drm/i915: Remove the vm open count

vms are not getting properly closed. Rather than fixing that,
Remove the vm open count and instead rely on the vm refcount.

The vm open count existed solely to break the strong references the
vmas had on the vms. Now instead make those references weak and
ensure vmas are destroyed when the vm is destroyed.

Unfortunately if the vm destructor and the object destructor both
wants to destroy a vma, that may lead to a race in that the vm
destructor just unbinds the vma and leaves the actual vma destruction
to the object destructor. However in order for the object destructor
to ensure the vma is unbound it needs to grab the vm mutex. In order
to keep the vm mutex alive until the object destructor is done with
it, somewhat hackishly grab a vm_resv refcount that is released late
in the vma destruction process, when the vm mutex is no longer needed.

v2: Address review-comments from Niranjana
- Clarify that the struct i915_address_space::skip_pte_rewrite is a hack
  and should ideally be replaced in an upcoming patch.
- Remove an unneeded continue in clear_vm_list and update comment.

v3:
- Documentation update
- Commit message formatting

Co-developed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220304082641.308069-2-thomas.hellstrom@linux.intel.com

1 files changed, 7 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/i915_vma_types.h b/drivers/gpu/drm/i915/i915_vma_types.h
index 88370dadca82..eac36be184e5 100644
--- a/drivers/gpu/drm/i915/i915_vma_types.h
+++ b/drivers/gpu/drm/i915/i915_vma_types.h
@@ -272,6 +272,13 @@ struct i915_vma {
 	atomic_t pages_count; /* number of active binds to the pages */
 
 	/**
+	 * Whether we hold a reference on the vm dma_resv lock to temporarily
+	 * block vm freeing until the vma is destroyed.
+	 * Protected by the vm mutex.
+	 */
+	bool vm_ddestroy;
+
+	/**
 	 * Support different GGTT views into the same object.
 	 * This means there can be multiple VMA mappings per object and per VM.
 	 * i915_ggtt_view_type is used to distinguish between those entries.