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, 6 insertions, 6 deletions

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h b/drivers/gpu/drm/i915/gem/i915_gem_object.h
index a5479ac7a4ad..ba224598ed69 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.h
+++ b/drivers/gpu/drm/i915/gem/i915_gem_object.h
@@ -620,12 +620,12 @@ int i915_gem_object_wait_migration(struct drm_i915_gem_object *obj,
 bool i915_gem_object_placement_possible(struct drm_i915_gem_object *obj,
 					enum intel_memory_type type);
 
-struct sg_table *shmem_alloc_st(struct drm_i915_private *i915,
-				size_t size, struct intel_memory_region *mr,
-				struct address_space *mapping,
-				unsigned int max_segment);
-void shmem_free_st(struct sg_table *st, struct address_space *mapping,
-		   bool dirty, bool backup);
+int shmem_sg_alloc_table(struct drm_i915_private *i915, struct sg_table *st,
+			 size_t size, struct intel_memory_region *mr,
+			 struct address_space *mapping,
+			 unsigned int max_segment);
+void shmem_sg_free_table(struct sg_table *st, struct address_space *mapping,
+			 bool dirty, bool backup);
 void __shmem_writeback(size_t size, struct address_space *mapping);
 
 #ifdef CONFIG_MMU_NOTIFIER