[pulseaudio-discuss] What's the purpose of "rewind" on corking and uncorking?

[Lin, Mengdong]

Why a sink input and its sink need "rewind" on corking and uncorking?

I don't quite understand the following code. Could somebody kindly shed light on it? And take ALSA sink for example, are there any valid (non-silent) data left in ALSA buffer after the rewinding?

void pa_sink_input_set_state_within_thread(pa_sink_input *i, pa_sink_input_state_t state) {
    ...
    corking = state == PA_SINK_INPUT_CORKED && i->thread_info.state == PA_SINK_INPUT_RUNNING;
   uncorking = i->thread_info.state == PA_SINK_INPUT_CORKED && state == PA_SINK_INPUT_RUNNING;

    if (i->state_change)
        i->state_change(i, state);

    i->thread_info.state = state;

    if (corking) {

        pa_log_debug("Requesting rewind due to corking");

        /* This will tell the implementing sink input driver to rewind
         * so that the unplayed already mixed data is not lost */
        pa_sink_input_request_rewind(i, 0, TRUE, TRUE, FALSE); ...

    } else if (uncorking) {

        i->thread_info.underrun_for = (uint64_t) -1;
        i->thread_info.playing_for = 0;

        pa_log_debug("Requesting rewind due to uncorking");

        /* OK, we're being uncorked. Make sure we're not rewound when
         * the hw buffer is remixed and request a remix. */
        pa_sink_input_request_rewind(i, 0, FALSE, TRUE, TRUE);
    }
}

Thanks
Amanda

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