ALSA: timer: Introduce virtual userspace-driven timers

Implement two ioctl calls in order to support virtual userspace-driven
ALSA timers.

The first ioctl is SNDRV_TIMER_IOCTL_CREATE, which gets the
snd_timer_uinfo struct as a parameter and puts a file descriptor of a
virtual timer into the `fd` field of the snd_timer_unfo structure. It
also updates the `id` field of the snd_timer_uinfo struct, which
provides a unique identifier for the timer (basically, the subdevice
number which can be used when creating timer instances).

This patch also introduces a tiny id allocator for the userspace-driven
timers, which guarantees that we don't have more than 128 of them in the
system.

Another ioctl is SNDRV_TIMER_IOCTL_TRIGGER, which allows us to trigger
the virtual timer (and calls snd_timer_interrupt for the timer under
the hood), causing all of the timer instances binded to this timer to
execute their callbacks.

The maximum amount of ticks available for the timer is 1 for the sake of
simplicity of the userspace API. 'start', 'stop', 'open' and 'close'
callbacks for the userspace-driven timers are empty since we don't
really do any hardware initialization here.

Suggested-by: Axel Holzinger <aholzinger@gmx.de>
Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20240813120701.171743-4-ivan.orlov0322@gmail.com

include/uapi/sound/asound.h

@@ -869,7 +869,7 @@ struct snd_ump_block_info {
  *  Timer section - /dev/snd/timer
  */
 
-#define SNDRV_TIMER_VERSION		SNDRV_PROTOCOL_VERSION(2, 0, 7)
+#define SNDRV_TIMER_VERSION		SNDRV_PROTOCOL_VERSION(2, 0, 8)
 
 enum {
 	SNDRV_TIMER_CLASS_NONE = -1,
@@ -894,6 +894,7 @@ enum {
 #define SNDRV_TIMER_GLOBAL_RTC		1	/* unused */
 #define SNDRV_TIMER_GLOBAL_HPET		2
 #define SNDRV_TIMER_GLOBAL_HRTIMER	3
+#define SNDRV_TIMER_GLOBAL_UDRIVEN	4
 
 /* info flags */
 #define SNDRV_TIMER_FLG_SLAVE		(1<<0)	/* cannot be controlled */
@@ -974,6 +975,18 @@ struct snd_timer_status {
 };
 #endif
 
+/*
+ * This structure describes the userspace-driven timer. Such timers are purely virtual,
+ * and can only be triggered from software (for instance, by userspace application).
+ */
+struct snd_timer_uinfo {
+	/* To pretend being a normal timer, we need to know the resolution in ns. */
+	__u64 resolution;
+	int fd;
+	unsigned int id;
+	unsigned char reserved[16];
+};
+
 #define SNDRV_TIMER_IOCTL_PVERSION	_IOR('T', 0x00, int)
 #define SNDRV_TIMER_IOCTL_NEXT_DEVICE	_IOWR('T', 0x01, struct snd_timer_id)
 #define SNDRV_TIMER_IOCTL_TREAD_OLD	_IOW('T', 0x02, int)
@@ -990,6 +1003,8 @@ struct snd_timer_status {
 #define SNDRV_TIMER_IOCTL_CONTINUE	_IO('T', 0xa2)
 #define SNDRV_TIMER_IOCTL_PAUSE		_IO('T', 0xa3)
 #define SNDRV_TIMER_IOCTL_TREAD64	_IOW('T', 0xa4, int)
+#define SNDRV_TIMER_IOCTL_CREATE	_IOWR('T', 0xa5, struct snd_timer_uinfo)
+#define SNDRV_TIMER_IOCTL_TRIGGER	_IO('T', 0xa6)
 
 #if __BITS_PER_LONG == 64
 #define SNDRV_TIMER_IOCTL_TREAD SNDRV_TIMER_IOCTL_TREAD_OLD

sound/core/Kconfig

@@ -242,6 +242,16 @@ config SND_JACK_INJECTION_DEBUG
 	  Say Y if you are debugging via jack injection interface.
 	  If unsure select "N".
 
