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Date: Thu, 17 Jan 2008 21:03:17 -0500
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Kyle McMartin <kyle@mcmartin.ca>
CC: akpm@linux-foundation.org, airlied@linux.ie, linux-kernel@vger.kernel.org
Subject: Re: [mm patch] i915: fix invalid opcode exception on cpus without
 clflush
References: <20080117235203.GB21681@phobos.i.cabal.ca>
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Kyle McMartin wrote:
> i915_flush_ttm was unconditionally executing a clflush instruction
> to (obviously) flush the cache. Instead, check if the cpu supports
> clflush, and if not, fall back to calling wbinvd to flush the entire
> cache.
> 
> Signed-off-by: Kyle McMartin <kmcmartin@redhat.com>
> 
> --- a/drivers/char/drm/i915_buffer.c
> +++ b/drivers/char/drm/i915_buffer.c
> @@ -286,7 +286,18 @@ void i915_flush_ttm(struct drm_ttm *ttm)
>  		return;
>  
>  	DRM_MEMORYBARRIER();
> +
> +#ifdef CONFIG_X86_32
> +	/* Hopefully nobody has built an x86-64 processor without clflush */
> +	if (!cpu_has_clflush) {
> +		wbinvd();
> +		DRM_MEMORYBARRIER();
> +		return;
> +	}
> +#endif
> +
>  	for (i = ttm->num_pages - 1; i >= 0; i--)
>  		drm_cache_flush_page(drm_ttm_get_page(ttm, i));
> +
>  	DRM_MEMORYBARRIER();
>  }

The #ifdef is bogus.  If it's required, it should go into 
asm-x86/required_features.h and then cpu_has_clflush is static; 
otherwise it's just plain wrong.

	-hpa