad idea.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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