drm/i915: Introduce refcounted sg-tables

As we start to introduce asynchronous failsafe object migration,
where we update the object state and then submit asynchronous
commands we need to record what memory resources are actually used
by various part of the command stream. Initially for three purposes:

1) Error capture.
2) Asynchronous migration error recovery.
3) Asynchronous vma bind.

At the time where these happens, the object state may have been updated
to be several migrations ahead and object sg-tables discarded.

In order to make it possible to keep sg-tables with memory resource
information for these operations, introduce refcounted sg-tables that
aren't freed until the last user is done with them.

The alternative would be to reference information sitting on the
corresponding ttm_resources which typically have the same lifetime as
these refcountes sg_tables, but that leads to other awkward constructs:
Due to the design direction chosen for ttm resource managers that would
lead to diamond-style inheritance, the LMEM resources may sometimes be
prematurely freed, and finally the subclassed struct ttm_resource would
have to bleed into the asynchronous vma bind code.

v3:
- Address a number of style issues (Matthew Auld)
v4:
- Dont check for st->sgl being NULL in i915_ttm_tt__shmem_unpopulate(),
  that should never happen. (Matthew Auld)
v5:
- Fix a Potential double-free (Matthew Auld)

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211101122444.114607-1-thomas.hellstrom@linux.intel.com

1 files changed, 46 insertions, 16 deletions

diff --git a/drivers/gpu/drm/i915/i915_scatterlist.c b/drivers/gpu/drm/i915/i915_scatterlist.c
index 4a6712dca838..41f2adb6a583 100644
--- a/drivers/gpu/drm/i915/i915_scatterlist.c
+++ b/drivers/gpu/drm/i915/i915_scatterlist.c
@@ -41,8 +41,32 @@ bool i915_sg_trim(struct sg_table *orig_st)
 	return true;
 }
 
+static void i915_refct_sgt_release(struct kref *ref)
+{
+	struct i915_refct_sgt *rsgt =
+		container_of(ref, typeof(*rsgt), kref);
+
+	sg_free_table(&rsgt->table);
+	kfree(rsgt);
+}
+
+static const struct i915_refct_sgt_ops rsgt_ops = {
+	.release = i915_refct_sgt_release
+};
+
+/**
+ * i915_refct_sgt_init - Initialize a struct i915_refct_sgt with default ops
+ * @rsgt: The struct i915_refct_sgt to initialize.
+ * size: The size of the underlying memory buffer.
+ */
+void i915_refct_sgt_init(struct i915_refct_sgt *rsgt, size_t size)
+{
+	__i915_refct_sgt_init(rsgt, size, &rsgt_ops);
+}
+
 /**
- * i915_sg_from_mm_node - Create an sg_table from a struct drm_mm_node
+ * i915_rsgt_from_mm_node - Create a refcounted sg_table from a struct
+ * drm_mm_node
  * @node: The drm_mm_node.
  * @region_start: An offset to add to the dma addresses of the sg list.
  *
@@ -50,25 +74,28 @@ bool i915_sg_trim(struct sg_table *orig_st)
  * taking a maximum segment length into account, splitting into segments
  * if necessary.
  *
- * Return: A pointer to a kmalloced struct sg_table on success, negative
+ * Return: A pointer to a kmalloced struct i915_refct_sgt on success, negative
  * error code cast to an error pointer on failure.
  */
-struct sg_table *i915_sg_from_mm_node(const struct drm_mm_node *node,
-				      u64 region_start)
+struct i915_refct_sgt *i915_rsgt_from_mm_node(const struct drm_mm_node *node,
+					      u64 region_start)
 {
 	const u64 max_segment = SZ_1G; /* Do we have a limit on this? */
 	u64 segment_pages = max_segment >> PAGE_SHIFT;
 	u64 block_size, offset, prev_end;
+	struct i915_refct_sgt *rsgt;
 	struct sg_table *st;
 	struct scatterlist *sg;
 
-	st = kmalloc(sizeof(*st), GFP_KERNEL);
-	if (!st)
+	rsgt = kmalloc(sizeof(*rsgt), GFP_KERNEL);
+	if (!rsgt)
 		return ERR_PTR(-ENOMEM);
 
+	i915_refct_sgt_init(rsgt, node->size << PAGE_SHIFT);
+	st = &rsgt->table;
 	if (sg_alloc_table(st, DIV_ROUND_UP(node->size, segment_pages),
 			   GFP_KERNEL)) {
-		kfree(st);
+		i915_refct_sgt_put(rsgt);
 		return ERR_PTR(-ENOMEM);
 	}
 
@@ -104,11 +131,11 @@ struct sg_table *i915_sg_from_mm_node(const struct drm_mm_node *node,
 	sg_mark_end(sg);
 	i915_sg_trim(st);
 
-	return st;
+	return rsgt;
 }
 
 /**
- * i915_sg_from_buddy_resource - Create an sg_table from a struct
+ * i915_rsgt_from_buddy_resource - Create a refcounted sg_table from a struct
  * i915_buddy_block list
  * @res: The struct i915_ttm_buddy_resource.
  * @region_start: An offset to add to the dma addresses of the sg list.
@@ -117,11 +144,11 @@ struct sg_table *i915_sg_from_mm_node(const struct drm_mm_node *node,
  * taking a maximum segment length into account, splitting into segments
  * if necessary.
  *
- * Return: A pointer to a kmalloced struct sg_table on success, negative
+ * Return: A pointer to a kmalloced struct i915_refct_sgts on success, negative
  * error code cast to an error pointer on failure.
  */
-struct sg_table *i915_sg_from_buddy_resource(struct ttm_resource *res,
-					     u64 region_start)
+struct i915_refct_sgt *i915_rsgt_from_buddy_resource(struct ttm_resource *res,
+						     u64 region_start)
 {
 	struct i915_ttm_buddy_resource *bman_res = to_ttm_buddy_resource(res);
 	const u64 size = res->num_pages << PAGE_SHIFT;
@@ -129,18 +156,21 @@ struct sg_table *i915_sg_from_buddy_resource(struct ttm_resource *res,
 	struct i915_buddy_mm *mm = bman_res->mm;
 	struct list_head *blocks = &bman_res->blocks;
 	struct i915_buddy_block *block;
+	struct i915_refct_sgt *rsgt;
 	struct scatterlist *sg;
 	struct sg_table *st;
 	resource_size_t prev_end;
 
 	GEM_BUG_ON(list_empty(blocks));
 
-	st = kmalloc(sizeof(*st), GFP_KERNEL);
-	if (!st)
+	rsgt = kmalloc(sizeof(*rsgt), GFP_KERNEL);
+	if (!rsgt)
 		return ERR_PTR(-ENOMEM);
 
+	i915_refct_sgt_init(rsgt, size);
+	st = &rsgt->table;
 	if (sg_alloc_table(st, res->num_pages, GFP_KERNEL)) {
-		kfree(st);
+		i915_refct_sgt_put(rsgt);
 		return ERR_PTR(-ENOMEM);
 	}
 
@@ -181,7 +211,7 @@ struct sg_table *i915_sg_from_buddy_resource(struct ttm_resource *res,
 	sg_mark_end(sg);
 	i915_sg_trim(st);
 
-	return st;
+	return rsgt;
 }
 
 #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)