ALSA: core: Add managed card creation

As a second step for preliminary to widen the devres usages among
sound drivers, this patch adds a new ALSA core API function,
snd_devm_card_new(), to create a snd_card object via devres.
When a card object is created by this new function, snd_card_free() is
called automatically and the card object resource gets released at the
device unbinding time.

However, the story isn't that simple.  A caveat is that we have to
call snd_card_free() at the very first of the whole resource release
procedure, in order to assure that the all exposed devices on
user-space are deleted and sync with processes accessing those devices
before releasing resources.

For achieving it, snd_card_register() adds a new devres action to
trigger snd_card_free() automatically when the given card object is a
"managed" one.  Since usually snd_card_register() is the last step of
the initialization, this should work in most cases.

With all these tricks, some drivers can get rid of the whole driver
remove callback code.

About a bit of implementation details: the patch adds two new flags to
snd_card object: managed and releasing.  The former indicates that the
object was created via snd_devm_card_new(), and the latter is used for
avoiding the double-free of snd_card_free() calls.  Both flags are
fairly internal and likely uninteresting to normal users.

Link: https://lore.kernel.org/r/20210715075941.23332-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/core.h

@@ -117,6 +117,8 @@ struct snd_card {
 	struct device card_dev;		/* cardX object for sysfs */
 	const struct attribute_group *dev_groups[4]; /* assigned sysfs attr */
 	bool registered;		/* card_dev is registered? */
+	bool managed;			/* managed via devres */
+	bool releasing;			/* during card free process */
 	int sync_irq;			/* assigned irq, used for PCM sync */
 	wait_queue_head_t remove_sleep;
 
@@ -274,6 +276,9 @@ extern int (*snd_mixer_oss_notify_callback)(struct snd_card *card, int cmd);
 int snd_card_new(struct device *parent, int idx, const char *xid,
 		 struct module *module, int extra_size,
 		 struct snd_card **card_ret);
+int snd_devm_card_new(struct device *parent, int idx, const char *xid,
+		      struct module *module, size_t extra_size,
+		      struct snd_card **card_ret);
 
 int snd_card_disconnect(struct snd_card *card);
 void snd_card_disconnect_sync(struct snd_card *card);

sound/core/init.c

@@ -134,6 +134,9 @@ void snd_device_initialize(struct device *dev, struct snd_card *card)
 }
 EXPORT_SYMBOL_GPL(snd_device_initialize);
 
+static int snd_card_init(struct snd_card *card, struct device *parent,
+			 int idx, const char *xid, struct module *module,
+			 size_t extra_size);
 static int snd_card_do_free(struct snd_card *card);
 static const struct attribute_group card_dev_attr_group;
 
@@ -163,9 +166,6 @@ int snd_card_new(struct device *parent, int idx, const char *xid,
 {
 	struct snd_card *card;
 	int err;
-#ifdef CONFIG_SND_DEBUG
-	char name[8];
-#endif
 
