On 03/06/2018 02:52 PM, Takashi Iwai wrote:
On Tue, 06 Mar 2018 13:05:16 +0100,
Oleksandr Andrushchenko wrote:
On 03/06/2018 01:32 PM, Takashi Iwai wrote:
On Tue, 06 Mar 2018 12:25:07 +0100,
Oleksandr Andrushchenko wrote:
On 03/06/2018 12:52 PM, Takashi Iwai wrote:
On Mon, 05 Feb 2018 09:24:58 +0100,
Oleksandr Andrushchenko wrote:
From: Oleksandr Andrushchenko <oleksandr_andrushche...@epam.com>
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 ,  and gather opinions from the relevant communities
(ALSA, Xen) on the change.
The relevant backend is implemented as a user-space application 
and uses accompanying helper library .
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.
During the first attempt to upstream the Linux front driver  number
of comments and concerns were raised, one of the biggest flaws in the
design were questioned by both Clemens Ladisch  and
Takashi Sakamoto : 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
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
4. Playback/capture buffer size is set on the backend side via
XENSND_FIELD_BUFFER_SIZE XenStore entry
So the new addition looks serving well for the point that was
suggested in the previous thread. As I see no frontend driver
implementation, it's hard to tell about the details, but through a
quick glance, the protocol should be OK.
Thank you, the driver is at 
Now, going back to a big picture: I took a look at the previous
patchset, and wonder what about the hw_params setup. Basically the
(frontend) application may request any size of buffer and periods
unless the driver sets up the hw constraints at open callback. That
is, app may request even the 16 bytes of buffer size, or 1GB of
buffer. The periods aren't always integer, so it can be 1024 bytes of
buffer with 400 bytes of periods.
And, if such parameters are set up freely in the frontend side, how
the backend is supposed to behave? From the frontend POV, it expects
receiving the wakeup/notification at each period processing (e.g. 400
bytes in the case above). But, the backend is another application, so
how would it work for such requirements? Am I missing something here?
Well, the frontend is not that free to decide as it might look like,
e.g. please see . Basically part of hw_params configuration is written
to XenStore  as a part of domain configuration which depends on
capabilities. E.g., we usually set buffer sizes to match real HW at
if we use ALSA and we have more freedom if we use PulseAudio there.
Finally, if backend decides that the requested buffer/period sizes are
not acceptable it will reject such a configuration.
OK, that restricts minimally. So at least there is the restriction /
communication about the buffer size. But it merely means the
*maximum* buffer size is set. Application may request still any
shorter value than that.
And, there are no restriction about period sizes (except for the
periods_max, which is calculated from buffer_bytes_max).
That is, application may request any size between them; and it expects
the wake up by this value.
I think that's a still missing stone in the design.
Well, so what would a real HW driver do in that case?
My understanding is that in this case SW can still request
something that HW can't do and driver will reject such configurations.
In my case, the role of that HW driver code which judges on if configuration
is acceptable just runs on the backend side, e.g. frontend driver is just
a proxy which talks to the backend to check if the backend can do what
And it is up to backend to decide.
Does that sound reasonable or you have something else on your mind?
Usually the hardware driver knows already the restrictions and sets up
the rules via hw constraints at open callback. There are lots of
snd_pcm_hw_constraint_*() helpers (and the relevant ones) to give more
additional rules for the parameter restrictions. For example, if the
periods must be aligned with the buffer size (i.e. buffer_size %
period_size == 0) as on many devices, you can call like:
in the open callback.
You are right, I saw those in other drivers
And, now an open question for XEN comes: what kind of restriction
should be applied to the frontend. Obviously it depends on the
backend, so there must be some communication, and the restriction must
be propagated at open, i.e. *before* actually hw_params is performed.
Could you please give me a hint of what those restrictions
could look like? E.g. map of supported buffer/period sizes, what else?
Waiting for your valuable comments,
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(-)
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