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Re: [Xen-devel] [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized frontend driver

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  Re: [Xen-devel] [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen  para-virtualized frontend driver Oleksandr Grytsov <al1img@gmail.com> - 2017-08-17 12:10 +0200
    Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized  frontend driver Takashi Sakamoto <o-takashi@sakamocchi.jp> - 2017-08-18 07:50 +0200
      Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized  frontend driver Oleksandr Andrushchenko <andr2000@gmail.com> - 2017-08-18 08:00 +0200
        Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized  frontend driver Takashi Sakamoto <o-takashi@sakamocchi.jp> - 2017-08-18 09:20 +0200
          Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized  frontend driver Oleksandr Andrushchenko <andr2000@gmail.com> - 2017-08-18 09:30 +0200
            Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized  frontend driver Takashi Sakamoto <o-takashi@sakamocchi.jp> - 2017-08-22 04:50 +0200
              Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized  frontend driver Oleksandr Grytsov <al1img@gmail.com> - 2017-08-23 17:00 +0200
                Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized  frontend driver Takashi Sakamoto <o-takashi@sakamocchi.jp> - 2017-08-24 06:40 +0200
                  Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized  frontend driver Oleksandr Andrushchenko <andr2000@gmail.com> - 2017-08-24 09:10 +0200

#1713758 — Re: [Xen-devel] [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized frontend driver

On Thu, Aug 10, 2017 at 11:10 AM, Oleksandr Andrushchenko
<andr2000@gmail.com> wrote:
> Hi,
>
> thank you very much for valuable comments and your time!
>
>
> On 08/10/2017 06:14 AM, Takashi Sakamoto wrote:
>>
>> Hi,
>>
>> On Aug 7 2017 21:22, Oleksandr Andrushchenko wrote:
>>>
>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>>>
>>> This patch series adds support for Xen [1] para-virtualized
>>> sound frontend driver. It implements the protocol from
>>> include/xen/interface/io/sndif.h 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 scenarious where
>>> unprivileged OS (e.g. Android, AGL etc) use software mixers.
>>>
>>> Both capture and playback are supported.
>>>
>>> Thank you,
>>> Oleksandr
>>>
>>> Resending because of rebase onto [2] + added missing patch
>>>
>>> [1] https://xenproject.org/
>>> [2]
>>> https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/log/?h=for-next
>>>
>>> Oleksandr Andrushchenko (12):
>>>    ALSA: vsnd: Introduce Xen para-virtualized sound frontend driver
>>>    ALSA: vsnd: Implement driver's probe/remove
>>>    ALSA: vsnd: Implement Xen bus state handling
>>>    ALSA: vsnd: Read sound driver configuration from Xen store
>>>    ALSA: vsnd: Implement Xen event channel handling
>>>    ALSA: vsnd: Implement handling of shared buffers
>>>    ALSA: vsnd: Introduce ALSA virtual sound driver
>>>    ALSA: vsnd: Initialize virtul sound card
>>>    ALSA: vsnd: Add timer for period interrupt emulation
>>>    ALSA: vsnd: Implement ALSA PCM operations
>>>    ALSA: vsnd: Implement communication with backend
>>>    ALSA: vsnd: Introduce Kconfig option to enable Xen PV sound
>>>
>>>   sound/drivers/Kconfig     |   12 +
>>>   sound/drivers/Makefile    |    2 +
>>>   sound/drivers/xen-front.c | 2107
>>> +++++++++++++++++++++++++++++++++++++++++++++
>>>   3 files changed, 2121 insertions(+)
>>>   create mode 100644 sound/drivers/xen-front.c
>>
>>
>> For this patchset, I have the same concern which Clemens Ladisch
>> denoted[1]. If I can understand your explanation about queueing between
>> Dom0/DomU stuffs, the concern can be described in short words; this
>> driver works without any synchronization to data transmission by actual
>> sound hardwares.
>>
> Yes, both your concerns and understanding are correct
>>
>> 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.
>>
>> These two points comes from typical designs of actual hardwares, thus
>> they doesn't come from unfair, unreasonable, intrusive demands from
>> software side.
>>
> This clear, thank you
>>
>> In design of typical stuffs on para-virtualization, Dom0 stuffs are hard
>> to give enough abstraction of sound hardwares in these two points for
>> DomU stuffs. Especially, it cannot abstract point 2) at all because the
>> value of position should be accurate against actual time frame, while
>> there's an overhead for DomU stuffs to read it. When DomU stuffs handles
>> the value, the value is enough past due to context switches between
>> Dom0/DomU. Therefore, this driver must rely on point 1) to synchronize
>> to actual sound hardwares.

