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

[Oleksandr Andrushchenko <andr2000@gmail.com>]

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] 
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/xen/interface/io/sndif.h?h=v4.14-rc1
>>
>> On 09/12/2017 10:52 AM, Oleksandr Grytsov wrote:
>>> On Tue, Sep 5, 2017 at 10:24 AM, Clemens Ladisch 
>>> <clemens@ladisch.de> 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.
>>
>