+config SND_UTIMER
+	bool "Enable support for userspace-controlled virtual timers"
+	depends on SND_TIMER
+	help
+	  Say Y to enable the support of userspace-controlled timers. These
+	  timers are purely virtual, and they are supposed to be triggered
+	  from userspace. They could be quite useful when synchronizing the
+	  sound timing with userspace applications (for instance, when sending
+	  data through snd-aloop).
+
 config SND_VMASTER
 	bool
 

sound/core/timer.c

@@ -13,6 +13,8 @@
 #include <linux/module.h>
 #include <linux/string.h>
 #include <linux/sched/signal.h>
+#include <linux/anon_inodes.h>
+#include <linux/idr.h>
 #include <sound/core.h>
 #include <sound/timer.h>
 #include <sound/control.h>
@@ -109,6 +111,16 @@ struct snd_timer_status64 {
 	unsigned char reserved[64];	/* reserved */
 };
 
+#ifdef CONFIG_SND_UTIMER
+#define SNDRV_UTIMERS_MAX_COUNT 128
+/* Internal data structure for keeping the state of the userspace-driven timer */
+struct snd_utimer {
+	char *name;
+	struct snd_timer *timer;
+	unsigned int id;
+};
+#endif
+
 #define SNDRV_TIMER_IOCTL_STATUS64	_IOR('T', 0x14, struct snd_timer_status64)
 
 /* list of timers */
@@ -2009,6 +2021,217 @@ enum {
 	SNDRV_TIMER_IOCTL_PAUSE_OLD = _IO('T', 0x23),
 };
 