In order to implement option 1) discussed (Interrupts to respond events from
actual hardwares) we did number of experiments to find out if it can be
implemented in the way it satisfies the requirements with respect to latency,
interrupt number and use-cases.

First of all the sound backend is a user-space application which uses either
ALSA or PulseAudio to play/capture audio depending on configuration.
Most of the use-cases we have are using PulseAudio as it allows to
implement more complex use cases then just plain ALSA.

We started to look at how can we get such an event so it can be used as
a period elapsed notification to the backend.

In case of ALSA we used poll mechanism to wait for events from ALSA:
we configured SW params to have period event, but the problem here is that
it is notified not only when period elapses, but also when ALSA is ready to
consume more data. There is no mechanism to distinguish between these
two events (please correct us if there is one). Anyways, even if ALSA provides
period event to user-space (again, backend is a user-space application)
latency will consist of: time from kernel to user-space, user-space Dom0 to
frontend driver DomU. Both are variable and depend on many factors,
so the latency is not deterministic.

(We were also thinking that we can implement a helper driver in Dom0 to have
a dedicated channel from ALSA to the backend to deliver period elapsed event,
so for instance, it can have some kind of a hook on snd_pcm_period_elapsed,
but it will not solve the use-case with PulseAudio discussed below.
Also it is unclear how to handle scenario when multiple DomU plays through
mixer with different frame rates, channels etc.).

In case of PulseAudio it is even worse. PulseAudio can’t provide any period
related information (again, please let us know if this is not true). From our
understanding, event if get such an event from PulseAudio it will be software
emulated in user-space: for example, when PulseAudio uses a single ALSA
sink and mixes two streams with different frame rates, it will generate at least
for one of the streams a software period elapsed event.

So, from the above we think that period elapsed event derived in the described
ways may not improve latency and will complicate the system. So, for that
reason we are thinking of the option 2) (Positions of actual data transmission
in any serial sound interfaces of actual hardwares.)

In both ALSA and PulseAudio cases we can get timestamp information
(current sample timestamp). It can be converted to frames or bytes or
whatever that has been processed by the HW. The backend can get
timestamp periodically (with polling period configured with respect
to framerates being played) and pass it to the frontend. Due to context
switch and other factors this information will be outdated as well, but it seems
to be the best sync approach the backend can provide.

So, from the above, both options for synchronization cannot guarantee that
we will indeed be synchronous or latency is deterministic, but may improve
things comparing to the kernel timer on frontend’s side.

All, could you please tell us your opinion on the above and suggest what
could be the right way to go?

