Re: [PATCH 1/6] drm/i915: Clearing buffer objects via CPU/GTT

[Dave Gordon <david.s.gordon@intel.com>]

On 09/12/15 12:46, ankitprasad.r.sharma@intel.com wrote:
> From: Ankitprasad Sharma <ankitprasad.r.sharma@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)
>
> Testcase: igt/gem_stolen
>
> Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_drv.h |  1 +
>   drivers/gpu/drm/i915/i915_gem.c | 79 +++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 80 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 548a0eb..8e554d3 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2856,6 +2856,7 @@ 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 9d2e6e3..d57e850 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -5244,3 +5244,82 @@ fail:
>   	drm_gem_object_unreference(&obj->base);
>   	return ERR_PTR(ret);
>   }
> +
> +/**
> + * i915_gem_clear_object() - 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;
> +	struct drm_i915_private *i915 = to_i915(obj->base.dev);
> +	struct drm_mm_node node;
> +
> +	WARN_ON(!mutex_is_locked(&obj->base.dev->struct_mutex));
> +	ret = i915_gem_obj_ggtt_pin(obj, 0, PIN_MAPPABLE | PIN_NONBLOCK);
> +	if (ret) {
> +		memset(&node, 0, sizeof(node));
> +		ret = drm_mm_insert_node_in_range_generic(&i915->gtt.base.mm,
> +							  &node, 4096, 0,
> +							  I915_CACHE_NONE, 0,
> +							  i915->gtt.mappable_end,
> +							  DRM_MM_SEARCH_DEFAULT,
> +							  DRM_MM_CREATE_DEFAULT);
> +		if (ret)
> +			goto out;
> +
> +		i915_gem_object_pin_pages(obj);
> +	} else {
> +		node.start = i915_gem_obj_ggtt_offset(obj);
> +		node.allocated = false;
> +	}
> +
> +	ret = i915_gem_object_put_fence(obj);
> +	if (ret)
> +		goto unpin;
> +
> +	if (node.allocated) {
> +		for (i = 0; i < size/PAGE_SIZE; i++) {
> +			wmb();
> +			i915->gtt.base.insert_page(&i915->gtt.base,
> +					i915_gem_object_get_dma_address(obj, i),
> +					node.start,
> +					I915_CACHE_NONE,
> +					0);
> +			wmb();
> +			base = ioremap_wc(i915->gtt.mappable_base + node.start, 4096);
> +			memset_io(base, 0, 4096);
> +			iounmap(base);
> +		}
> +	} else {
> +		/* Get the CPU virtual address of the buffer */
> +		base = ioremap_wc(i915->gtt.mappable_base +
> +				  node.start, size);
> +		if (base == NULL) {
> +			DRM_ERROR("Mapping of gem object to CPU failed!\n");
> +			ret = -ENOSPC;
> +			goto unpin;
> +		}
> +
> +		memset_io(base, 0, size);
> +		iounmap(base);
> +	}
> +unpin:
> +	if (node.allocated) {
> +		wmb();
> +		i915->gtt.base.clear_range(&i915->gtt.base,
> +				node.start, node.size,
> +				true);
> +		drm_mm_remove_node(&node);
> +		i915_gem_object_unpin_pages(obj);
> +	}
> +	else {
> +		i915_gem_object_ggtt_unpin(obj);
> +	}
> +out:
> +	return ret;
> +}

This is effectively two functions interleaved, as shown by the repeated 
if (node.allocated) tests. Would it not be clearer to have the mainline 
function deal only with the GTT-pinned case, and a separate function for 
the page-by-page version, called as a fallback if pinning fails?

int i915_gem_object_clear(struct drm_i915_gem_object *obj)
{
	int ret, i;
	char __iomem *base;
	size_t size = obj->base.size;
	struct drm_i915_private *i915 = to_i915(obj->base.dev);

	WARN_ON(!mutex_is_locked(&obj->base.dev->struct_mutex));
	ret = i915_gem_obj_ggtt_pin(obj, 0, PIN_MAPPABLE|PIN_NONBLOCK);
	if (ret)
		return __i915_obj_clear_by_pages(...);

	... mainline (fast) code here ...

	return ret;
}

static int __i915_obj_clear_by_pages(...);
{
	... complicated page-by-page fallback code here ...
}

.Dave.


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