Re: [PATCH] drm/i915: Use i915_gem_object_get_dma_address() to populate rotated vmas

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 16/10/2018 16:04, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
>
> Replace the kvmalloc_array() with i915_gem_object_get_dma_address() when
> populating rotated vmas. One random access mechanism ought to be enough
> for everyone?
>
> To calculate the size of the radix tree I think we can do
> something like this (assuming 64bit pointers):
>   num_pages = obj_size / 4096
>   tree_height = ceil(log64(num_pages))
>   num_nodes = sum(64^n, n, 0, tree_height-1)
>   tree_size = num_nodes * 576

For 1920x1080x4 I get around 19k in real life - does that match your 
math? For something like two UHD screens with double buffering I guess 
that's around 4 * 2 * 2 = 16 times more, which is perhaps a worst case 
setup? So ~300KiB permanently pinned and likely not used post first pin 
to display. If that is OK with you then you have my ack for the patch. 
Or a review if you can live with some delay.

Regards,

Tvrtko

> If we compare that with the object size we should get a relative
> overhead of around .2% to 1% for reasonable sized objects,
> which framebuffers tend to be.
>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
> Suggested-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> ---
>   drivers/gpu/drm/i915/i915_gem_gtt.c | 31 ++++++-----------------------
>   1 file changed, 6 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index 29ca9007a704..98d9a1eb1ed2 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -3637,7 +3637,7 @@ void i915_gem_restore_gtt_mappings(struct drm_i915_private *dev_priv)
>   }
>   
>   static struct scatterlist *
> -rotate_pages(const dma_addr_t *in, unsigned int offset,
> +rotate_pages(struct drm_i915_gem_object *obj, unsigned int offset,
>   	     unsigned int width, unsigned int height,
>   	     unsigned int stride,
>   	     struct sg_table *st, struct scatterlist *sg)
> @@ -3646,7 +3646,7 @@ rotate_pages(const dma_addr_t *in, unsigned int offset,
>   	unsigned int src_idx;
>   
>   	for (column = 0; column < width; column++) {
> -		src_idx = stride * (height - 1) + column;
> +		src_idx = stride * (height - 1) + column + offset;
>   		for (row = 0; row < height; row++) {
>   			st->nents++;
>   			/* We don't need the pages, but need to initialize
> @@ -3654,7 +3654,8 @@ rotate_pages(const dma_addr_t *in, unsigned int offset,
>   			 * The only thing we need are DMA addresses.
>   			 */
>   			sg_set_page(sg, NULL, I915_GTT_PAGE_SIZE, 0);
> -			sg_dma_address(sg) = in[offset + src_idx];
> +			sg_dma_address(sg) =
> +				i915_gem_object_get_dma_address(obj, src_idx);
>   			sg_dma_len(sg) = I915_GTT_PAGE_SIZE;
>   			sg = sg_next(sg);
>   			src_idx -= stride;
> @@ -3668,22 +3669,11 @@ static noinline struct sg_table *
>   intel_rotate_pages(struct intel_rotation_info *rot_info,
>   		   struct drm_i915_gem_object *obj)
>   {
> -	const unsigned long n_pages = obj->base.size / I915_GTT_PAGE_SIZE;
>   	unsigned int size = intel_rotation_info_size(rot_info);
> -	struct sgt_iter sgt_iter;
> -	dma_addr_t dma_addr;
> -	unsigned long i;
> -	dma_addr_t *page_addr_list;
>   	struct sg_table *st;
>   	struct scatterlist *sg;
>   	int ret = -ENOMEM;
> -
> -	/* Allocate a temporary list of source pages for random access. */
> -	page_addr_list = kvmalloc_array(n_pages,
> -					sizeof(dma_addr_t),
> -					GFP_KERNEL);
> -	if (!page_addr_list)
> -		return ERR_PTR(ret);
> +	int i;
>   
>   	/* Allocate target SG list. */
>   	st = kmalloc(sizeof(*st), GFP_KERNEL);
> @@ -3694,29 +3684,20 @@ intel_rotate_pages(struct intel_rotation_info *rot_info,
>   	if (ret)
>   		goto err_sg_alloc;
>   
> -	/* Populate source page list from the object. */
> -	i = 0;
> -	for_each_sgt_dma(dma_addr, sgt_iter, obj->mm.pages)
> -		page_addr_list[i++] = dma_addr;
> -
> -	GEM_BUG_ON(i != n_pages);
>   	st->nents = 0;
>   	sg = st->sgl;
>   
>   	for (i = 0 ; i < ARRAY_SIZE(rot_info->plane); i++) {
> -		sg = rotate_pages(page_addr_list, rot_info->plane[i].offset,
> +		sg = rotate_pages(obj, rot_info->plane[i].offset,
>   				  rot_info->plane[i].width, rot_info->plane[i].height,
>   				  rot_info->plane[i].stride, st, sg);
>   	}
>   
> -	kvfree(page_addr_list);
> -
>   	return st;
>   
>   err_sg_alloc:
>   	kfree(st);
>   err_st_alloc:
> -	kvfree(page_addr_list);
>   
>   	DRM_DEBUG_DRIVER("Failed to create rotated mapping for object size %zu! (%ux%u tiles, %u pages)\n",
>   			 obj->base.size, rot_info->plane[0].width, rot_info->plane[0].height, size);
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