[RFC v1] ALSA: xen-front: Add Xen para-virtualized frontend driver

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@epam.com>]

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/include/xen/interface/io/sndif.h
[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/095d7feae00bf00c852c67c4f1044de5601678ed