Hello, Konrad, Takashi, Takashi and Clemens!

Could you please take a look at this series if it meets ALSA expectations on para-virtualized sound for Xen?

Thank you, Oleksandr

On 02/05/2018 10:24 AM, Oleksandr Andrushchenko wrote:

From: Oleksandr Andrushchenko oleksandr_andrushchenko@epam.com

Hi, all!

Foreword

This change is aimed to add support for explicit back and front synchronization during playback and capture in response to comments raised during upstream attempt of the para-virtualized sound frontend driver for Xen [1], [2] and gather opinions from the relevant communities (ALSA, Xen) on the change.

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) to make sure the proposed solution does work.

Rationale

During the first attempt to upstream the Linux front 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 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
4. Playback/capture buffer size is set on the backend side via XENSND_FIELD_BUFFER_SIZE XenStore entry

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

Oleksandr Andrushchenko (2): sndif: introduce protocol version sndif: add explicit back and front synchronization

xen/include/public/io/sndif.h | 173 +++++++++++++++++++++++++++++++++++++++++- 1 file changed, 170 insertions(+), 3 deletions(-)