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[alsa] Add low-tech sync with the player (and AirPlay)
This commit is contained in:
ejurgensen
parent
eca41e306e
commit
632bfd9a33
@ -38,6 +38,13 @@
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#include "outputs.h"
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#define PACKET_SIZE STOB(AIRTUNES_V2_PACKET_SAMPLES)
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// The maximum number of samples that the output is allowed to get behind (or
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// ahead) of the player position, before compensation is attempted
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#define ALSA_MAX_LATENCY 352
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// If latency is jumping up and down we don't do compensation since we probably
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// wouldn't do a good job. This sets the maximum the latency is allowed to vary
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// within the 10 seconds where we measure latency each second.
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#define ALSA_MAX_LATENCY_VARIANCE 100
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// TODO Unglobalise these and add support for multiple sound cards
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static char *card_name;
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@ -59,6 +66,13 @@ enum alsa_state
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ALSA_STATE_FAILED = -1,
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};
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enum alsa_sync_state
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{
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ALSA_SYNC_OK,
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ALSA_SYNC_AHEAD,
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ALSA_SYNC_BEHIND,
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};
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struct alsa_session
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{
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enum alsa_state state;
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@ -68,6 +82,9 @@ struct alsa_session
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uint64_t pos;
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uint64_t start_pos;
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int32_t last_latency;
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int sync_counter;
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// An array that will hold the packets we prebuffer. The length of the array
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// is prebuf_len (measured in rtp_packets)
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uint8_t *prebuf;
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@ -92,7 +109,6 @@ struct alsa_session
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extern struct event_base *evbase_player;
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static struct alsa_session *sessions;
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static int sync_counter;
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/* Forwards */
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static void
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@ -567,74 +583,33 @@ playback_start(struct alsa_session *as, uint64_t pos, uint64_t start_pos)
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as->state = ALSA_STATE_STREAMING;
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}
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static void
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playback_write(struct alsa_session *as, uint8_t *buf, uint64_t rtptime)
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// This function writes the sample buf into either the prebuffer or directly to
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// ALSA, depending on how much room there is in ALSA, and whether we are
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// prebuffering or not. It also transfers from the the prebuffer to ALSA, if
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// needed. Returns 0 on success, negative on error.
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static int
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buffer_write(struct alsa_session *as, uint8_t *buf, snd_pcm_sframes_t *avail, int prebuffering, int prebuf_empty)
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{
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snd_pcm_sframes_t ret;
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snd_pcm_sframes_t avail;
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snd_pcm_sframes_t delay;
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snd_pcm_sframes_t nsamp;
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struct timespec now;
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uint64_t pb_pos;
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uint64_t cur_pos;
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uint8_t *pkt;
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int prebuffering;
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int prebuf_empty;
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int npackets;
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int latency;
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int diff;
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snd_pcm_sframes_t nsamp;
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snd_pcm_sframes_t ret;
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prebuffering = (as->pos < as->start_pos);
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prebuf_empty = (as->prebuf_head == as->prebuf_tail);
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nsamp = AIRTUNES_V2_PACKET_SAMPLES;
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as->pos += AIRTUNES_V2_PACKET_SAMPLES;
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// We need to copy to the prebuffer if we are prebuffering OR if the
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// prebuffer has not been emptied yet
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if (prebuffering || !prebuf_empty)
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if (prebuffering || !prebuf_empty || *avail < AIRTUNES_V2_PACKET_SAMPLES)
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{
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pkt = &as->prebuf[as->prebuf_head * PACKET_SIZE];
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memcpy(pkt, buf, PACKET_SIZE);
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as->prebuf_head = (as->prebuf_head + 1) % as->prebuf_len;
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}
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if (prebuffering)
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return;
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if (prebuffering || *avail < AIRTUNES_V2_PACKET_SAMPLES)
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return 0; // No actual writing
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ret = snd_pcm_avail_delay(hdl, &avail, &delay);
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if (ret < 0)
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goto alsa_error;
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if (avail < AIRTUNES_V2_PACKET_SAMPLES)
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return;
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sync_counter++;
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if (sync_counter >= 126)
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{
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sync_counter = 0;
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if (!prebuf_empty)
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npackets = (as->prebuf_head - (as->prebuf_tail + 1) + as->prebuf_len) % as->prebuf_len + 1;
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else
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npackets = 0;
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pb_pos = rtptime - delay - AIRTUNES_V2_PACKET_SAMPLES * npackets;
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ret = player_get_current_pos(&cur_pos, &now, 0); // TODO commit?
