linux-kernel - Re: [PATCH] [11/20] x86: Use the correct cpuid method to detect MWAIT support for C states

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Date:	Thu, 3 Jan 2008 11:45:26 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <ak@...e.de>
Cc:	andreas.herrmann3@....com, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] [11/20] x86: Use the correct cpuid method to detect
	MWAIT support for C states


* Andi Kleen <ak@...e.de> wrote:

> +static int mwait_usable(const struct cpuinfo_x86 *c)
> +{
> +	if (force_mwait)
> +		return 1;
> +	/* Any C1 states supported? */
> +	return c->cpuid_level >= 5 && ((cpuid_edx(5) >> 4) & 0xf) > 0;
> +}
> +
>  void __cpuinit select_idle_routine(const struct cpuinfo_x86 *c)
>  {
> -	if (cpu_has(c, X86_FEATURE_MWAIT)) {
> +	if (cpu_has(c, X86_FEATURE_MWAIT) && mwait_usable(c)) {
>  		printk("monitor/mwait feature present.\n");

hm, why not clear FEATURE_MWAIT if it's "not usable"? That's the 
standard approach we do for CPU features that do not work.

	Ingo
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