> Which will also introduce some latency too: time needed to deliver and
> handle interrupt from Dom0 to DomU
>>
>> Typically, drivers configure hardwares to
>> generate interrupts per period of PCM buffer. This means that this
>> driver should notify to Dom0 about the value of period size requested
>> by applications.
>>
>> 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.
>
> Ok, then on "open" command I will pass from DomU to Dom0 an additional
> parameter, period size. Then Dom0 will respond with actual period size
> for DomU to use. So, this way period size will be negotiated.
> Does the above look ok to you?
>>
>> 2. notifications of the interrupts from actual hardwares to DomU.
>>
> Ok, I will introduce an event from Dom0 to DomU to signal period elapsed.
>
> Taking into account the fact that period size may be as small as,
> say, 1ms, do you think we can/need to mangle period size in 1) on Dom0 side
> to be reasonable, so we do not flood with interrupts/events from Dom0 to
> DomU?
> Do you see any "formula" to determine that reasonable/acceptable
> period limit, so both Dom0 and DomU are happy?
>
>> For the reasons, your driver used kernel's timer interface to generate
>> 'pseudo' interrupts for the purpose. However, it depends on Dom0's
>> abstraction different from sound hardwares and Linux kernel's
>> abstraction for timer functionality. In this case, gap between 'actual'
>> interrupts from hardware and the 'pseudo' interrupts from a combination
>> of several components brings unexpected result on several situations.
>>
> You are right
>>
>> I think this is defects of 'sndif' interface in Xen side. I think it
>> better for you to work in Xen community to improve the above interface
>> at first, then work for Linux stuffs.
>>
> Please see above for planned changes to the protocol
>>
>>
>> Additionally, in next time, please remind of several points below:
>>  * When a first patch adds an initial code for drivers, it should
>>    include entries for Makefile and Kconfig, so that the driver can be
>>    built even if it's still in an initial shape.
>
> Will do
>>
>> Each patch should be
>>    self-contained and should be in a shape so that developers easily run
>>    bisecting. In other words, your first patch[2] includes modification
>>   for Makefile and Kconfig in your last patch[3].
>
> Will do
>>
>>  * When any read-only symbols is added,  it should have 'const'
>>    qualifier so that the symbol places to .rodata section of ELF
>>    binaries. For example, in your code, 'alsa_sndif_formats' is such an
>>    symbol. In recent Linux development, some developers work for
>>    constifying such symbols. Please remind of their continuous works in
>>    upstream[4].
>
> Will do
>>
>>  * You can split your driver to several files. In
>>    'include/xen/interface/io/sndif.h', Dom0 produces functionalities for
>>    DomU to control gain/volume/mute and in future your driver may get
>>    more codes. If split to several files make it readable, it should be
>>    done.
>
> Will do. If I split, do you think it would be better to move the driver
> from sound/drivers to sound/xen folder, so all those files do not mix
> with the rest?
>>
>>  * In my taste, a prefix of the subject line should be 'xen-front',
>>   instead of 'vsnd'. It comes from name of your driver.
>>
> Will do
>>
>> [1] [alsa-devel] [PATCH 08/11] ALSA: vsnd: Add timer for period interrupt
>> emulation
>>
>> http://mailman.alsa-project.org/pipermail/alsa-devel/2017-August/123617.html
>> [2] [PATCH RESEND1 01/12] ALSA: vsnd: Introduce Xen para-virtualized sound
>> frontend driver
>>
>> http://mailman.alsa-project.org/pipermail/alsa-devel/2017-August/123654.html
>> [3] [alsa-devel] [PATCH RESEND1 12/12] ALSA: vsnd: Introduce Kconfig
>> option to enable Xen PV sound
>>
>> http://mailman.alsa-project.org/pipermail/alsa-devel/2017-August/123662.html
>> [4] You can see many posts for this; e.g. [alsa-devel] [PATCH 0/7]
>> constify ALSA usb_device_id.
>>
>> http://mailman.alsa-project.org/pipermail/alsa-devel/2017-August/123564.html
>>
>> Regards
>>
>> Takashi Sakamoto
>
> Thank you,
> Oleksandr
>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> https://lists.xen.org/xen-devel



-- 
Best Regards,
Oleksandr Grytsov.

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#1714585 — Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized frontend driver

On Aug 17 2017 19:05, Oleksandr Grytsov wrote:
> So, from the above we think that period elapsed event derived in the described
> ways may not improve latency and will complicate the system. So, for that
> reason we are thinking of the option 2) (Positions of actual data transmission
> in any serial sound interfaces of actual hardwares.)

Please declare the way to enable stuffs on DomU to get the positions 
from actual hardware.


