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

[Oleksandr Grytsov <al1img@xxxxxxxxx>]

On Tue, Sep 5, 2017 at 10:24 AM, Clemens Ladisch <clemens@xxxxxxxxxx> wrote:
> Oleksandr Andrushchenko wrote:
>>>> We understand that emulated interrupt on the frontend side is completely 
>>>> not
>>>> acceptable
>
> Allow me to expand on that:  Proper synchronization requires that the
> exact position is communicated, not estimated.  Just because the nominal
> rate of the stream is known does not imply that you know the actual rate.
> Forget for the moment that there even is a nominal rate; assume that it
> works like, e.g., a storage controller, and that you can know that a DMA
> buffer was consumed by the device only after it has told you.
>
> It's possible and likely that there is a latency when reporting the
> stream position, but that is still better than guessing what the DMA
> is doing.  (You would never just try to guess when writing data to
> disk, would you?)
>
>>>> 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).
>
> As far as I can see, all audio APIs (ALSA, PulseAudio, etc.) have poll()
> or callbacks or similar mechanisms to inform you when new data can be
> written, and always allow to query the current position.
>
>> [...]
>> 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?
>
> I doubt that the driver architecture will stay completely the same, so I
> do not think that this experimental driver would demonstrate how the
> solution would feel.
>
> As the first step, I would suggest creating a driver with proper
> synchronization, even if it has high latency.  Reducing the latency
> would then be 'just' an optimization.
>
>
> Regards,
> Clemens

Definitely feedback from the backend side is required. Currently
we are working on synchronized version on the backend
and frontend side. We will be back once we have the solution.

Thanks.

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