[PATCH 0/4] Introduce userspace-driven ALSA timers

[Ivan Orlov]

There are multiple possible timer sources which could be useful for
the sound stream synchronization: hrtimers, hardware clocks (e.g. PTP),
timer wheels (jiffies). Currently, using one of them to synchronize
the audio stream of snd-aloop module would require writing a
kernel-space driver which exports an ALSA timer through the
snd_timer interface.

However, it is not really convenient for application developers, who may
want to define their custom timer sources for audio synchronization.

For instance, we could have a network application which receives frames
and sends them to snd-aloop pcm device, and another application
listening on the other end of snd-aloop. It makes sense to transfer a
new period of data only when certain amount of frames is received
through the network, but definitely not when a certain amount of jiffies
on a local system elapses. Since all of the devices are purely virtual
it won't introduce any glitches and will help the application developers
to avoid using sample-rate conversion.

This patch series introduces userspace-driven ALSA timers: virtual
timers which are created and controlled from userspace. The timer can
be created from the userspace using the new ioctl SNDRV_TIMER_IOCTL_CREATE.
After creating a timer, it becomes available for use system-wide, so it
can be passed to snd-aloop as a timer source (timer_source parameter
would be "-1.SNDRV_TIMER_GLOBAL_UDRIVEN.{timer_id}"). When the userspace
app decides to trigger a timer, it calls another ioctl
SNDRV_TIMER_IOCTL_TRIGGER on the file descriptor of a timer. It
initiates a transfer of a new period of data.

Userspace-driven timers are associated with file descriptors. If the
application wishes to destroy the timer, it can simply release the file
descriptor of a virtual timer.

I believe introducing new ioctl calls is quite inconvenient (as we have
a limited amount of them), but other possible ways of app <-> kernel
communication (like virtual FS) seem completely inappropriate for this
task (but I'd love to discuss alternative solutions).

This patch series also updates the snd-aloop module so the global timers
can be used as a timer_source for it (it allows using userspace-driven
timers as timer source).

Ivan Orlov (4):
  ALSA: aloop: Allow using global timers
  Docs/sound: Add documentation for userspace-driven ALSA timers
  ALSA: timer: Introduce virtual userspace-driven timers
  selftests: ALSA: Cover userspace-driven timers with test

 Documentation/sound/index.rst               |   1 +
 Documentation/sound/utimers.rst             | 120 +++++++++++
 include/uapi/sound/asound.h                 |  17 ++
 sound/core/Kconfig                          |  11 +
 sound/core/timer.c                          | 226 ++++++++++++++++++++
 sound/drivers/aloop.c                       |   2 +
 tools/testing/selftests/alsa/Makefile       |   2 +-
 tools/testing/selftests/alsa/global-timer.c |  87 ++++++++
 tools/testing/selftests/alsa/utimer-test.c  | 133 ++++++++++++
 9 files changed, 598 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/sound/utimers.rst
 create mode 100644 tools/testing/selftests/alsa/global-timer.c
 create mode 100644 tools/testing/selftests/alsa/utimer-test.c

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