[alsa-devel] [Xen-devel][PATCH 0/2] sndif: add explicit back and front synchronization

[Oleksandr Andrushchenko]

Any chance ALSA community may also review the below?

BTW, the driver, with the changes proposed, is at [1]

Thank you,

Oleksandr

On 02/05/2018 10:24 AM, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko at 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/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
>
>
> 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(-)
>
[1] 
https://github.com/andr2000/linux/commits/tiwai_sound_for_next_pv_snd_upstream_v1