+#ifdef CONFIG_SND_UTIMER
+/*
+ * Since userspace-driven timers are passed to userspace, we need to have an identifier
+ * which will allow us to use them (basically, the subdevice number of udriven timer).
+ */
+static DEFINE_IDA(snd_utimer_ids);
+
+static void snd_utimer_put_id(struct snd_utimer *utimer)
+{
+	int timer_id = utimer->id;
+
+	snd_BUG_ON(timer_id < 0 || timer_id >= SNDRV_UTIMERS_MAX_COUNT);
+	ida_free(&snd_utimer_ids, timer_id);
+}
+
+static int snd_utimer_take_id(void)
+{
+	return ida_alloc_max(&snd_utimer_ids, SNDRV_UTIMERS_MAX_COUNT - 1, GFP_KERNEL);
+}
+
+static void snd_utimer_free(struct snd_utimer *utimer)
+{
+	snd_timer_free(utimer->timer);
+	snd_utimer_put_id(utimer);
+	kfree(utimer->name);
+	kfree(utimer);
+}
+
+static int snd_utimer_release(struct inode *inode, struct file *file)
+{
+	struct snd_utimer *utimer = (struct snd_utimer *)file->private_data;
+
+	snd_utimer_free(utimer);
+	return 0;
+}
+
+static int snd_utimer_trigger(struct file *file)
+{
+	struct snd_utimer *utimer = (struct snd_utimer *)file->private_data;
+
+	snd_timer_interrupt(utimer->timer, utimer->timer->sticks);
+	return 0;
+}
+
+static long snd_utimer_ioctl(struct file *file, unsigned int ioctl, unsigned long arg)
+{
+	switch (ioctl) {
+	case SNDRV_TIMER_IOCTL_TRIGGER:
+		return snd_utimer_trigger(file);
+	}
+
+	return -ENOTTY;
+}
+
+static const struct file_operations snd_utimer_fops = {
+	.llseek = noop_llseek,
+	.release = snd_utimer_release,
+	.unlocked_ioctl = snd_utimer_ioctl,
+};
+
+static int snd_utimer_start(struct snd_timer *t)
+{
+	return 0;
+}
+
+static int snd_utimer_stop(struct snd_timer *t)
+{
+	return 0;
+}
+
+static int snd_utimer_open(struct snd_timer *t)
+{
+	return 0;
+}
+
+static int snd_utimer_close(struct snd_timer *t)
+{
+	return 0;
+}
+
+static const struct snd_timer_hardware timer_hw = {
+	.flags = SNDRV_TIMER_HW_AUTO | SNDRV_TIMER_HW_WORK,
+	.open = snd_utimer_open,
+	.close = snd_utimer_close,
+	.start = snd_utimer_start,
+	.stop = snd_utimer_stop,
+};
+
+static int snd_utimer_create(struct snd_timer_uinfo *utimer_info,
+			     struct snd_utimer **r_utimer)
+{
+	struct snd_utimer *utimer;
+	struct snd_timer *timer;
+	struct snd_timer_id tid;
+	int utimer_id;
+	int err = 0;
+
+	if (!utimer_info || utimer_info->resolution == 0)
+		return -EINVAL;
+
+	utimer = kzalloc(sizeof(*utimer), GFP_KERNEL);
+	if (!utimer)
+		return -ENOMEM;
+
+	/* We hold the ioctl lock here so we won't get a race condition when allocating id */
+	utimer_id = snd_utimer_take_id();
+	if (utimer_id < 0) {
+		err = utimer_id;
+		goto err_take_id;
+	}
+
+	utimer->name = kasprintf(GFP_KERNEL, "snd-utimer%d", utimer_id);
+	if (!utimer->name) {
+		err = -ENOMEM;
+		goto err_get_name;
+	}
+
+	utimer->id = utimer_id;
+
+	tid.dev_sclass = SNDRV_TIMER_SCLASS_APPLICATION;
+	tid.dev_class = SNDRV_TIMER_CLASS_GLOBAL;
+	tid.card = -1;
+	tid.device = SNDRV_TIMER_GLOBAL_UDRIVEN;
+	tid.subdevice = utimer_id;
+
+	err = snd_timer_new(NULL, utimer->name, &tid, &timer);
+	if (err < 0) {
+		pr_err("Can't create userspace-driven timer\n");
+		goto err_timer_new;
+	}
+
+	timer->module = THIS_MODULE;
+	timer->hw = timer_hw;
+	timer->hw.resolution = utimer_info->resolution;
+	timer->hw.ticks = 1;
+	timer->max_instances = MAX_SLAVE_INSTANCES;
+
+	utimer->timer = timer;
+
+	err = snd_timer_global_register(timer);
+	if (err < 0) {
+		pr_err("Can't register a userspace-driven timer\n");
+		goto err_timer_reg;
+	}
+
+	*r_utimer = utimer;
+	return 0;
+
+err_timer_reg:
+	snd_timer_free(timer);
+err_timer_new:
+	kfree(utimer->name);
+err_get_name:
+	snd_utimer_put_id(utimer);
+err_take_id:
+	kfree(utimer);
+
+	return err;
+}
+
+static int snd_utimer_ioctl_create(struct file *file,
+				   struct snd_timer_uinfo __user *_utimer_info)
+{
+	struct snd_utimer *utimer;
+	struct snd_timer_uinfo *utimer_info __free(kfree) = NULL;
+	int err, timer_fd;
+
+	utimer_info = memdup_user(_utimer_info, sizeof(*utimer_info));
+	if (IS_ERR(utimer_info))
+		return PTR_ERR(no_free_ptr(utimer_info));
+
+	err = snd_utimer_create(utimer_info, &utimer);
+	if (err < 0)
+		return err;
+
+	utimer_info->id = utimer->id;
+
+	timer_fd = anon_inode_getfd(utimer->name, &snd_utimer_fops, utimer, O_RDWR | O_CLOEXEC);
+	if (timer_fd < 0) {
+		snd_utimer_free(utimer);
+		return timer_fd;
+	}
+
+	utimer_info->fd = timer_fd;
+
+	err = copy_to_user(_utimer_info, utimer_info, sizeof(*utimer_info));
+	if (err) {
+		/*
+		 * "Leak" the fd, as there is nothing we can do about it.
+		 * It might have been closed already since anon_inode_getfd
+		 * makes it available for userspace.
+		 *
+		 * We have to rely on the process exit path to do any
+		 * necessary cleanup (e.g. releasing the file).
+		 */
+		return -EFAULT;
+	}
+
+	return 0;
+}
+
+#else
+
+static int snd_utimer_ioctl_create(struct file *file,
+				   struct snd_timer_uinfo __user *_utimer_info)
+{
+	return -ENOTTY;
+}
+
+#endif
+
 static long __snd_timer_user_ioctl(struct file *file, unsigned int cmd,
 				 unsigned long arg, bool compat)
 {
@@ -2053,6 +2276,8 @@ static long __snd_timer_user_ioctl(struct file *file, unsigned int cmd,
 	case SNDRV_TIMER_IOCTL_PAUSE:
 	case SNDRV_TIMER_IOCTL_PAUSE_OLD:
 		return snd_timer_user_pause(file);
+	case SNDRV_TIMER_IOCTL_CREATE:
+		return snd_utimer_ioctl_create(file, argp);
 	}
 	return -ENOTTY;
 }