drm/i915/gt: describe the new tlb parameter at i915_vma_resource

TLB cache invalidation can happen on two different situations:

1. synchronously, at __vma_put_pages();
2. asynchronously.

On the first case, TLB cache invalidation happens inside
__vma_put_pages(). So, no need to do it later on.

However, on the second case, the pages will keep in memory
until __i915_vma_evict() is called.

So, we need to store the TLB data at struct i915_vma_resource,
in order to do a TLB cache invalidation before allowing
userspace to re-use the same memory.

So, i915_vma_resource_unbind() has gained a new parameter
in order to store the TLB data at the second case.

Document it.
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/aa55eef7e63b8f3d0f69b525db2dd2eb87e9db6b.1658924372.git.mchehab@kernel.org

drivers/gpu/drm/i915/i915_vma_resource.c

@@ -216,6 +216,10 @@ i915_vma_resource_fence_notify(struct i915_sw_fence *fence,
 /**
  * i915_vma_resource_unbind - Unbind a vma resource
  * @vma_res: The vma resource to unbind.
+ * @tlb: pointer to vma->obj->mm.tlb associated with the resource
+ *	 to be stored at vma_res->tlb. When not-NULL, it will be used
+ *	 to do TLB cache invalidation before freeing a VMA resource.
+ *	 Used only for async unbind.
  *
  * At this point this function does little more than publish a fence that
  * signals immediately unless signaling is held back.