 	if (snd_BUG_ON(!card_ret))
 		return -EINVAL;
@@ -176,6 +176,74 @@ int snd_card_new(struct device *parent, int idx, const char *xid,
 	card = kzalloc(sizeof(*card) + extra_size, GFP_KERNEL);
 	if (!card)
 		return -ENOMEM;
+
+	err = snd_card_init(card, parent, idx, xid, module, extra_size);
+	if (err < 0) {
+		kfree(card);
+		return err;
+	}
+
+	*card_ret = card;
+	return 0;
+}
+EXPORT_SYMBOL(snd_card_new);
+
+static void __snd_card_release(struct device *dev, void *data)
+{
+	snd_card_free(data);
+}
+
+/**
+ * snd_devm_card_new - managed snd_card object creation
+ * @parent: the parent device object
+ * @idx: card index (address) [0 ... (SNDRV_CARDS-1)]
+ * @xid: card identification (ASCII string)
+ * @module: top level module for locking
+ * @extra_size: allocate this extra size after the main soundcard structure
+ * @card_ret: the pointer to store the created card instance
+ *
+ * This function works like snd_card_new() but manages the allocated resource
+ * via devres, i.e. you don't need to free explicitly.
+ *
+ * When a snd_card object is created with this function and registered via
+ * snd_card_register(), the very first devres action to call snd_card_free()
+ * is added automatically.  In that way, the resource disconnection is assured
+ * at first, then released in the expected order.
+ */
+int snd_devm_card_new(struct device *parent, int idx, const char *xid,
+		      struct module *module, size_t extra_size,
+		      struct snd_card **card_ret)
+{
+	struct snd_card *card;
+	int err;
+
+	*card_ret = NULL;
+	card = devres_alloc(__snd_card_release, sizeof(*card) + extra_size,
+			    GFP_KERNEL);
+	if (!card)
+		return -ENOMEM;
+	card->managed = true;
+	err = snd_card_init(card, parent, idx, xid, module, extra_size);
+	if (err < 0) {
+		devres_free(card);
+		return err;
+	}
+
+	devres_add(parent, card);
+	*card_ret = card;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(snd_devm_card_new);
+
+static int snd_card_init(struct snd_card *card, struct device *parent,
+			 int idx, const char *xid, struct module *module,
+			 size_t extra_size)
+{
+	int err;
+#ifdef CONFIG_SND_DEBUG
+	char name[8];
+#endif
+
 	if (extra_size > 0)
 		card->private_data = (char *)card + sizeof(struct snd_card);
 	if (xid)
@@ -197,7 +265,6 @@ int snd_card_new(struct device *parent, int idx, const char *xid,
 		mutex_unlock(&snd_card_mutex);
 		dev_err(parent, "cannot find the slot for index %d (range 0-%i), error: %d\n",
 			 idx, snd_ecards_limit - 1, err);
-		kfree(card);
 		return err;
 	}
 	set_bit(idx, snd_cards_lock);		/* lock it */
@@ -256,8 +323,6 @@ int snd_card_new(struct device *parent, int idx, const char *xid,
 	sprintf(name, "card%d", idx);
 	card->debugfs_root = debugfs_create_dir(name, sound_debugfs_root);
 #endif
-
-	*card_ret = card;
 	return 0;
 
       __error_ctl:
@@ -266,7 +331,6 @@ int snd_card_new(struct device *parent, int idx, const char *xid,
 	put_device(&card->card_dev);
   	return err;
 }
-EXPORT_SYMBOL(snd_card_new);
 
 /**
  * snd_card_ref - Get the card object from the index
@@ -481,6 +545,7 @@ EXPORT_SYMBOL_GPL(snd_card_disconnect_sync);
 
 static int snd_card_do_free(struct snd_card *card)
 {
+	card->releasing = true;
 #if IS_ENABLED(CONFIG_SND_MIXER_OSS)
 	if (snd_mixer_oss_notify_callback)
 		snd_mixer_oss_notify_callback(card, SND_MIXER_OSS_NOTIFY_FREE);
@@ -498,7 +563,8 @@ static int snd_card_do_free(struct snd_card *card)
 #endif
 	if (card->release_completion)
 		complete(card->release_completion);
-	kfree(card);
+	if (!card->managed)
+		kfree(card);
 	return 0;
 }
 
@@ -745,6 +811,14 @@ int snd_card_add_dev_attr(struct snd_card *card,
 }
 EXPORT_SYMBOL_GPL(snd_card_add_dev_attr);
 
+static void trigger_card_free(void *data)
+{
+	struct snd_card *card = data;
+
+	if (!card->releasing)
+		snd_card_free(data);
+}
+
 /**
  *  snd_card_register - register the soundcard
  *  @card: soundcard structure
@@ -768,6 +842,15 @@ int snd_card_register(struct snd_card *card)
 		if (err < 0)
 			return err;
 		card->registered = true;
+	} else {
+		if (card->managed)
+			devm_remove_action(card->dev, trigger_card_free, card);
+	}
+
+	if (card->managed) {
+		err = devm_add_action(card->dev, trigger_card_free, card);
+		if (err < 0)
+			return err;
 	}
 
 	err = snd_device_register_all(card);