Regards

Takashi Sakamoto

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#1714588 — Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized frontend driver

Hello,

On 08/18/2017 08:43 AM, Takashi Sakamoto wrote:
> On Aug 17 2017 19:05, Oleksandr Grytsov wrote:
>> So, from the above we think that period elapsed event derived in the 
>> described
>> ways may not improve latency and will complicate the system. So, for 
>> that
>> reason we are thinking of the option 2) (Positions of actual data 
>> transmission
>> in any serial sound interfaces of actual hardwares.)
>
> Please declare the way to enable stuffs on DomU to get the positions 
> from actual hardware.
>
This is what we haven't investigated yet in detail as we were mostly 
focusing on
period elapsed approach, so we can report our findings to the community.

Taking into account the fact that the backend we have is a user-space 
application
running on top of ALSA/PulseAudio we cannot get HW pointers from actual
hardware by any means (PulseAudio use-case is the most tough thing in 
equation)

At the moment we are seeking for any advise from more experienced developers
in the field on possible ways to solve the problem of Dom0/DomU 
synchronization.
Hope, together we can identify such a way that would be accepted by the 
community.

If you have something on your mind could you please share your thoughts?
>
> Regards
>
> Takashi Sakamoto
Thank you,
Oleksandr Andrushchenko

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#1714624 — Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized frontend driver

On Aug 18 2017 14:56, Oleksandr Andrushchenko wrote:
> Taking into account the fact that the backend we have is a user-space 
> application
> running on top of ALSA/PulseAudio we cannot get HW pointers from actual
> hardware by any means (PulseAudio use-case is the most tough thing in 
> equation)

You mean that any alsa-lib or libpulse applications run on Dom0 as a
backend driver for the frontend driver on DomU?


Regards

Takashi Sakamoto

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#1714627 — Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized frontend driver

On 08/18/2017 10:17 AM, Takashi Sakamoto wrote:
> On Aug 18 2017 14:56, Oleksandr Andrushchenko wrote:
>> Taking into account the fact that the backend we have is a user-space 
>> application
>> running on top of ALSA/PulseAudio we cannot get HW pointers from actual
>> hardware by any means (PulseAudio use-case is the most tough thing in 
>> equation)
>
> You mean that any alsa-lib or libpulse applications run on Dom0 as a
> backend driver for the frontend driver on DomU?
>
No, the sound backend [1] is a user-space application (ALSA/PulseAudio 
client)
which runs as a Xen para-virtual backend in Dom0 and serves all the
frontends running in DomU(s).
Other ALSA/PulseAudio clients in Dom0 are also allowed to run at the
same time.
>
> Regards
>
> Takashi Sakamoto
Thank you,
Oleksandr

[1] https://github.com/xen-troops/snd_be

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#1716990 — Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized frontend driver

Hi,

On Aug 18 2017 16:23, Oleksandr Andrushchenko wrote:
 >> You mean that any alsa-lib or libpulse applications run on Dom0 as a
 >> backend driver for the frontend driver on DomU?
 >>
 > No, the sound backend [1] is a user-space application (ALSA/PulseAudio
 > client)
 > which runs as a Xen para-virtual backend in Dom0 and serves all the
 > frontends running in DomU(s).
 > Other ALSA/PulseAudio clients in Dom0 are also allowed to run at the
 > same time.
 >
 > [1] https://github.com/xen-troops/snd_be

Actually, you did what I meant.

Playback                                                     Capture
  delay       DomU-A    DomU-B    DomU-C                      delay
             --------- --------- ---------
             |       | |       | |       |
(queueing)  | App-A | | App-B | | App-C |                   (handling)
     |       |       | |       | |       |                       ^
     |       | (TSS) | | (TSS) | | (TSS) |                       |
     |       |       | |       | |       |                       |
     |       ---^----- ----^---- ----^----                       |
     |       ===|==========|=========|==== XenBus and            |
     |       ---|----------|-------- |----   mapped page frame   |
     |  Dom0 |  v          v         v   |                       |
     |       |App-0      App-1     App-2 |                       |
     |       |  ^          ^         ^   |                       |
     |       |  |-> App-3<-|         |   |                       |
     |       |(IPC)  ^   (IPC)       |   |                       |
     |       |       v               v   |                       |
     |       |==HW abstraction for TSS ==|                       |
     |       |          ^     ^          |                       |
     |       -----------|-----|-----------                       |
     |                  |     |    (TSS = Time Sharing System)   |
     |                  v     v                                  |
     |                  Hardwares                                |
     v                     v                                     |
(presenting)          physical part                         (sampling)

