ALSA: control - add layer registration routines

The layer registration allows to handle an extra functionality
on top of the control API. It can be used for the audio
LED control for example.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20210317172945.842280-3-perex@perex.cz
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/control.h

@@ -108,6 +108,14 @@ struct snd_ctl_file {
 	struct list_head events;	/* waiting events for read */
 };
 
+struct snd_ctl_layer_ops {
+	struct snd_ctl_layer_ops *next;
+	const char *module_name;
+	void (*lregister)(struct snd_card *card);
+	void (*ldisconnect)(struct snd_card *card);
+	void (*lnotify)(struct snd_card *card, unsigned int mask, struct snd_kcontrol *kctl, unsigned int ioff);
+};
+
 #define snd_ctl_file(n) list_entry(n, struct snd_ctl_file, list)
 
 typedef int (*snd_kctl_ioctl_func_t) (struct snd_card * card,
@@ -140,6 +148,10 @@ int snd_ctl_unregister_ioctl_compat(snd_kctl_ioctl_func_t fcn);
 #define snd_ctl_unregister_ioctl_compat(fcn)
 #endif
 
+int snd_ctl_request_layer(const char *module_name);
+void snd_ctl_register_layer(struct snd_ctl_layer_ops *lops);
+void snd_ctl_disconnect_layer(struct snd_ctl_layer_ops *lops);
+
 int snd_ctl_get_preferred_subdevice(struct snd_card *card, int type);
 
 static inline unsigned int snd_ctl_get_ioffnum(struct snd_kcontrol *kctl, struct snd_ctl_elem_id *id)

sound/core/control.c

@@ -28,10 +28,12 @@ struct snd_kctl_ioctl {
 };
 
 static DECLARE_RWSEM(snd_ioctl_rwsem);
+static DECLARE_RWSEM(snd_ctl_layer_rwsem);
 static LIST_HEAD(snd_control_ioctls);
 #ifdef CONFIG_COMPAT
 static LIST_HEAD(snd_control_compat_ioctls);
 #endif
+static struct snd_ctl_layer_ops *snd_ctl_layer;
 
 static int snd_ctl_open(struct inode *inode, struct file *file)
 {
@@ -195,10 +197,15 @@ void snd_ctl_notify_one(struct snd_card *card, unsigned int mask,
 			struct snd_kcontrol *kctl, unsigned int ioff)
 {
 	struct snd_ctl_elem_id id = kctl->id;
+	struct snd_ctl_layer_ops *lops;
 
 	id.index += ioff;
 	id.numid += ioff;
 	snd_ctl_notify(card, mask, &id);
+	down_read(&snd_ctl_layer_rwsem);
+	for (lops = snd_ctl_layer; lops; lops = lops->next)
+		lops->lnotify(card, mask, kctl, ioff);
+	up_read(&snd_ctl_layer_rwsem);
 }
 EXPORT_SYMBOL(snd_ctl_notify_one);
 
@@ -1997,6 +2004,86 @@ EXPORT_SYMBOL_GPL(snd_ctl_get_preferred_subdevice);
 #define snd_ctl_ioctl_compat	NULL
 #endif
 
