[PATCH] drm/radeon: Don't try to enable write-combining without PAT

[Christian König]

Am 04.02.2015 um 02:19 schrieb Michel Dänzer:
> From: Michel Dänzer <michel.daenzer at amd.com>
>
> Doing so can cause things to become slow.
>
> Print a warning at compile time and an informative message at runtime in
> that case.
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88758
> Cc: stable at vger.kernel.org
> Signed-off-by: Michel Dänzer <michel.daenzer at amd.com>

Interesting I wonder what the rational behind this is. I mean 
CONFIG_X86_PAT will obviously affect write combining, but why does it 
slow down things if we request something that the kernel isn't 
configured for?

Anyway, patch is Reviewed-by: Christian König <christian.koenig at amd.com>

Regards,
Christian.

> ---
>   drivers/gpu/drm/radeon/radeon_object.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
>
> diff --git a/drivers/gpu/drm/radeon/radeon_object.c b/drivers/gpu/drm/radeon/radeon_object.c
> index 7d68223..bd3df10 100644
> --- a/drivers/gpu/drm/radeon/radeon_object.c
> +++ b/drivers/gpu/drm/radeon/radeon_object.c
> @@ -238,6 +238,18 @@ int radeon_bo_create(struct radeon_device *rdev,
>   	 * See https://bugs.freedesktop.org/show_bug.cgi?id=84627
>   	 */
>   	bo->flags &= ~RADEON_GEM_GTT_WC;
> +#elif defined(CONFIG_X86) && !defined(CONFIG_X86_PAT)
> +	/* Don't try to enable write-combining when it can't work, or things
> +	 * may be slow
> +	 * See https://bugs.freedesktop.org/show_bug.cgi?id=88758
> +	 */
> +
> +#warning Please enable CONFIG_MTRR and CONFIG_X86_PAT for better performance \
> +	 thanks to write-combining
> +
> +	DRM_INFO_ONCE("Please enable CONFIG_MTRR and CONFIG_X86_PAT for "
> +		      "better performance thanks to write-combining\n");
> +	bo->flags &= ~RADEON_GEM_GTT_WC;
>   #endif
>   
>   	radeon_ttm_placement_from_domain(bo, domain);