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/Makefile

@@ -50,6 +50,7 @@ i915-y += i915_drv.o \
 	  intel_device_info.o \
 	  intel_pm.o \
 	  intel_runtime_pm.o \
+	  intel_wakeref.o \
 	  intel_uncore.o
 
 # core library code

drivers/gpu/drm/i915/Makefile.header-test

@@ -31,7 +31,8 @@ header_test := \
 	intel_psr.h \
 	intel_sdvo.h \
 	intel_sprite.h \
-	intel_tv.h
+	intel_tv.h \
+	intel_wakeref.h
 
 quiet_cmd_header_test = HDRTEST $@
       cmd_header_test = echo "\#include \"$(<F)\"" > $@

drivers/gpu/drm/i915/i915_drv.h

@@ -74,6 +74,7 @@
 #include "intel_opregion.h"
 #include "intel_uc.h"
 #include "intel_uncore.h"
+#include "intel_wakeref.h"
 #include "intel_wopcm.h"
 
 #include "i915_gem.h"
@@ -134,8 +135,6 @@ bool i915_error_injected(void);
 	__i915_printk(i915, i915_error_injected() ? KERN_DEBUG : KERN_ERR, \
 		      fmt, ##__VA_ARGS__)
 
-typedef depot_stack_handle_t intel_wakeref_t;
-
 enum hpd_pin {
 	HPD_NONE = 0,
 	HPD_TV = HPD_NONE,     /* TV is known to be unreliable */

drivers/gpu/drm/i915/intel_wakeref.c

+/*
+ * SPDX-License-Identifier: MIT
+ *
+ * Copyright © 2019 Intel Corporation
+ */
+
+#include "intel_drv.h"
+#include "intel_wakeref.h"
+
+int __intel_wakeref_get_first(struct drm_i915_private *i915,
+			      struct intel_wakeref *wf,
+			      int (*fn)(struct intel_wakeref *wf))
+{
+	/*
+	 * Treat get/put as different subclasses, as we may need to run
+	 * the put callback from under the shrinker and do not want to
+	 * cross-contanimate that callback with any extra work performed
+	 * upon acquiring the wakeref.
+	 */
+	mutex_lock_nested(&wf->mutex, SINGLE_DEPTH_NESTING);
+	if (!atomic_read(&wf->count)) {
+		int err;
+
+		wf->wakeref = intel_runtime_pm_get(i915);
+
+		err = fn(wf);
+		if (unlikely(err)) {
+			intel_runtime_pm_put(i915, wf->wakeref);
+			mutex_unlock(&wf->mutex);
+			return err;
+		}
+
+		smp_mb__before_atomic(); /* release wf->count */
+	}
+	atomic_inc(&wf->count);
+	mutex_unlock(&wf->mutex);
+
+	return 0;
+}
+
+int __intel_wakeref_put_last(struct drm_i915_private *i915,
+			     struct intel_wakeref *wf,
+			     int (*fn)(struct intel_wakeref *wf))
+{
+	int err;
+
+	err = fn(wf);
+	if (likely(!err))
+		intel_runtime_pm_put(i915, wf->wakeref);
+	else
+		atomic_inc(&wf->count);
+	mutex_unlock(&wf->mutex);
+
+	return err;
+}
+
+void __intel_wakeref_init(struct intel_wakeref *wf, struct lock_class_key *key)
+{
+	__mutex_init(&wf->mutex, "wakeref", key);
+	atomic_set(&wf->count, 0);
+}

drivers/gpu/drm/i915/intel_wakeref.h

+/*
+ * 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 */