+/*
+ * control layers (audio LED etc.)
+ */
+
+/**
+ * snd_ctl_request_layer - request to use the layer
+ * @module_name: Name of the kernel module (NULL == build-in)
+ *
+ * Return an error code when the module cannot be loaded.
+ */
+int snd_ctl_request_layer(const char *module_name)
+{
+	struct snd_ctl_layer_ops *lops;
+
+	if (module_name == NULL)
+		return 0;
+	down_read(&snd_ctl_layer_rwsem);
+	for (lops = snd_ctl_layer; lops; lops = lops->next)
+		if (strcmp(lops->module_name, module_name) == 0)
+			break;
+	up_read(&snd_ctl_layer_rwsem);
+	if (lops)
+		return 0;
+	return request_module(module_name);
+}
+EXPORT_SYMBOL_GPL(snd_ctl_request_layer);
+
+/**
+ * snd_ctl_register_layer - register new control layer
+ * @lops: operation structure
+ *
+ * The new layer can track all control elements and do additional
+ * operations on top (like audio LED handling).
+ */
+void snd_ctl_register_layer(struct snd_ctl_layer_ops *lops)
+{
+	struct snd_card *card;
+	int card_number;
+
+	down_write(&snd_ctl_layer_rwsem);
+	lops->next = snd_ctl_layer;
+	snd_ctl_layer = lops;
+	up_write(&snd_ctl_layer_rwsem);
+	for (card_number = 0; card_number < SNDRV_CARDS; card_number++) {
+		card = snd_card_ref(card_number);
+		if (card) {
+			down_read(&card->controls_rwsem);
+			lops->lregister(card);
+			up_read(&card->controls_rwsem);
+			snd_card_unref(card);
+		}
+	}
+}
+EXPORT_SYMBOL_GPL(snd_ctl_register_layer);
+
+/**
+ * snd_ctl_disconnect_layer - disconnect control layer
+ * @lops: operation structure
+ *
+ * It is expected that the information about tracked cards
+ * is freed before this call (the disconnect callback is
+ * not called here).
+ */
+void snd_ctl_disconnect_layer(struct snd_ctl_layer_ops *lops)
+{
+	struct snd_ctl_layer_ops *lops2, *prev_lops2;
+
+	down_write(&snd_ctl_layer_rwsem);
+	for (lops2 = snd_ctl_layer, prev_lops2 = NULL; lops2; lops2 = lops2->next)
+		if (lops2 == lops) {
+			if (!prev_lops2)
+				snd_ctl_layer = lops->next;
+			else
+				prev_lops2->next = lops->next;
+			break;
+		}
+	up_write(&snd_ctl_layer_rwsem);
+}
+EXPORT_SYMBOL_GPL(snd_ctl_disconnect_layer);
+
 /*
  *  INIT PART
  */
@@ -2020,9 +2107,20 @@ static const struct file_operations snd_ctl_f_ops =
 static int snd_ctl_dev_register(struct snd_device *device)
 {
 	struct snd_card *card = device->device_data;
+	struct snd_ctl_layer_ops *lops;
+	int err;
 
-	return snd_register_device(SNDRV_DEVICE_TYPE_CONTROL, card, -1,
-				   &snd_ctl_f_ops, card, &card->ctl_dev);
+	err = snd_register_device(SNDRV_DEVICE_TYPE_CONTROL, card, -1,
+				  &snd_ctl_f_ops, card, &card->ctl_dev);
+	if (err < 0)
+		return err;
+	down_read(&card->controls_rwsem);
+	down_read(&snd_ctl_layer_rwsem);
+	for (lops = snd_ctl_layer; lops; lops = lops->next)
+		lops->lregister(card);
+	up_read(&snd_ctl_layer_rwsem);
+	up_read(&card->controls_rwsem);
+	return 0;
 }
 
 /*
@@ -2032,6 +2130,7 @@ static int snd_ctl_dev_disconnect(struct snd_device *device)
 {
 	struct snd_card *card = device->device_data;
 	struct snd_ctl_file *ctl;
+	struct snd_ctl_layer_ops *lops;
 	unsigned long flags;
 
 	read_lock_irqsave(&card->ctl_files_rwlock, flags);
@@ -2041,6 +2140,13 @@ static int snd_ctl_dev_disconnect(struct snd_device *device)
 	}
 	read_unlock_irqrestore(&card->ctl_files_rwlock, flags);
 
+	down_read(&card->controls_rwsem);
+	down_read(&snd_ctl_layer_rwsem);
+	for (lops = snd_ctl_layer; lops; lops = lops->next)
+		lops->ldisconnect(card);
+	up_read(&snd_ctl_layer_rwsem);
+	up_read(&card->controls_rwsem);
+
 	return snd_unregister_device(&card->ctl_dev);
 }