Add the documentation which describes the new userspace-driven timers API introduced in this patch series. The documentation contains:
- Description of userspace-driven ALSA timers, what they are for - Description of the timers API - Example of how the timers can be created and triggered - How the timers can be used as a timer sources for snd-aloop module
Suggested-by: Axel Holzinger aholzinger@gmx.de Signed-off-by: Ivan Orlov ivan.orlov0322@gmail.com --- Documentation/sound/index.rst | 1 + Documentation/sound/utimers.rst | 120 ++++++++++++++++++++++++++++++++ 2 files changed, 121 insertions(+) create mode 100644 Documentation/sound/utimers.rst
diff --git a/Documentation/sound/index.rst b/Documentation/sound/index.rst index 7e67e12730d3..c437f2a4bc85 100644 --- a/Documentation/sound/index.rst +++ b/Documentation/sound/index.rst @@ -13,6 +13,7 @@ Sound Subsystem Documentation alsa-configuration hd-audio/index cards/index + utimers
.. only:: subproject and html
diff --git a/Documentation/sound/utimers.rst b/Documentation/sound/utimers.rst new file mode 100644 index 000000000000..0bd875ea9986 --- /dev/null +++ b/Documentation/sound/utimers.rst @@ -0,0 +1,120 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======================= +Userspace-driven timers +======================= + +:Author: Ivan Orlov ivan.orlov0322@gmail.com + +Preface +======= + +This document describes the userspace-driven timers: virtual ALSA timers +which could be created and controlled by userspace applications using +IOCTL calls. Such timers could be useful when synchronizing audio +stream with timer sources which we don't have ALSA timers exported for +(e.g. PTP clocks), and when synchronizing the audio stream going through +two virtual sound devices using ``snd-aloop`` (for instance, when +we have a network application sending frames to one snd-aloop device, +and another sound application listening on the other end of snd-aloop). + +Enabling userspace-driven timers +================================ + +The userspace-driven timers could be enabled in the kernel using the +``CONFIG_SND_UTIMER`` configuration option. It depends on the +``CONFIG_SND_TIMER`` option, so it also should be enabled. + +Userspace-driven timers API +=========================== + +Userspace application can create a userspace-driven ALSA timer by +executing the ``SNDRV_TIMER_IOCTL_CREATE`` ioctl call on the +``/dev/snd/timer`` device file descriptor. The ``snd_utimer_info`` +structure should be passed as an ioctl argument: + +:: + + struct snd_utimer_info { + snd_pcm_uframes_t frame_rate; + snd_pcm_uframes_t period_size; + unsigned int id; + } + +``frame_rate`` and ``period_size`` set the desired frame rate and period +size emulated by the virtual timer respectively. ``id`` field gets +overwritten by the ioctl, and the identifier you get in this field after +the call can be used as a timer subdevice number when passing the timer +to ``snd-aloop`` kernel module or other userspace applications. There +could be up to 128 userspace-driven timers in the system at one moment +of time, thus the id value ranges from 0 to 127. + +Besides from overwriting the ``snd_utimer_info`` struct, ioctl returns +a timer file descriptor, which can be used to trigger the timer. This +guarantees that the timer can only be triggered by the process which +created it. The timer then can be triggered with +``SNDRV_TIMER_IOCTL_TRIGGER`` ioctl call on the timer file descriptor. + +So, the example code for creating and triggering the timer would be: + +:: + + static const struct snd_utimer_info utimer_info = { + .frame_rate = 8000, + .period_size = 4410, + .id = -1, + }; + + int timer_device_fd = open("/dev/snd/timer", O_RDWR | O_CLOEXEC); + int utimer_fd = ioctl(timer_device_fd, SNDRV_TIMER_IOCTL_CREATE, &utimer_info); + + if (utimer_fd < 0) { + perror("Failed to create the timer"); + return -1; + } + + ... + + /* + * Now we want to trigger the timer. Callbacks of all of the + * timer instances binded to this timer will be executed after + * this call. + */ + ioctl(utimer_fd, SNDRV_TIMER_IOCTL_TRIGGER, NULL); + + ... + + /* Now, destroy the timer */ + close(utimer_fd); + + +More detailed example of creating and ticking the timer could be found +in the utimer ALSA selftest. + +Userspace-driven timers and snd-aloop +------------------------------------- + +Userspace-driven timers could be easily used with ``snd-aloop`` module +when synchronizing two sound applications on both ends of the virtual +sound loopback. For instance, if one of the applications receives sound +frames from network and sends them to snd-aloop pcm device, and another +application listens for frames on the other snd-aloop pcm device, it +makes sense that the ALSA middle layer should initiate a data +transaction when the new period of data is received through network, but +not when the certain amount of jiffies elapses. Userspace-driven ALSA +timers could be used to achieve this. + +To use userspace-driven ALSA timer as a timer source of snd-aloop, pass +the following string as the snd-aloop ``timer_source`` parameter: + +:: + + # modprobe snd-aloop timer_source="-1.4.<utimer_id>" + +Where ``utimer_id`` is the id of the timer you created with +``SNDRV_TIMER_IOCTL_CREATE``, and ``4`` is the number of +userspace-driven timers device (``SNDRV_TIMER_GLOBAL_UDRIVEN``). + +After that, each time you trigger the timer with +``SNDRV_TIMER_IOCTL_TRIGGER`` the new period of data will be transferred +from one snd-aloop device to another.