Use ALSA timestamps for marking input data.

This reportedly gives much more stable delay, as the timer is sampled
when the audio actually arrives in the kernel. Patch by Yann Dubreuil,
from the BreizhCamp repository.

diff --git a/nageru/alsa_input.cpp b/nageru/alsa_input.cpp
index a64f8107d3a7f34bd6d16db929c6b8b5940531ca..40fba43c12a19192a0c0fdcc07f430355df88619 100644 (file)
--- a/nageru/alsa_input.cpp
+++ b/nageru/alsa_input.cpp
@@ -111,6 +111,9 @@ bool ALSAInput::open_device()
        snd_pcm_sw_params_alloca(&sw_params);
        RETURN_FALSE_ON_ERROR("snd_pcm_sw_params_current()", snd_pcm_sw_params_current(pcm_handle, sw_params));
        RETURN_FALSE_ON_ERROR("snd_pcm_sw_params_set_start_threshold", snd_pcm_sw_params_set_start_threshold(pcm_handle, sw_params, num_periods * period_size / 2));
+       RETURN_FALSE_ON_ERROR("snd_pcm_sw_params_set_tstamp_mode", snd_pcm_sw_params_set_tstamp_mode(pcm_handle, sw_params, SND_PCM_TSTAMP_ENABLE));
+       RETURN_FALSE_ON_ERROR("snd_pcm_sw_params_set_tstamp_type", snd_pcm_sw_params_set_tstamp_type(pcm_handle, sw_params, SND_PCM_TSTAMP_TYPE_MONOTONIC));
+
        RETURN_FALSE_ON_ERROR("snd_pcm_sw_params()", snd_pcm_sw_params(pcm_handle, sw_params));
 
        RETURN_FALSE_ON_ERROR("snd_pcm_nonblock()", snd_pcm_nonblock(pcm_handle, 1));
@@ -221,6 +224,8 @@ ALSAInput::CaptureEndReason ALSAInput::do_capture()
        RETURN_ON_ERROR("snd_pcm_start()", snd_pcm_start(pcm_handle));
        parent_pool->set_card_state(internal_dev_index, ALSAPool::Device::State::RUNNING);
 
+       snd_pcm_status_t *status;
+       snd_pcm_status_alloca(&status);
        while (!should_quit.should_quit()) {
                int ret = snd_pcm_wait(pcm_handle, /*timeout=*/100);
                if (ret == 0) continue;  // Timeout.
@@ -232,7 +237,14 @@ ALSAInput::CaptureEndReason ALSAInput::do_capture()
                }
                RETURN_ON_ERROR("snd_pcm_wait()", ret);
 
-               snd_pcm_sframes_t frames = snd_pcm_readi(pcm_handle, buffer.get(), buffer_frames);
+               ret = snd_pcm_status(pcm_handle, status);
+               RETURN_ON_ERROR("snd_pcm_status()", ret);
+
+               snd_pcm_sframes_t avail = snd_pcm_status_get_avail(status);
+               snd_htimestamp_t alsa_ts;
+               snd_pcm_status_get_htstamp(status, &alsa_ts);
+
+               snd_pcm_sframes_t frames = snd_pcm_readi(pcm_handle, buffer.get(), avail);
                if (frames == -EPIPE) {
                        fprintf(stderr, "[%s] ALSA overrun\n", device.c_str());
                        snd_pcm_prepare(pcm_handle);
@@ -245,11 +257,12 @@ ALSAInput::CaptureEndReason ALSAInput::do_capture()
                }
                RETURN_ON_ERROR("snd_pcm_readi()", frames);
 
-               const steady_clock::time_point now = steady_clock::now();
+               // NOTE: This assumes steady_clock::time_point is the same as clock_gettime(CLOCK_MONOTONIC).
+               const steady_clock::time_point ts = steady_clock::time_point(seconds(alsa_ts.tv_sec) + nanoseconds(alsa_ts.tv_nsec));
                bool success;
                do {
                        if (should_quit.should_quit()) return CaptureEndReason::REQUESTED_QUIT;
-                       success = audio_callback(buffer.get(), frames, audio_format, now);
+                       success = audio_callback(buffer.get(), frames, audio_format, ts);
                } while (!success);
        }
        return CaptureEndReason::REQUESTED_QUIT;