Hi, all!
Foreword ========
This RFC is aimed to introduce support of para-virtualized sound frontend driver for Xen [1] and gather opinions from the relevant communities (ALSA, Xen). It implements the protocol from [2] with the following limitations: - mute/unmute is not supported - get/set volume is not supported Volume control is not supported for the reason that most of the use-cases (at the moment) are based on scenarios where unprivileged OS (e.g. Android, AGL etc) uses software mixers. Both capture and playback are supported.
The relevant backend is implemented as a user-space application [3] and uses accompanying helper library [4].
Both frontend driver and backend were tested on real HW running Xen hypervisor (Renesas R-Car ARM based H3/M3 boards, x86).
Discussion ==========
During the first attempt to upstream the driver [5] number of comments and concerns were raised, one of the biggest flaws in the design were questioned by both Clemens Ladisch [6] and Takashi Sakamoto [7]: the absence of synchronization between frontend and backend during capture/playback. Two options were discussed:
“In design of ALSA PCM core, drivers are expected to synchronize to actual hardwares for semi-realtime data transmission. The synchronization is done by two points: 1) Interrupts to respond events from actual hardwares. 2) Positions of actual data transmission in any serial sound interfaces of actual hardwares. “
and finally a change to the existing protocol was suggested:
“In 'include/xen/interface/io/sndif.h', there's no functionalities I described the above: 1. notifications from DomU to Dom0 about the size of period for interrupts from actual hardwares. Or no way from Dom0 to DomU about the configured size of the period. 2. notifications of the interrupts from actual hardwares to DomU.”
This is implemented as a change to the sndif protocol [8] and allows removing period emulation: 1. Introduced a new event channel from back to front 2. New event with number of bytes played/captured (XENSND_EVT_CUR_POS, to be used for sending snd_pcm_period_elapsed at frontend (in Linux implementation). Sent in bytes, not frames to make the protocol generic and consistent) 3. New request for playback/capture control (XENSND_OP_TRIGGER) with start/pause/stop/resume sub-ops.
Along with these changes other comments on the driver were addressed, e.g. split into smaller chunks, moved the driver from misc to xen etc.
Hope, this helps to get the full picture of what was discussed and makes it possible to move forward.
Waiting for your valuable comments,
Thank you, Oleksandr
[1] https://github.com/andr2000/linux/commits/snd_upstream_v1 [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/incl... [3] https://github.com/xen-troops/snd_be [4] https://github.com/xen-troops/libxenbe [5] https://lkml.org/lkml/2017/8/7/363 [6] http://mailman.alsa-project.org/pipermail/alsa-devel/2017-August/123617.html [7] http://mailman.alsa-project.org/pipermail/alsa-devel/2017-August/123744.html [8] https://github.com/andr2000/linux/commit/095d7feae00bf00c852c67c4f1044de5601...