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

On 10/04/2017 09:50 AM, Oleksandr Andrushchenko wrote:
> gentle reminder
> On 09/26/2017 02:35 PM, Oleksandr Andrushchenko wrote:
>> Clemens, Sakamoto-san,
>> could you please review the below if you by chance have a minute?
>> Thank you,
>> Oleksandr
>> On 09/19/2017 11:57 AM, Oleksandr Andrushchenko wrote:
>>> Hi, all!
>>> We did some work on implementing the idea with
>>> feedback events from the backend to the frontend.
>>> Please see attached the changes to the existing sndif protocol [1]:
>>> 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.
>>> 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.
>>> The implementation we have showed that this is sufficient to
>>> successfully play/capture w/o using emulated interrupts.
>>> Clemens, Sakamoto-san,
>>> could you please review the changes and confirm that these are ok to
>>> be upstreamed to the sndif protocol and are enough for the frontend
>>> driver we want to upstream (we have it implemented, just need to make
>>> sure the general approach is accepted by the ALSA community).
>>> Thank you very much for your time,
>>> Oleksandr Andrushchenko
>>> Oleksandr Grytsov
>>> [1]
>>> On 09/12/2017 10:52 AM, Oleksandr Grytsov wrote:
>>>> On Tue, Sep 5, 2017 at 10:24 AM, Clemens Ladisch
>>>> <> 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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