I can easily imagine that several applications (App[0|1|2]) run in Dom0
as backend drivers of this context, to add several 'virtual' sound
device for DomU, via Xenbus. The backend drivers can handle different
hardware for the 'virtual' sound devices; e.g. it can be BSD socket
applications.  Of course, this is a sample based on my imagination.
Actually, you assume that your application exclusively produces the
'virtual' sound cards, I guess.  Anyway, it's not a point of this
discussion.

 > In order to implement option 1) discussed (Interrupts to respond 
events from
 > actual hardware) we did number of experiments to find out if it can be
 > implemented in the way it satisfies the requirements with respect to 
latency,
 > interrupt number and use-cases.
 >
 > First of all the sound backend is a user-space application which uses 
either
 > ALSA or PulseAudio to play/capture audio depending on configuration.
 > Most of the use-cases we have are using PulseAudio as it allows to
 > implement more complex use cases then just plain ALSA.

When assuming App-3 in the above diagram as PulseAudio, a combination
of App-0/App-1/App-3 may correspond to the backend driver in your
use-case.

 > We started to look at how can we get such an event so it can be used as
 > a period elapsed notification to the backend.
 >
 > In case of ALSA we used poll mechanism to wait for events from ALSA:
 > we configured SW params to have period event, but the problem here is 
that
 > it is notified not only when period elapses, but also when ALSA is 
ready to
 > consume more data. There is no mechanism to distinguish between these
 > two events (please correct us if there is one). Anyways, even if ALSA 
provides
 > period event to user-space (again, backend is a user-space application)
 > latency will consist of: time from kernel to user-space, user-space 
Dom0 to
 > frontend driver DomU. Both are variable and depend on many factors,
 > so the latency is not deterministic.
 >
 > (We were also thinking that we can implement a helper driver in Dom0 
to have
 > a dedicated channel from ALSA to the backend to deliver period 
elapsed event,
 > so for instance, it can have some kind of a hook on 
snd_pcm_period_elapsed,
 > but it will not solve the use-case with PulseAudio discussed below.
 > Also it is unclear how to handle scenario when multiple DomU plays 
through
 > mixer with different frame rates, channels etc.).

In design of ALSA PCM core, processes are awakened from poll wait by
the other running tasks, which calculate available space on PCM buffer.
This is done by a call of 'snd_pcm_hw_prw0()' in 'sound/core/pcm_lib.c'
in kernel land. In this function, ALSA PCM core calls implementation of
'struct snd_pcm_ops.pointer()' in each driver and get current position
of data transmission within buffer size, then 'hw_ptr' of PCM buffer
is updated, then calculates the avail space.

Typical ALSA PCM drivers call the function in any hw IRQ context for
interrupts generated by hardware, or sw IRQ context for interrupts
generated by packet-oriented drivers for general-purpose buses such as
USB. This is a reason that the drivers configure hardware to generate
interrupts.

Actually, the value of 'avail_min' can be configured by user threads
as 'struct snd_pcm_sw_params'. As a default, this equals to the size of
period of PCM buffer.

On the other hand, any user thread can also call the function in a
call graph of ioctl(2) with some commands; e.g. SNDRV_PCM_IOCTL_HWSYNC.
Even if a user thread is on poll wait, the other user thread can awake
the thread by calling ioctl(2) with such commands. But usual program
processes I/O in one user thread and this scenario is rare.

