Re: [PATCH v5 4/7] drm/i915: optimise i915_gem_object_vmap_range() for small objects

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 29/02/16 11:13, Dave Gordon wrote:
> We're using this function for ringbuffers and other "small" objects, so
> it's worth avoiding an extra malloc()/free() cycle if the page array is
> small enough to put on the stack. Here we've chosen an arbitrary cutoff
> of 32 (4k) pages, which is big enough for a ringbuffer (4 pages) or a
> context image (currently up to 22 pages).
>
> v5:
>      change name of local array [Chris Wilson]
>
> Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Alex Dai <yu.dai@intel.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_gem.c | 17 +++++++++++------
>   1 file changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index c621b3e..c126211 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2417,7 +2417,8 @@ void *i915_gem_object_vmap_range(struct drm_i915_gem_object *obj,
>   				 unsigned int npages)
>   {
>   	struct sg_page_iter sg_iter;
> -	struct page **pages;
> +	struct page *stack_pages[32];
> +	struct page **pages = stack_pages;
>   	void *addr;
>   	int i;
>
> @@ -2426,10 +2427,13 @@ void *i915_gem_object_vmap_range(struct drm_i915_gem_object *obj,
>   		return NULL;
>   	}
>
> -	pages = drm_malloc_gfp(npages, sizeof(*pages), GFP_TEMPORARY);
> -	if (pages == NULL) {
> -		DRM_DEBUG_DRIVER("Failed to get space for pages\n");
> -		return NULL;
> +	if (npages > ARRAY_SIZE(stack_pages)) {
> +		/* Too big for stack -- allocate temporary array instead */
> +		pages = drm_malloc_gfp(npages, sizeof(*pages), GFP_TEMPORARY);
> +		if (pages == NULL) {
> +			DRM_DEBUG_DRIVER("Failed to get space for pages\n");
> +			return NULL;
> +		}
>   	}
>
>   	i = 0;
> @@ -2442,7 +2446,8 @@ void *i915_gem_object_vmap_range(struct drm_i915_gem_object *obj,
>   	addr = vmap(pages, npages, 0, PAGE_KERNEL);
>   	if (addr == NULL)
>   		DRM_DEBUG_DRIVER("Failed to vmap pages\n");
> -	drm_free_large(pages);
> +	if (pages != stack_pages)
> +		drm_free_large(pages);
>
>   	return addr;
>   }
>

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko

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