1 .. SPDX-License-Identifier: GPL-2.0
2
3 =======================
4 Userspace-driven timers
5 =======================
6
7 :Author: Ivan Orlov <ivan.orlov0322@gmail.com>
8
9 Preface
10 =======
11
12 This document describes the userspace-driven timers: virtual ALSA timers
13 which could be created and controlled by userspace applications using
14 IOCTL calls. Such timers could be useful when synchronizing audio
15 stream with timer sources which we don't have ALSA timers exported for
16 (e.g. PTP clocks), and when synchronizing the audio stream going through
17 two virtual sound devices using ``snd-aloop`` (for instance, when
18 we have a network application sending frames to one snd-aloop device,
19 and another sound application listening on the other end of snd-aloop).
20
21 Enabling userspace-driven timers
22 ================================
23
24 The userspace-driven timers could be enabled in the kernel using the
25 ``CONFIG_SND_UTIMER`` configuration option. It depends on the
26 ``CONFIG_SND_TIMER`` option, so it also should be enabled.
27
28 Userspace-driven timers API
29 ===========================
30
31 Userspace application can create a userspace-driven ALSA timer by
32 executing the ``SNDRV_TIMER_IOCTL_CREATE`` ioctl call on the
33 ``/dev/snd/timer`` device file descriptor. The ``snd_timer_uinfo``
34 structure should be passed as an ioctl argument:
35
36 ::
37
38 struct snd_timer_uinfo {
39 __u64 resolution;
40 int fd;
41 unsigned int id;
42 unsigned char reserved[16];
43 }
44
45 The ``resolution`` field sets the desired resolution in nanoseconds for
46 the virtual timer. ``resolution`` field simply provides an information
47 about the virtual timer, but does not affect the timing itself. ``id``
48 field gets overwritten by the ioctl, and the identifier you get in this
49 field after the call can be used as a timer subdevice number when
50 passing the timer to ``snd-aloop`` kernel module or other userspace
51 applications. There could be up to 128 userspace-driven timers in the
52 system at one moment of time, thus the id value ranges from 0 to 127.
53
54 Besides from overwriting the ``snd_timer_uinfo`` struct, ioctl stores
55 a timer file descriptor, which can be used to trigger the timer, in the
56 ``fd`` field of the ``snd_timer_uinfo`` struct. Allocation of a file
57 descriptor for the timer guarantees that the timer can only be triggered
58 by the process which created it. The timer then can be triggered with
59 ``SNDRV_TIMER_IOCTL_TRIGGER`` ioctl call on the timer file descriptor.
60
61 So, the example code for creating and triggering the timer would be:
62
63 ::
64
65 static struct snd_timer_uinfo utimer_info = {
66 /* Timer is going to tick (presumably) every 1000000 ns */
67 .resolution = 1000000ULL,
68 .id = -1,
69 };
70
71 int timer_device_fd = open("/dev/snd/timer", O_RDWR | O_CLOEXEC);
72
73 if (ioctl(timer_device_fd, SNDRV_TIMER_IOCTL_CREATE, &utimer_info)) {
74 perror("Failed to create the timer");
75 return -1;
76 }
77
78 ...
79
80 /*
81 * Now we want to trigger the timer. Callbacks of all of the
82 * timer instances binded to this timer will be executed after
83 * this call.
84 */
85 ioctl(utimer_info.fd, SNDRV_TIMER_IOCTL_TRIGGER, NULL);
86
87 ...
88
89 /* Now, destroy the timer */
90 close(timer_info.fd);
91
92
93 More detailed example of creating and ticking the timer could be found
94 in the utimer ALSA selftest.
95
96 Userspace-driven timers and snd-aloop
97 -------------------------------------
98
99 Userspace-driven timers could be easily used with ``snd-aloop`` module
100 when synchronizing two sound applications on both ends of the virtual
101 sound loopback. For instance, if one of the applications receives sound
102 frames from network and sends them to snd-aloop pcm device, and another
103 application listens for frames on the other snd-aloop pcm device, it
104 makes sense that the ALSA middle layer should initiate a data
105 transaction when the new period of data is received through network, but
106 not when the certain amount of jiffies elapses. Userspace-driven ALSA
107 timers could be used to achieve this.
108
109 To use userspace-driven ALSA timer as a timer source of snd-aloop, pass
110 the following string as the snd-aloop ``timer_source`` parameter:
111
112 ::
113
114 # modprobe snd-aloop timer_source="-1.4.<utimer_id>"
115
116 Where ``utimer_id`` is the id of the timer you created with
117 ``SNDRV_TIMER_IOCTL_CREATE``, and ``4`` is the number of
118 userspace-driven timers device (``SNDRV_TIMER_GLOBAL_UDRIVEN``).
119
120 ``resolution`` for the userspace-driven ALSA timer used with snd-aloop
121 should be calculated as ``1000000000ULL / frame_rate * period_size`` as
122 the timer is going to tick every time a new period of frames is ready.
123
124 After that, each time you trigger the timer with
125 ``SNDRV_TIMER_IOCTL_TRIGGER`` the new period of data will be transferred
126 from one snd-aloop device to another.
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