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, 72 insertions, 4 deletions

diff --git a/drivers/gpu/drm/i915/i915_scatterlist.h b/drivers/gpu/drm/i915/i915_scatterlist.h
index b8bd5925b03f..12c6a1684081 100644
--- a/drivers/gpu/drm/i915/i915_scatterlist.h
+++ b/drivers/gpu/drm/i915/i915_scatterlist.h
@@ -144,10 +144,78 @@ static inline unsigned int i915_sg_segment_size(void)
 
 bool i915_sg_trim(struct sg_table *orig_st);
 
-struct sg_table *i915_sg_from_mm_node(const struct drm_mm_node *node,
-				      u64 region_start);
+/**
+ * struct i915_refct_sgt_ops - Operations structure for struct i915_refct_sgt
+ */
+struct i915_refct_sgt_ops {
+	/**
+	 * release() - Free the memory of the struct i915_refct_sgt
+	 * @ref: struct kref that is embedded in the struct i915_refct_sgt
+	 */
+	void (*release)(struct kref *ref);
+};
+
+/**
+ * struct i915_refct_sgt - A refcounted scatter-gather table
+ * @kref: struct kref for refcounting
+ * @table: struct sg_table holding the scatter-gather table itself. Note that
+ * @table->sgl = NULL can be used to determine whether a scatter-gather table
+ * is present or not.
+ * @size: The size in bytes of the underlying memory buffer
+ * @ops: The operations structure.
+ */
+struct i915_refct_sgt {
+	struct kref kref;
+	struct sg_table table;
+	size_t size;
+	const struct i915_refct_sgt_ops *ops;
+};
+
+/**
+ * i915_refct_sgt_put - Put a refcounted sg-table
+ * @rsgt the struct i915_refct_sgt to put.
+ */
+static inline void i915_refct_sgt_put(struct i915_refct_sgt *rsgt)
+{
+	if (rsgt)
+		kref_put(&rsgt->kref, rsgt->ops->release);
+}
+
+/**
+ * i915_refct_sgt_get - Get a refcounted sg-table
+ * @rsgt the struct i915_refct_sgt to get.
+ */
+static inline struct i915_refct_sgt *
+i915_refct_sgt_get(struct i915_refct_sgt *rsgt)
+{
+	kref_get(&rsgt->kref);
+	return rsgt;
+}
+
+/**
+ * __i915_refct_sgt_init - Initialize a refcounted sg-list with a custom
+ * operations structure
+ * @rsgt The struct i915_refct_sgt to initialize.
+ * @size: Size in bytes of the underlying memory buffer.
+ * @ops: A customized operations structure in case the refcounted sg-list
+ * is embedded into another structure.
+ */
+static inline void __i915_refct_sgt_init(struct i915_refct_sgt *rsgt,
+					 size_t size,
+					 const struct i915_refct_sgt_ops *ops)
+{
+	kref_init(&rsgt->kref);
+	rsgt->table.sgl = NULL;
+	rsgt->size = size;
+	rsgt->ops = ops;
+}
+
+void i915_refct_sgt_init(struct i915_refct_sgt *rsgt, size_t size);
+
+struct i915_refct_sgt *i915_rsgt_from_mm_node(const struct drm_mm_node *node,
+					      u64 region_start);
 
-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);
 
 #endif