The above is a typical scenario to use ALSA stuffs for semi-realtime
data transmission for sound hardware. Programs rely on the IRQ
generated by hardware. Drivers are programmed to configure the
hardware generating the IRQ. ALSA PCM applications are awakened by IRQ
handlers and queue/handle PCM frames in avail space on PCM buffer.

For efficiency, the interval of IRQ is configured as the same size
as a period of PCM buffer in frame unit. This is a concept of the
'period'. But there's a rest not to configure the interval per period;
e.g. IEC 61883-1/6 engine in ALSA firewire stack configures 1394 OHCI
isochronous context for callback per 2msec in its sw IRQ context while
the size of period is restricted to get one interrupt at least.
Therefore, the interval of interrupt is not necessarily as the same as
the size of period as long as IRQ handler enables applications to handle
avail space.


In a recent decade, ALSA PCM core supports the other scenario, which
rely on system timer with enough accuracy. In this scenario,
applications get an additional descriptor for system timer and
configure the timer to wake up as applications' convenience, or use
precise system call for multiplexed I/O such as ppoll(2). Applications
wake up as they prefer, the applications call ioctl(2) with
SNDRV_PCM_IOCTL_HWSYNC and calculate the avail space, then process PCM
frames. When all of handled PCM frames are queued, they schedule to
wake up far enough. Else, they schedule to wake up soon to reduce delay
for handled PCM frames.

In this scenario, any hw/sw interrupt is not necessarily required as
long as system timer is enough accurate and data transmission
automatically runs regardless of IRQ handlers. For this scenario, a few
drivers have conditional code to suppress hw/sw intervals; e.g. drivers
for 'Intel HDA' and 'C-Media 87xx' because this scenario requires
actual hardware to transfer data frames automatically but make it
available for drivers to get precise position of the transmission.
Furthermore, there's a application which supports this scenario. As
long as I know, excluding PulseAudio, nothing.

As a supplement, I note that audio timestamp is also calculated in the
function, 'snd_pcm_hw_prw0()'.


Well, as I indicated, the frontend driver works without any
synchronization to data transmission by actual sound hardware. It
relies on system timer on each of DomU and Dom0. I note my concern
against this design at last.

Linux is a kind of Time Sharing System. CPU time is divided for each
tasks. Thus there's delay of scheduling. ALSA is designed to rely on
hw/sw interrupts, because IRQ context can run regardless of the task
scheduling. (actually many exceptions I know.). This design dedicates
data transmission for actual time frame.

In a diagram of top of this message, the frontend driver runs on each
of DomU. Timer functionality of the DomU is based on scheduling on Dom0
somehow, thus there's a delay due to scheduling. At least, it has a
restriction for its preciseness. Additionally, applications on DomU are
schedulable tasks, thus they're dominated by task scheduler on DomU.
There's no reliance for actual time frame. Furthermore, libpulse
applications on Dom0 perform IPC to pulseaudio daemon. This brings
an additional overhead to synchronize to the other processes.

This is not an issue for usual applications. But for applications to
transfer data against actual time frame, it's a problem. Totally,
there's no guarantee of the data transmission for semi-realtime
capability. Any applications on DomU must run with large delay for safe
against timing gap.


Regards

Takashi Sakamoto

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#1718434 — Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized frontend driver

Hi,

Thank you for detailed explanation.

We understand that emulated interrupt on the frontend side is completely not
acceptable and definitely we need to provide some feedback mechanism from
Dom0 to DomU.

In our case it is technically impossible to provide precise period interrupt
(mostly because our backend is a user space application).
The best we can implement it is provide number of frames (time, bytes etc.)
consumed by real HW. This info will be outdated due to different delays but
we can provide precise timestamps when this info was acquired.

Would this info be useful to update the frontend driver state?

If you have in mind any other solution we would appreciate.

-- 
Best Regards,
Oleksandr Grytsov.

