Re: [PATCH] drm/i915/selftests: Provide full mb() around clflush

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Fri, Jul 06, 2018 at 06:49:26PM +0100, Chris Wilson wrote:
> clflush is an unserialised instruction and the IA manual strongly advises
> you to serialise it with a mb. To be cautious, apply one before and one
> after,

my understanding is that we need one before and one after anyways,
not just a matter of being cautious for being cautious..

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>


> so that it is serialised with both writes and reads without
> worrying too much about the required direction.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  .../drm/i915/selftests/i915_gem_coherency.c   | 21 ++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/selftests/i915_gem_coherency.c b/drivers/gpu/drm/i915/selftests/i915_gem_coherency.c
> index 294c58aba2c1..df44c302a9fe 100644
> --- a/drivers/gpu/drm/i915/selftests/i915_gem_coherency.c
> +++ b/drivers/gpu/drm/i915/selftests/i915_gem_coherency.c
> @@ -42,11 +42,21 @@ static int cpu_set(struct drm_i915_gem_object *obj,
>  
>  	page = i915_gem_object_get_page(obj, offset >> PAGE_SHIFT);
>  	map = kmap_atomic(page);
> -	if (needs_clflush & CLFLUSH_BEFORE)
> +
> +	if (needs_clflush & CLFLUSH_BEFORE) {
> +		mb();
>  		clflush(map+offset_in_page(offset) / sizeof(*map));
> +		mb();
> +	}
> +
>  	map[offset_in_page(offset) / sizeof(*map)] = v;
> -	if (needs_clflush & CLFLUSH_AFTER)
> +
> +	if (needs_clflush & CLFLUSH_AFTER) {
> +		mb();
>  		clflush(map+offset_in_page(offset) / sizeof(*map));
> +		mb();
> +	}
> +
>  	kunmap_atomic(map);
>  
>  	i915_gem_obj_finish_shmem_access(obj);
> @@ -68,8 +78,13 @@ static int cpu_get(struct drm_i915_gem_object *obj,
>  
>  	page = i915_gem_object_get_page(obj, offset >> PAGE_SHIFT);
>  	map = kmap_atomic(page);
> -	if (needs_clflush & CLFLUSH_BEFORE)
> +
> +	if (needs_clflush & CLFLUSH_BEFORE) {
> +		mb();
>  		clflush(map+offset_in_page(offset) / sizeof(*map));
> +		mb();
> +	}
> +
>  	*v = map[offset_in_page(offset) / sizeof(*map)];
>  	kunmap_atomic(map);
>  
> -- 
> 2.18.0
> 
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