[pulseaudio-discuss] [PATCH] echo-cancel: Try to minimise in-flight chunks in snapshot latency

[Georg Chini]

On 09.03.2017 05:37, Arun Raghavan wrote:
> We don't always know whether the in-flight memory chunks will be
> rendered or skipped (if the source is not in RUNNING). This can cause us
> to have an erroneous estimate of drift, particularly when the canceller
> starts.
>
> To avoid this, we explicitly flush out the send and receive sides of the
> message queue of audio chunks going from the sink to the source before
> trying to perform a resync.
> ---
>   src/modules/echo-cancel/module-echo-cancel.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/src/modules/echo-cancel/module-echo-cancel.c b/src/modules/echo-cancel/module-echo-cancel.c
> index dfd05b6..ed75e0c 100644
> --- a/src/modules/echo-cancel/module-echo-cancel.c
> +++ b/src/modules/echo-cancel/module-echo-cancel.c
> @@ -683,8 +683,13 @@ static void do_resync(struct userdata *u) {
>       pa_log("Doing resync");
>   
>       /* update our snapshot */
> -    source_output_snapshot_within_thread(u, &latency_snapshot);
> +    /* 1. Get sink input latency snapshot, might cause buffers to be sent to source thread */
>       pa_asyncmsgq_send(u->sink_input->sink->asyncmsgq, PA_MSGOBJECT(u->sink_input), SINK_INPUT_MESSAGE_LATENCY_SNAPSHOT, &latency_snapshot, 0, NULL);
> +    /* 2. Pick up any in-flight buffers (and discard if needed) */
> +    while (pa_asyncmsgq_process_one(u->asyncmsgq))
> +        ;
> +    /* 3. Now get the source output latency snapshot */
> +    source_output_snapshot_within_thread(u, &latency_snapshot);
>   
>       /* calculate drift between capture and playback */
>       diff_time = calc_diff(u, &latency_snapshot);

While taking a look at the patch I noticed something else in the 
do_resync().
You are doing:

     source_output_snapshot_within_thread(u, &latency_snapshot);
     pa_asyncmsgq_send(u->sink_input->sink->asyncmsgq, 
PA_MSGOBJECT(u->sink_input), SINK_INPUT_MESSAGE_LATENCY_SNAPSHOT, 
&latency_snapshot, 0, NULL);

from the source thread. I tried something similar in module loopback and
found that you should not send a message to the sink thread from there.

At least for bluetooth it looks like input and output is done in the 
same thread,
so the pa_asyncmsg_send() will hang. I tested it with my headset and it 
hangs
indeed.