[Intel-gfx] [PATCH 04/10] drm/i915: Clearing buffer objects via CPU/GTT

[Chris Wilson]

On Tue, Jan 26, 2016 at 01:13:30AM +0530, ankitprasad.r.sharma at intel.com wrote:
> From: Ankitprasad Sharma <ankitprasad.r.sharma at intel.com>
> 
> This patch adds support for clearing buffer objects via CPU/GTT. This
> is particularly useful for clearing out the non shmem backed objects.
> Currently intend to use this only for buffers allocated from stolen
> region.
> 
> v2: Added kernel doc for i915_gem_clear_object(), corrected/removed
> variable assignments (Tvrtko)
> 
> v3: Map object page by page to the gtt if the pinning of the whole object
> to the ggtt fails, Corrected function name (Chris)
> 
> v4: Clear the buffer page by page, and not map the whole object in the gtt
> aperture. Use i915 wrapper function in place of drm_mm_insert_node_in_range.
> 
> v5: Use renamed wrapper function for drm_mm_insert_node_in_range,
> updated barrier positioning (Chris)
> 
> v6: Use PAGE_SIZE instead of 4096, use get_pages call before pinning pages
> (Tvrtko)
> 
> v7: Fixed the onion (undo operation in reverse order) (Chris)
> 
> Testcase: igt/gem_stolen
> 
> Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma at intel.com>
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_drv.h |  1 +
>  drivers/gpu/drm/i915/i915_gem.c | 46 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 47 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index c9b8a63..fcaea77 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2892,6 +2892,7 @@ int i915_gem_obj_prepare_shmem_read(struct drm_i915_gem_object *obj,
>  				    int *needs_clflush);
>  
>  int __must_check i915_gem_object_get_pages(struct drm_i915_gem_object *obj);
> +int i915_gem_object_clear(struct drm_i915_gem_object *obj);
>  
>  static inline int __sg_page_count(struct scatterlist *sg)
>  {
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 9c751f3..ad70bd3 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -5398,3 +5398,49 @@ fail:
>  	drm_gem_object_unreference(&obj->base);
>  	return ERR_PTR(ret);
>  }
> +
> +/**
> + * i915_gem_object_clear() - Clear buffer object via CPU/GTT
> + * @obj: Buffer object to be cleared
> + *
> + * Return: 0 - success, non-zero - failure
> + */
> +int i915_gem_object_clear(struct drm_i915_gem_object *obj)
> +{
> +	int ret, i;
> +	char __iomem *base;
> +	size_t size = obj->base.size;

size_t!!!!

> +	struct drm_i915_private *i915 = to_i915(obj->base.dev);

I have a different preference for my variable ordering, but meh.

> +	struct drm_mm_node node;
> +
> +	lockdep_assert_held(&obj->base.dev->struct_mutex);
> +	ret = insert_mappable_node(i915, &node, PAGE_SIZE);
> +	if (ret)
> +		return ret;
> +
> +	ret = i915_gem_object_get_pages(obj);
> +	if (ret)
> +		goto err_remove_node;
> +
> +	i915_gem_object_pin_pages(obj);
> +	base = io_mapping_map_wc(i915->gtt.mappable, node.start);
> +	for (i = 0; i < size/PAGE_SIZE; i++) {
> +		i915->gtt.base.insert_page(&i915->gtt.base,
> +					   i915_gem_object_get_dma_address(obj, i),
> +					   node.start,
> +					   I915_CACHE_NONE, 0);
> +		wmb();

Again, let's document that these provide explicit barriers with the
insert-page WC into the GGTT versus the WC through the GGTT.

> +		memset_io(base, 0, PAGE_SIZE);
> +		wmb();
> +	}
> +
> +	io_mapping_unmap(base);

So whitespace after the loop + unmap, but none before? That's
inconsistent. Either you mean the loop is part of the map-wc critical
section or it is not.

Fix the size_t, and you have my r-b.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre