[PATCH 1/2] drm: make idr_mutex a spinlock

[David Herrmann]

There is no reason to use a heavy mutex for idr protection. Use a spinlock
and make idr-allocation use idr_preload().

This patch also makes mode-object lookup irq-save, in case you ever wanna
lookup modeset objects from interrupts. This is just a side-effect of
avoiding a mutex.

Signed-off-by: David Herrmann <dh.herrmann at gmail.com>
---
 drivers/gpu/drm/drm_crtc.c | 34 ++++++++++++++++++++--------------
 include/drm/drm_crtc.h     |  4 ++--
 2 files changed, 22 insertions(+), 16 deletions(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 61b6978..97eba56 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -283,8 +283,10 @@ static int drm_mode_object_get_reg(struct drm_device *dev,
 {
 	int ret;
 
-	mutex_lock(&dev->mode_config.idr_mutex);
-	ret = idr_alloc(&dev->mode_config.crtc_idr, register_obj ? obj : NULL, 1, 0, GFP_KERNEL);
+	idr_preload(GFP_KERNEL);
+	spin_lock_irq(&dev->mode_config.idr_lock);
+
+	ret = idr_alloc(&dev->mode_config.crtc_idr, register_obj ? obj : NULL, 1, 0, GFP_NOWAIT);
 	if (ret >= 0) {
 		/*
 		 * Set up the object linking under the protection of the idr
@@ -293,7 +295,9 @@ static int drm_mode_object_get_reg(struct drm_device *dev,
 		obj->id = ret;
 		obj->type = obj_type;
 	}
-	mutex_unlock(&dev->mode_config.idr_mutex);
+
+	spin_unlock_irq(&dev->mode_config.idr_lock);
+	idr_preload_end();
 
 	return ret < 0 ? ret : 0;
 }
@@ -322,9 +326,9 @@ int drm_mode_object_get(struct drm_device *dev,
 static void drm_mode_object_register(struct drm_device *dev,
 				     struct drm_mode_object *obj)
 {
-	mutex_lock(&dev->mode_config.idr_mutex);
+	spin_lock_irq(&dev->mode_config.idr_lock);
 	idr_replace(&dev->mode_config.crtc_idr, obj, obj->id);
-	mutex_unlock(&dev->mode_config.idr_mutex);
+	spin_unlock_irq(&dev->mode_config.idr_lock);
 }
 
 /**
@@ -339,17 +343,18 @@ static void drm_mode_object_register(struct drm_device *dev,
 void drm_mode_object_put(struct drm_device *dev,
 			 struct drm_mode_object *object)
 {
-	mutex_lock(&dev->mode_config.idr_mutex);
+	spin_lock_irq(&dev->mode_config.idr_lock);
 	idr_remove(&dev->mode_config.crtc_idr, object->id);
-	mutex_unlock(&dev->mode_config.idr_mutex);
+	spin_unlock_irq(&dev->mode_config.idr_lock);
 }
 
 static struct drm_mode_object *_object_find(struct drm_device *dev,
 		uint32_t id, uint32_t type)
 {
 	struct drm_mode_object *obj = NULL;
+	unsigned long flags;
 
-	mutex_lock(&dev->mode_config.idr_mutex);
+	spin_lock_irqsave(&dev->mode_config.idr_lock, flags);
 	obj = idr_find(&dev->mode_config.crtc_idr, id);
 	if (obj && type != DRM_MODE_OBJECT_ANY && obj->type != type)
 		obj = NULL;
@@ -358,7 +363,7 @@ static struct drm_mode_object *_object_find(struct drm_device *dev,
 	/* don't leak out unref'd fb's */
 	if (obj && (obj->type == DRM_MODE_OBJECT_FB))
 		obj = NULL;
-	mutex_unlock(&dev->mode_config.idr_mutex);
+	spin_unlock_irqrestore(&dev->mode_config.idr_lock, flags);
 
 	return obj;
 }
@@ -433,9 +438,9 @@ EXPORT_SYMBOL(drm_framebuffer_init);
 static void __drm_framebuffer_unregister(struct drm_device *dev,
 					 struct drm_framebuffer *fb)
 {
-	mutex_lock(&dev->mode_config.idr_mutex);
+	spin_lock_irq(&dev->mode_config.idr_lock);
 	idr_remove(&dev->mode_config.crtc_idr, fb->base.id);
-	mutex_unlock(&dev->mode_config.idr_mutex);
+	spin_unlock_irq(&dev->mode_config.idr_lock);
 
 	fb->base.id = 0;
 }
@@ -465,14 +470,15 @@ static struct drm_framebuffer *__drm_framebuffer_lookup(struct drm_device *dev,
 {
 	struct drm_mode_object *obj = NULL;
 	struct drm_framebuffer *fb;
+	unsigned long flags;
 
-	mutex_lock(&dev->mode_config.idr_mutex);
+	spin_lock_irqsave(&dev->mode_config.idr_lock, flags);
 	obj = idr_find(&dev->mode_config.crtc_idr, id);
 	if (!obj || (obj->type != DRM_MODE_OBJECT_FB) || (obj->id != id))
 		fb = NULL;
 	else
 		fb = obj_to_fb(obj);
-	mutex_unlock(&dev->mode_config.idr_mutex);
+	spin_unlock_irqrestore(&dev->mode_config.idr_lock, flags);
 
 	return fb;
 }
@@ -5049,7 +5055,7 @@ void drm_mode_config_init(struct drm_device *dev)
 {
 	mutex_init(&dev->mode_config.mutex);
 	drm_modeset_lock_init(&dev->mode_config.connection_mutex);
-	mutex_init(&dev->mode_config.idr_mutex);
+	spin_lock_init(&dev->mode_config.idr_lock);
 	mutex_init(&dev->mode_config.fb_lock);
 	INIT_LIST_HEAD(&dev->mode_config.fb_list);
 	INIT_LIST_HEAD(&dev->mode_config.crtc_list);
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index 77d9763..9c57b56 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -742,7 +742,7 @@ struct drm_mode_group {
 /**
  * drm_mode_config - Mode configuration control structure
  * @mutex: mutex protecting KMS related lists and structures
- * @idr_mutex: mutex for KMS ID allocation and management
+ * @idr_lock: lock for KMS ID allocation and management
  * @crtc_idr: main KMS ID tracking object
  * @num_fb: number of fbs available
  * @fb_list: list of framebuffers available
@@ -772,7 +772,7 @@ struct drm_mode_config {
 	struct mutex mutex; /* protects configuration (mode lists etc.) */
 	struct drm_modeset_lock connection_mutex; /* protects connector->encoder and encoder->crtc links */
 	struct drm_modeset_acquire_ctx *acquire_ctx; /* for legacy _lock_all() / _unlock_all() */
-	struct mutex idr_mutex; /* for IDR management */
+	spinlock_t idr_lock; /* for IDR management */
 	struct idr crtc_idr; /* use this idr for all IDs, fb, crtc, connector, modes - just makes life easier */
 	/* this is limited to one for now */
 
-- 
2.1.0