[pulseaudio-discuss] [PATCH v12 06/13] bluetooth: Parse remote timestamp from A2DP RTP packets when available

[Pali Rohár]

Some A2DP codecs (e.g. SBC or aptX-HD) use RTP packets. For sources use
timestamps from RTP packets to calculate read index and therefore remote
timestamp for synchronization.
---
 src/modules/bluetooth/a2dp-codec-api.h       |  4 ++--
 src/modules/bluetooth/a2dp-codec-sbc.c       |  3 ++-
 src/modules/bluetooth/module-bluez5-device.c | 14 +++++++++++---
 3 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/src/modules/bluetooth/a2dp-codec-api.h b/src/modules/bluetooth/a2dp-codec-api.h
index bc4844596..d39f8aafa 100644
--- a/src/modules/bluetooth/a2dp-codec-api.h
+++ b/src/modules/bluetooth/a2dp-codec-api.h
@@ -90,8 +90,8 @@ typedef struct pa_a2dp_codec {
     size_t (*encode_buffer)(void *codec_info, uint32_t timestamp, const uint8_t *input_buffer, size_t input_size, uint8_t *output_buffer, size_t output_size, size_t *processed);
     /* Decode input_buffer of input_size to output_buffer of output_size,
      * returns size of filled ouput_buffer and set processed to size of
-     * processed input_buffer */
-    size_t (*decode_buffer)(void *codec_info, const uint8_t *input_buffer, size_t input_size, uint8_t *output_buffer, size_t output_size, size_t *processed);
+     * processed input_buffer and set timestamp */
+    size_t (*decode_buffer)(void *codec_info, uint32_t *timestamp, const uint8_t *input_buffer, size_t input_size, uint8_t *output_buffer, size_t output_size, size_t *processed);
 } pa_a2dp_codec;
 
 #endif
diff --git a/src/modules/bluetooth/a2dp-codec-sbc.c b/src/modules/bluetooth/a2dp-codec-sbc.c
index 89c647fbe..733c1a9ab 100644
--- a/src/modules/bluetooth/a2dp-codec-sbc.c
+++ b/src/modules/bluetooth/a2dp-codec-sbc.c
@@ -597,7 +597,7 @@ static size_t encode_buffer(void *codec_info, uint32_t timestamp, const uint8_t
     return d - output_buffer;
 }
 
-static size_t decode_buffer(void *codec_info, const uint8_t *input_buffer, size_t input_size, uint8_t *output_buffer, size_t output_size, size_t *processed) {
+static size_t decode_buffer(void *codec_info, uint32_t *timestamp, const uint8_t *input_buffer, size_t input_size, uint8_t *output_buffer, size_t output_size, size_t *processed) {
     struct sbc_info *sbc_info = (struct sbc_info *) codec_info;
 
     struct rtp_header *header;
@@ -657,6 +657,7 @@ static size_t decode_buffer(void *codec_info, const uint8_t *input_buffer, size_
         frame_count--;
     }
 
+    *timestamp = ntohl(header->timestamp);
     *processed = p - input_buffer;
     return d - output_buffer;
 }
diff --git a/src/modules/bluetooth/module-bluez5-device.c b/src/modules/bluetooth/module-bluez5-device.c
index a97c6c703..0bfe33d3a 100644
--- a/src/modules/bluetooth/module-bluez5-device.c
+++ b/src/modules/bluetooth/module-bluez5-device.c
@@ -552,6 +552,7 @@ static int a2dp_process_push(struct userdata *u) {
         struct msghdr m;
         bool found_tstamp = false;
         pa_usec_t tstamp;
+        uint32_t timestamp;
         uint8_t *ptr;
         ssize_t l;
         size_t processed;
@@ -587,8 +588,6 @@ static int a2dp_process_push(struct userdata *u) {
 
         pa_assert((size_t) l <= u->decoder_buffer_size);
 
-        /* TODO: get timestamp from rtp */
-
         for (cm = CMSG_FIRSTHDR(&m); cm; cm = CMSG_NXTHDR(&m, cm)) {
             if (cm->cmsg_level == SOL_SOCKET && cm->cmsg_type == SO_TIMESTAMP) {
                 struct timeval *tv = (struct timeval*) CMSG_DATA(cm);
@@ -609,7 +608,8 @@ static int a2dp_process_push(struct userdata *u) {
         ptr = pa_memblock_acquire(memchunk.memblock);
         memchunk.length = pa_memblock_get_length(memchunk.memblock);
 
-        memchunk.length = u->a2dp_codec->decode_buffer(u->decoder_info, u->decoder_buffer, l, ptr, memchunk.length, &processed);
+        timestamp = 0; /* Decoder does not have to fill RTP timestamp */
+        memchunk.length = u->a2dp_codec->decode_buffer(u->decoder_info, &timestamp, u->decoder_buffer, l, ptr, memchunk.length, &processed);
 
         pa_memblock_release(memchunk.memblock);
 
@@ -619,6 +619,14 @@ static int a2dp_process_push(struct userdata *u) {
             break;
         }
 
+        /* Some codecs may provide RTP timestamp, so use it to update read_index for calculation of remote tstamp */
+        if (timestamp) {
+            /* RTP timestamp is only 32bit and may overflow, avoid it by calculating high 32bits from the last read_index */
+            size_t frame_size = pa_frame_size(&u->decoder_sample_spec);
+            uint64_t timestamp_hi = (u->read_index / frame_size) & 0xFFFFFFFF00000000ULL;
+            u->read_index = (timestamp_hi | timestamp) * frame_size;
+        }
+
         u->read_index += (uint64_t) memchunk.length;
         pa_smoother_put(u->read_smoother, tstamp, pa_bytes_to_usec(u->read_index, &u->decoder_sample_spec));
         pa_smoother_resume(u->read_smoother, tstamp, true);
-- 
2.11.0