ALSA: control: Add kcontrol callbacks for lock/unlock

This allows drivers to implement policy around locking/unlocking
controls, such as enforcing that a group of controls may only be locked
by the same process/file, and taking actions when the controls
lock/unlock (such as granting special access on lock and resetting
values on unlock).

This is, in particular, useful to implement volume safety controls, such
that only a particular process (that locks controls and completes a
handshake) may increase volumes above a given safe limit. It also allows
the volume to be automatically lowered if that process dies (which will
trigger an implicit unlock).

Signed-off-by: Hector Martin <marcan@marcan.st>

Author: marcan

include/sound/control.h

@@ -14,9 +14,12 @@
 #define snd_kcontrol_chip(kcontrol) ((kcontrol)->private_data)
 
 struct snd_kcontrol;
+struct snd_ctl_file;
 typedef int (snd_kcontrol_info_t) (struct snd_kcontrol * kcontrol, struct snd_ctl_elem_info * uinfo);
 typedef int (snd_kcontrol_get_t) (struct snd_kcontrol * kcontrol, struct snd_ctl_elem_value * ucontrol);
 typedef int (snd_kcontrol_put_t) (struct snd_kcontrol * kcontrol, struct snd_ctl_elem_value * ucontrol);
+typedef int (snd_kcontrol_lock_t) (struct snd_kcontrol * kcontrol, struct snd_ctl_file *owner);
+typedef void (snd_kcontrol_unlock_t) (struct snd_kcontrol * kcontrol);
 typedef int (snd_kcontrol_tlv_rw_t)(struct snd_kcontrol *kcontrol,
 				    int op_flag, /* SNDRV_CTL_TLV_OP_XXX */
 				    unsigned int size,
@@ -55,6 +58,8 @@ struct snd_kcontrol_new {
 	snd_kcontrol_info_t *info;
 	snd_kcontrol_get_t *get;
 	snd_kcontrol_put_t *put;
+	snd_kcontrol_lock_t *lock;
+	snd_kcontrol_unlock_t *unlock;
 	union {
 		snd_kcontrol_tlv_rw_t *c;
 		const unsigned int *p;
@@ -74,6 +79,8 @@ struct snd_kcontrol {
 	snd_kcontrol_info_t *info;
 	snd_kcontrol_get_t *get;
 	snd_kcontrol_put_t *put;
+	snd_kcontrol_lock_t *lock;
+	snd_kcontrol_unlock_t *unlock;
 	union {
 		snd_kcontrol_tlv_rw_t *c;
 		const unsigned int *p;

sound/core/control.c

@@ -123,10 +123,12 @@ static int snd_ctl_release(struct inode *inode, struct file *file)
 	scoped_guard(rwsem_write, &card->controls_rwsem) {
 		list_for_each_entry(control, &card->controls, list)
 			for (idx = 0; idx < control->count; idx++)
-				if (control->vd[idx].owner == ctl)
+				if (control->vd[idx].owner == ctl) {
 					control->vd[idx].owner = NULL;
+					if (control->unlock)
+						control->unlock(control);
+				}
 	}
-
 	snd_fasync_free(ctl->fasync);
 	snd_ctl_empty_read_queue(ctl);
 	put_pid(ctl->pid);
@@ -303,6 +305,8 @@ struct snd_kcontrol *snd_ctl_new1(const struct snd_kcontrol_new *ncontrol,
 	kctl->info = ncontrol->info;
 	kctl->get = ncontrol->get;
 	kctl->put = ncontrol->put;
+	kctl->lock = ncontrol->lock;
+	kctl->unlock = ncontrol->unlock;
 	kctl->tlv.p = ncontrol->tlv.p;
 
 	kctl->private_value = ncontrol->private_value;
@@ -1359,6 +1363,12 @@ static int snd_ctl_elem_lock(struct snd_ctl_file *file,
 	vd = &kctl->vd[snd_ctl_get_ioff(kctl, &id)];
 	if (vd->owner)
 		return -EBUSY;
+
+	if (kctl->lock) {
+		int err = kctl->lock(kctl, file);
+		if (err < 0)
+			return err;
+	}
 	vd->owner = file;
 	return 0;
 }
@@ -1383,6 +1393,8 @@ static int snd_ctl_elem_unlock(struct snd_ctl_file *file,
 	if (vd->owner != file)
 		return -EPERM;
 	vd->owner = NULL;
+	if (kctl->unlock)
+		kctl->unlock(kctl);
 	return 0;
 }