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if (ret == 0)
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latency = cur_pos - pb_pos;
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else
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latency = 0;
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diff = cur_pos - as->pos;
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if (latency)
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DPRINTF(E_DBG, L_LAUDIO, "Sync to cur_pos %" PRIu64 ", pb_pos %" PRIu64 " (diff %d, delay %li), pos %" PRIu64 " (diff %d)\n", cur_pos, pb_pos, latency, delay, as->pos, diff);
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}
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// If we have data in prebuf we send as much as we can
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if (!prebuf_empty)
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{
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// We will now set buf so that we will transfer as much as possible to ALSA
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buf = &as->prebuf[as->prebuf_tail * PACKET_SIZE];
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if (as->prebuf_head > as->prebuf_tail)
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@ -643,7 +618,7 @@ playback_write(struct alsa_session *as, uint8_t *buf, uint64_t rtptime)
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npackets = as->prebuf_len - as->prebuf_tail;
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nsamp = npackets * AIRTUNES_V2_PACKET_SAMPLES;
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while (nsamp > avail)
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while (nsamp > *avail)
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{
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npackets -= 1;
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nsamp -= AIRTUNES_V2_PACKET_SAMPLES;
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@ -651,15 +626,111 @@ playback_write(struct alsa_session *as, uint8_t *buf, uint64_t rtptime)
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as->prebuf_tail = (as->prebuf_tail + npackets) % as->prebuf_len;
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}
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else
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nsamp = AIRTUNES_V2_PACKET_SAMPLES;
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ret = snd_pcm_writei(hdl, buf, nsamp);
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if (ret < 0)
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goto alsa_error;
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else if (ret != nsamp)
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return ret;
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if (ret != nsamp)
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DPRINTF(E_WARN, L_LAUDIO, "ALSA partial write detected\n");
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*avail -= ret;
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return 0;
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}
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// Checks if ALSA's playback position is ahead or behind the player's
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enum alsa_sync_state
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sync_check(struct alsa_session *as, uint64_t rtptime, snd_pcm_sframes_t delay, int prebuf_empty)
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{
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enum alsa_sync_state sync;
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struct timespec now;
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uint64_t cur_pos;
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uint64_t pb_pos;
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int32_t latency;
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int npackets;
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sync = ALSA_SYNC_OK;
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if (player_get_current_pos(&cur_pos, &now, 0) != 0)
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return sync;
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if (!prebuf_empty)
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npackets = (as->prebuf_head - (as->prebuf_tail + 1) + as->prebuf_len) % as->prebuf_len + 1;
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else
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npackets = 0;
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pb_pos = rtptime - delay - AIRTUNES_V2_PACKET_SAMPLES * npackets;
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latency = cur_pos - pb_pos;
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// If the latency is low or very different from our last measurement, we reset the sync_counter
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if (abs(latency) < ALSA_MAX_LATENCY || abs(as->last_latency - latency) > ALSA_MAX_LATENCY_VARIANCE)
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{
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as->sync_counter = 0;
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sync = ALSA_SYNC_OK;
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}
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// If we have measured a consistent latency for 10 seconds, then we take action
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else if (as->sync_counter >= 10 * 126)
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{
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DPRINTF(E_INFO, L_LAUDIO, "Taking action to compensate for ALSA latency of %d samples\n", latency);
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as->sync_counter = 0;
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if (latency > 0)
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sync = ALSA_SYNC_BEHIND;
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else
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sync = ALSA_SYNC_AHEAD;
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}
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as->last_latency = latency;
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if (latency)
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DPRINTF(E_DBG, L_LAUDIO, "Sync %d cur_pos %" PRIu64 ", pb_pos %" PRIu64 " (diff %d, delay %li), pos %" PRIu64 "\n", sync, cur_pos, pb_pos, latency, delay, as->pos);
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return sync;
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}
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static void
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playback_write(struct alsa_session *as, uint8_t *buf, uint64_t rtptime)
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{
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snd_pcm_sframes_t ret;
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snd_pcm_sframes_t avail;
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snd_pcm_sframes_t delay;
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enum alsa_sync_state sync;
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int prebuffering;
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int prebuf_empty;
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prebuffering = (as->pos < as->start_pos);
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prebuf_empty = (as->prebuf_head == as->prebuf_tail);
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as->pos += AIRTUNES_V2_PACKET_SAMPLES;
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if (prebuffering)
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{
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buffer_write(as, buf, NULL, prebuffering, prebuf_empty);
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return;
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}
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ret = snd_pcm_avail_delay(hdl, &avail, &delay);
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if (ret < 0)
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goto alsa_error;
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// Every second we do a sync check
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sync = ALSA_SYNC_OK;
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as->sync_counter++;
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if (as->sync_counter % 126 == 0)
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sync = sync_check(as, rtptime, delay, prebuf_empty);
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// Skip write -> reduce the delay
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if (sync == ALSA_SYNC_BEHIND)
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return;
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ret = buffer_write(as, buf, &avail, prebuffering, prebuf_empty);
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// Double write -> increase the delay
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if (sync == ALSA_SYNC_AHEAD && (ret == 0))
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ret = buffer_write(as, buf, &avail, prebuffering, prebuf_empty);
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if (ret < 0)
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goto alsa_error;
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return;
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alsa_error:
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