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#1718819 — Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized frontend driver

On Aug 23 2017 23:51, Oleksandr Grytsov wrote:
> Hi,
> 
> Thank you for detailed explanation.
> 
> We understand that emulated interrupt on the frontend side is completely not
> acceptable and definitely we need to provide some feedback mechanism from
> Dom0 to DomU.
> 
> In our case it is technically impossible to provide precise period interrupt
> (mostly because our backend is a user space application).
> The best we can implement it is provide number of frames (time, bytes etc.)
> consumed by real HW. This info will be outdated due to different delays but
> we can provide precise timestamps when this info was acquired.

Stuffs of ALSA PCM in kernel land is an abstraction layer for actual
hardware for data transmission. The stuffs get affects from a design of
actual hardware. Furthermore, sound subsystems on the other operating
systems such as Microsoft Windows are also designed with a consideration
about actual hardware. When you design any interfaces as an abstraction
for such software layer, it's better to understand actual hardware and
design of low-level software layer somehow.

Actually the 'sndif' has no good abstraction for actual hardware,
therefore an idea to implement frontend driver as an ALSA driver is not
reasonable at all. It's better to implement it as an application in
the other software layer, e.g. sinks/sources of PulseAudio in DomU via
Xenbus. This idea is nearer an original concept of Xen framework, I
guess. But I don't know we can write any applications of Xenbus in user
land of DomU or not.

Anyway, it's not a good idea to have an ALSA driver for the present 
'sndif', in my opinion.


Regards

Takashi Sakamoto

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#1718892 — Re: [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized frontend driver

Hello,

On 08/24/2017 07:38 AM, Takashi Sakamoto wrote:
> On Aug 23 2017 23:51, Oleksandr Grytsov wrote:
>> Hi,
>>
>> Thank you for detailed explanation.
>>
>> We understand that emulated interrupt on the frontend side is 
>> completely not
>> acceptable and definitely we need to provide some feedback mechanism 
>> from
>> Dom0 to DomU.
>>
>> In our case it is technically impossible to provide precise period 
>> interrupt
>> (mostly because our backend is a user space application).
>> The best we can implement it is provide number of frames (time, bytes 
>> etc.)
>> consumed by real HW. This info will be outdated due to different 
>> delays but
>> we can provide precise timestamps when this info was acquired.
>
> Stuffs of ALSA PCM in kernel land is an abstraction layer for actual
> hardware for data transmission. The stuffs get affects from a design of
> actual hardware. Furthermore, sound subsystems on the other operating
> systems such as Microsoft Windows are also designed with a consideration
> about actual hardware. When you design any interfaces as an abstraction
> for such software layer, it's better to understand actual hardware and
> design of low-level software layer somehow.
>
> Actually the 'sndif' has no good abstraction for actual hardware,
> therefore an idea to implement frontend driver as an ALSA driver is not
> reasonable at all. 
the reason for that is that you can use the same frontend driver for various
DomUs without the need to write yet another HAL/application, e.g. if one 
of the
DomUs has no PulseAudio (uses ALSA) and yet another DomU has PulseAudio,
then using the same driver allows you to enable both out of the box with the
same codebase.
If we can imagine something else running on top of ALSA (say some other
mixing software other than PulseAudio) then we will have to support that 
as well

> It's better to implement it as an application in
> the other software layer, e.g. sinks/sources of PulseAudio in DomU
please see our reasoning above
> via
> Xenbus. This idea is nearer an original concept of Xen framework, I
> guess. But I don't know we can write any applications of Xenbus in user
> land of DomU or not.
>
> Anyway, it's not a good idea to have an ALSA driver for the present 
> 'sndif', in my opinion.
>
ok, so the main concern here is that we cannot properly synchronize 
Dom0-DomU.
If we put this apart for a second are there any other concerns on having 
ALSA
frontend driver? If not, can we have the driver with timer 
implementation upstreamed
as experimental until we have some acceptable synchronization solution?
This will allow broader audience to try and feel the solution and 
probably contribute?
>
> Regards
>
> Takashi Sakamoto
Thank you very much for your time,
Oleksandr Andrushchenko

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