drm/xe: add lockdep annotation for xe_device_mem_access_get()

The atomics here might hide potential issues, also rpm core is not
holding any lock when calling our rpm resume callback, so add a dummy lock
with the idea that xe_pm_runtime_resume() is eventually going to be
called when we are holding it. This only needs to happen once and then
lockdep can validate all callers and their locks.

v2: (Thomas Hellström)
 - Prefer static lockdep_map instead of full blown mutex.
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_device.c

@@ -34,6 +34,12 @@
 #include "xe_vm_madvise.h"
 #include "xe_wait_user_fence.h"
 
+#ifdef CONFIG_LOCKDEP
+static struct lockdep_map xe_device_mem_access_lockdep_map = {
+	.name = "xe_device_mem_access_lockdep_map"
+};
+#endif
+
 static int xe_file_open(struct drm_device *dev, struct drm_file *file)
 {
 	struct xe_file *xef;
@@ -410,10 +416,28 @@ void xe_device_mem_access_get(struct xe_device *xe)
 	if (xe_pm_read_callback_task(xe) == current)
 		return;
 
+	/*
+	 * Since the resume here is synchronous it can be quite easy to deadlock
+	 * if we are not careful. Also in practice it might be quite timing
+	 * sensitive to ever see the 0 -> 1 transition with the callers locks
+	 * held, so deadlocks might exist but are hard for lockdep to ever see.
+	 * With this in mind, help lockdep learn about the potentially scary
+	 * stuff that can happen inside the runtime_resume callback by acquiring
+	 * a dummy lock (it doesn't protect anything and gets compiled out on
+	 * non-debug builds).  Lockdep then only needs to see the
+	 * mem_access_lockdep_map -> runtime_resume callback once, and then can
+	 * hopefully validate all the (callers_locks) -> mem_access_lockdep_map.
+	 * For example if the (callers_locks) are ever grabbed in the
+	 * runtime_resume callback, lockdep should give us a nice splat.
+	 */
+	lock_map_acquire(&xe_device_mem_access_lockdep_map);
+
 	xe_pm_runtime_get(xe);
 	ref = atomic_inc_return(&xe->mem_access.ref);
 
 	XE_WARN_ON(ref == S32_MAX);
+
+	lock_map_release(&xe_device_mem_access_lockdep_map);
 }
 
 void xe_device_mem_access_put(struct xe_device *xe)