drm/i915: Introduce struct intel_wakeref

For controlling runtime pm of the GT and engines, we would like to have
a callback to do extra work the first time we wake up and the last time
we drop the wakeref. This first/last access needs serialisation and so
we encompass a mutex with the regular intel_wakeref_t tracker.

v2: Drop the _once naming and report the errors.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc; Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190424200717.1686-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_wakeref.h

@@ -0,0 +1,133 @@
+/*
+ * SPDX-License-Identifier: MIT
+ *
+ * Copyright © 2019 Intel Corporation
+ */
+
+#ifndef INTEL_WAKEREF_H
+#define INTEL_WAKEREF_H
+
+#include <linux/atomic.h>
+#include <linux/mutex.h>
+#include <linux/stackdepot.h>
+
+struct drm_i915_private;
+
+typedef depot_stack_handle_t intel_wakeref_t;
+
+struct intel_wakeref {
+	atomic_t count;
+	struct mutex mutex;
+	intel_wakeref_t wakeref;
+};
+
+void __intel_wakeref_init(struct intel_wakeref *wf,
+			  struct lock_class_key *key);
+#define intel_wakeref_init(wf) do {					\
+	static struct lock_class_key __key;				\
+									\
+	__intel_wakeref_init((wf), &__key);				\
+} while (0)
+
+int __intel_wakeref_get_first(struct drm_i915_private *i915,
+			      struct intel_wakeref *wf,
+			      int (*fn)(struct intel_wakeref *wf));
+int __intel_wakeref_put_last(struct drm_i915_private *i915,
+			     struct intel_wakeref *wf,
+			     int (*fn)(struct intel_wakeref *wf));
+
+/**
+ * intel_wakeref_get: Acquire the wakeref
+ * @i915: the drm_i915_private device
+ * @wf: the wakeref
+ * @fn: callback for acquired the wakeref, called only on first acquire.
+ *
+ * Acquire a hold on the wakeref. The first user to do so, will acquire
+ * the runtime pm wakeref and then call the @fn underneath the wakeref
+ * mutex.
+ *
+ * Note that @fn is allowed to fail, in which case the runtime-pm wakeref
+ * will be released and the acquisition unwound, and an error reported.
+ *
+ * Returns: 0 if the wakeref was acquired successfully, or a negative error
+ * code otherwise.
+ */
+static inline int
+intel_wakeref_get(struct drm_i915_private *i915,
+		  struct intel_wakeref *wf,
+		  int (*fn)(struct intel_wakeref *wf))
+{
+	if (unlikely(!atomic_inc_not_zero(&wf->count)))
+		return __intel_wakeref_get_first(i915, wf, fn);
+
+	return 0;
+}
+
+/**
+ * intel_wakeref_put: Release the wakeref
+ * @i915: the drm_i915_private device
+ * @wf: the wakeref
+ * @fn: callback for releasing the wakeref, called only on final release.
+ *
+ * Release our hold on the wakeref. When there are no more users,
+ * the runtime pm wakeref will be released after the @fn callback is called
+ * underneath the wakeref mutex.
+ *
+ * Note that @fn is allowed to fail, in which case the runtime-pm wakeref
+ * is retained and an error reported.
+ *
+ * Returns: 0 if the wakeref was released successfully, or a negative error
+ * code otherwise.
+ */
+static inline int
+intel_wakeref_put(struct drm_i915_private *i915,
+		  struct intel_wakeref *wf,
+		  int (*fn)(struct intel_wakeref *wf))
+{
+	if (atomic_dec_and_mutex_lock(&wf->count, &wf->mutex))
+		return __intel_wakeref_put_last(i915, wf, fn);
+
+	return 0;
+}
+
+/**
+ * intel_wakeref_lock: Lock the wakeref (mutex)
+ * @wf: the wakeref
+ *
+ * Locks the wakeref to prevent it being acquired or released. New users
+ * can still adjust the counter, but the wakeref itself (and callback)
+ * cannot be acquired or released.
+ */
+static inline void
+intel_wakeref_lock(struct intel_wakeref *wf)
+	__acquires(wf->mutex)
+{
+	mutex_lock(&wf->mutex);
+}
+
+/**
+ * intel_wakeref_unlock: Unlock the wakeref
+ * @wf: the wakeref
+ *
+ * Releases a previously acquired intel_wakeref_lock().
+ */
+static inline void
+intel_wakeref_unlock(struct intel_wakeref *wf)
+	__releases(wf->mutex)
+{
+	mutex_unlock(&wf->mutex);
+}
+
+/**
+ * intel_wakeref_active: Query whether the wakeref is currently held
+ * @wf: the wakeref
+ *
+ * Returns: true if the wakeref is currently held.
+ */
+static inline bool
+intel_wakeref_active(struct intel_wakeref *wf)
+{
+	return atomic_read(&wf->count);
+}
+
+#endif /* INTEL_WAKEREF_H */