Revert "drm/i915/lmem: Limit block size to 4G"

Mixing I915_ALLOC_CONTIGUOUS and I915_ALLOC_MAX_SEGMENT_SIZE fared
badly. The two directives conflict, with the contiguous request setting
the min_order to the full size of the object, and the max-segment-size
setting the max_order to the limit of the DMA mapper. This results in a
situation where max_order < min_order, causing our sanity checks to
fail.

Instead of limiting the buddy block size, in the previous patch we split
the oversized buddy into multiple scatterlist elements.

Fixes: d2cf0125 ("drm/i915/lmem: Limit block size to 4G")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201202173444.14903-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_region.c

@@ -43,7 +43,7 @@ i915_gem_object_get_pages_buddy(struct drm_i915_gem_object *obj)
 		return -ENOMEM;
 	}
 
-	flags = I915_ALLOC_MIN_PAGE_SIZE | I915_ALLOC_MAX_SEGMENT_SIZE;
+	flags = I915_ALLOC_MIN_PAGE_SIZE;
 	if (obj->flags & I915_BO_ALLOC_CONTIGUOUS)
 		flags |= I915_ALLOC_CONTIGUOUS;
 

drivers/gpu/drm/i915/intel_memory_region.c

@@ -72,7 +72,6 @@ __intel_memory_region_get_pages_buddy(struct intel_memory_region *mem,
 				      struct list_head *blocks)
 {
 	unsigned int min_order = 0;
-	unsigned int max_order;
 	unsigned long n_pages;
 
 	GEM_BUG_ON(!IS_ALIGNED(size, mem->mm.chunk_size));
@@ -93,28 +92,13 @@ __intel_memory_region_get_pages_buddy(struct intel_memory_region *mem,
 
 	n_pages = size >> ilog2(mem->mm.chunk_size);
 
-	/*
-	 * If we going to feed this into an sg list we should limit the block
-	 * sizes such that we don't exceed the i915_sg_segment_size().
-	 */
-	if (flags & I915_ALLOC_MAX_SEGMENT_SIZE) {
-		unsigned int max_segment = i915_sg_segment_size();
-
-		if (GEM_WARN_ON(max_segment < mem->mm.chunk_size))
-			max_order = 0;
-		else
-			max_order = ilog2(max_segment) - ilog2(mem->mm.chunk_size);
-	} else {
-		max_order = mem->mm.max_order;
-	}
-
 	mutex_lock(&mem->mm_lock);
 
 	do {
 		struct i915_buddy_block *block;
 		unsigned int order;
 
-		order = min_t(u32, fls(n_pages) - 1, max_order);
+		order = fls(n_pages) - 1;
 		GEM_BUG_ON(order > mem->mm.max_order);
 		GEM_BUG_ON(order < min_order);
 

drivers/gpu/drm/i915/intel_memory_region.h

@@ -46,7 +46,6 @@ enum intel_region_id {
 
 #define I915_ALLOC_MIN_PAGE_SIZE  BIT(0)
 #define I915_ALLOC_CONTIGUOUS     BIT(1)
-#define I915_ALLOC_MAX_SEGMENT_SIZE	BIT(2)
 
 #define for_each_memory_region(mr, i915, id) \
 	for (id = 0; id < ARRAY_SIZE((i915)->mm.regions); id++) \

drivers/gpu/drm/i915/selftests/intel_memory_region.c

@@ -356,21 +356,21 @@ static int igt_mock_splintered_region(void *arg)
 
 static int igt_mock_max_segment(void *arg)
 {
+	const unsigned int max_segment = i915_sg_segment_size();
 	struct intel_memory_region *mem = arg;
 	struct drm_i915_private *i915 = mem->i915;
 	struct drm_i915_gem_object *obj;
 	struct i915_buddy_block *block;
+	struct scatterlist *sg;
 	LIST_HEAD(objects);
 	u64 size;
 	int err = 0;
 
 	/*
-	 * The size of block are only limited by the largest power-of-two that
-	 * will fit in the region size, but to construct an object we also
-	 * require feeding it into an sg list, where the upper limit of the sg
-	 * entry is at most UINT_MAX, therefore when allocating with
-	 * I915_ALLOC_MAX_SEGMENT_SIZE we shouldn't see blocks larger than
-	 * i915_sg_segment_size().
+	 * While we may create very large contiguous blocks, we may need
+	 * to break those down for consumption elsewhere. In particular,
+	 * dma-mapping with scatterlist elements have an implicit limit of
+	 * UINT_MAX on each element.
 	 */
 
 	size = SZ_8G;
@@ -384,12 +384,23 @@ static int igt_mock_max_segment(void *arg)
 		goto out_put;
 	}
 
+	err = -EINVAL;
 	list_for_each_entry(block, &obj->mm.blocks, link) {
-		if (i915_buddy_block_size(&mem->mm, block) > i915_sg_segment_size()) {
-			pr_err("%s found block size(%llu) larger than max sg_segment_size(%u)",
-			       __func__,
-			       i915_buddy_block_size(&mem->mm, block),
-			       i915_sg_segment_size());
+		if (i915_buddy_block_size(&mem->mm, block) > max_segment) {
+			err = 0;
+			break;
+		}
+	}
+	if (err) {
+		pr_err("%s: Failed to create a huge contiguous block\n",
+		       __func__);
+		goto out_close;
+	}
+
+	for (sg = obj->mm.pages->sgl; sg; sg = sg_next(sg)) {
+		if (sg->length > max_segment) {
+			pr_err("%s: Created an oversized scatterlist entry, %u > %u\n",
+			       __func__, sg->length, max_segment);
 			err = -EINVAL;
 			goto out_close;
 		}