Re: Fwd: VirtioSound device emulation implementation

[Shreyansh Chouhan <chouhan.shreyansh2702@gmail.com>]

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On Mon, 19 Apr 2021 at 18:00, Shreyansh Chouhan <
chouhan.shreyansh2702@gmail.com> wrote:

>
> On Fri, 16 Apr 2021 at 17:02, Gerd Hoffmann <kraxel@redhat.com> wrote:
>
>>   Hi,
>>
>> > I learned that the callback passed in AUD_open_out, (lets call it the
>> write
>> > audio callback,)  is supposed to mix and write the
>> > buffers to HWVoiceOut. I have written that, the basic algorithm being:
>> >
>> > 1. Pop element from tx virtqueue.
>> > 2. Get the xfer header from the elem->out_sg (iov_to_buf(elem->out_sg,
>> 1,
>> > 0, &hdr, sizeof(hdr)))
>> > 3. Get the buffer from elem->out_sg (iov_to_buf(elem->out_sg, 1,
>> > sizeof(hdr), &mixbuf, period_bytes))
>> > 4. AUD_write the buffer
>>
>> AUD_write returns the number of bytes actually accepted.
>>
>> In case the audio backend consumed the complete buffer you can go ahead
>> as described.  Otherwise stop here and resume (try AUD_write() the
>> remaining data) when the callback is called again.
>>
> The callback that is supposed to write the buffers to HWVoiceOut has to
> do it such that it only writes the buffers for the streams that it was
> called by. The tx
> virtqueue is going to have buffers for all the streams. How do I handle
> this situation?
>
> Say, I have 3 output streams 0, 1 and 2, the callback was called on stream
> 0, how do
> I only play buffers corresponding to this stream? Is there a way by which
> I could iterate over
> the virtqeueue?
>
> If not then I should probably store the VirtQueueElements corresponding to
> a specific stream
> in the stream itself. I have to store the elements because I'd have to
> write to the response and
> mark these elements as used, (via virtqueue_push i think.)
>
> Also, is virtqueue a 'queue'? I remember reading in the specs that the
> device can consume
> the buffers in any order it prefers unless a feature bit is negotiated. So
> does virtqueue_pop
> always return the first element that entered the queue? (FIFO?)
>
> If it is FIFO, then I think the only way would be to iterate over the
> virtqueue. However if `virtqueu_pop`
> does return the last inserted element, I can use `handle_tx`
>
to get the last element inserted and then add it to the corresponding
stream.

Sorry for the incomplete mail. I don't know how I hit send.

>
>
>> No problem.  I'm likewise busy or on (easter) vacation at times and fail
>> to send timely answers (sorry for that).
>>
> Also that's fine, I just got a bit worried :)

I think once this `tx` part is done I'll have the pcm streams ready, and I
can send in patches by tonight
or tomorrow evening latest.

>
>> HTH & take care,
>>   Gerd
>>
>>

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