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, 6 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/i915_vma_resource.h b/drivers/gpu/drm/i915/i915_vma_resource.h
index 25913913baa6..5d8427caa2ba 100644
--- a/drivers/gpu/drm/i915/i915_vma_resource.h
+++ b/drivers/gpu/drm/i915/i915_vma_resource.h
@@ -62,6 +62,11 @@ struct i915_page_sizes {
  * deferred to a work item awaiting unsignaled fences. This is a hack.
  * (dma_fence_work uses a fence flag for this, but this seems slightly
  * cleaner).
+ * @needs_wakeref: Whether a wakeref is needed during unbind. Since we can't
+ * take a wakeref in the dma-fence signalling critical path, it needs to be
+ * taken when the unbind is scheduled.
+ * @skip_pte_rewrite: During ggtt suspend and vm takedown pte rewriting
+ * needs to be skipped for unbind.
  *
  * The lifetime of a struct i915_vma_resource is from a binding request to
  * the actual possible asynchronous unbind has completed.
@@ -113,6 +118,7 @@ struct i915_vma_resource {
 	bool allocated:1;
 	bool immediate_unbind:1;
 	bool needs_wakeref:1;
+	bool skip_pte_rewrite:1;
 };
 
 bool i915_vma_resource_hold(struct i915_vma_resource *vma_res,