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).
V1 -> V2: - Fix some problems found by Christophe Jaillet christophe.jaillet@wanadoo.fr V2 -> V3: - Add improvements suggested by Takashi Iwai tiwai@suse.de V3 -> V4: - Address comments from Jaroslav Kysela perex@perex.cz and Mark Brown broonie@kernel.org V4 -> V5: - Add missing error processing noticed by Takashi Iwai tiwai@suse.de - Return timer file descriptor as part of the snd_timer_uinfo structure. This is a more standard way of using ioctl interface, where the return value of the ioctl is either 0 or an error code.
Please, find the patch-specific changelog in the following patches.
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 | 126 +++++++++++ include/uapi/sound/asound.h | 17 +- sound/core/Kconfig | 10 + sound/core/timer.c | 225 ++++++++++++++++++++ sound/drivers/aloop.c | 2 + tools/testing/selftests/alsa/Makefile | 4 +- tools/testing/selftests/alsa/global-timer.c | 87 ++++++++ tools/testing/selftests/alsa/utimer-test.c | 164 ++++++++++++++ 9 files changed, 633 insertions(+), 3 deletions(-) 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