diff options
Diffstat (limited to 'Documentation/sound')
| -rw-r--r-- | Documentation/sound/designs/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/sound/designs/timestamping.rst (renamed from Documentation/sound/alsa/timestamping.txt) | 143 |
2 files changed, 80 insertions, 64 deletions
diff --git a/Documentation/sound/designs/index.rst b/Documentation/sound/designs/index.rst index f7ca11307033..798b1a44bbbd 100644 --- a/Documentation/sound/designs/index.rst +++ b/Documentation/sound/designs/index.rst @@ -7,6 +7,7 @@ Designs and Implementations control-names channel-mapping-api compress-offload + timestamping procfile powersave oss-emulation diff --git a/Documentation/sound/alsa/timestamping.txt b/Documentation/sound/designs/timestamping.rst index 9d579aefbffd..2b0fff503415 100644 --- a/Documentation/sound/alsa/timestamping.txt +++ b/Documentation/sound/designs/timestamping.rst @@ -1,18 +1,22 @@ +===================== +ALSA PCM Timestamping +===================== + The ALSA API can provide two different system timestamps: - Trigger_tstamp is the system time snapshot taken when the .trigger -callback is invoked. This snapshot is taken by the ALSA core in the -general case, but specific hardware may have synchronization -capabilities or conversely may only be able to provide a correct -estimate with a delay. In the latter two cases, the low-level driver -is responsible for updating the trigger_tstamp at the most appropriate -and precise moment. Applications should not rely solely on the first -trigger_tstamp but update their internal calculations if the driver -provides a refined estimate with a delay. + callback is invoked. This snapshot is taken by the ALSA core in the + general case, but specific hardware may have synchronization + capabilities or conversely may only be able to provide a correct + estimate with a delay. In the latter two cases, the low-level driver + is responsible for updating the trigger_tstamp at the most appropriate + and precise moment. Applications should not rely solely on the first + trigger_tstamp but update their internal calculations if the driver + provides a refined estimate with a delay. - tstamp is the current system timestamp updated during the last -event or application query. -The difference (tstamp - trigger_tstamp) defines the elapsed time. + event or application query. + The difference (tstamp - trigger_tstamp) defines the elapsed time. The ALSA API provides two basic pieces of information, avail and delay, which combined with the trigger and current system @@ -22,15 +26,15 @@ the ring buffer and the amount of queued samples. The use of these different pointers and time information depends on the application needs: -- 'avail' reports how much can be written in the ring buffer -- 'delay' reports the time it will take to hear a new sample after all -queued samples have been played out. +- ``avail`` reports how much can be written in the ring buffer +- ``delay`` reports the time it will take to hear a new sample after all + queued samples have been played out. When timestamps are enabled, the avail/delay information is reported along with a snapshot of system time. Applications can select from -CLOCK_REALTIME (NTP corrections including going backwards), -CLOCK_MONOTONIC (NTP corrections but never going backwards), -CLOCK_MONOTIC_RAW (without NTP corrections) and change the mode +``CLOCK_REALTIME`` (NTP corrections including going backwards), +``CLOCK_MONOTONIC`` (NTP corrections but never going backwards), +``CLOCK_MONOTIC_RAW`` (without NTP corrections) and change the mode dynamically with sw_params @@ -38,17 +42,18 @@ The ALSA API also provide an audio_tstamp which reflects the passage of time as measured by different components of audio hardware. In ascii-art, this could be represented as follows (for the playback case): +:: + --------------------------------------------------------------> time + ^ ^ ^ ^ ^ + | | | | | + analog link dma app FullBuffer + time time time time time + | | | | | + |< codec delay >|<--hw delay-->|<queued samples>|<---avail->| + |<----------------- delay---------------------->| | + |<----ring buffer length---->| ---------------------------------------------------------------> time - ^ ^ ^ ^ ^ - | | | | | - analog link dma app FullBuffer - time time time time time - | | | | | - |< codec delay >|<--hw delay-->|<queued samples>|<---avail->| - |<----------------- delay---------------------->| | - |<----ring buffer length---->| The analog time is taken at the last stage of the playback, as close as possible to the actual transducer @@ -113,11 +118,11 @@ audio applications... Due to the varied nature of timestamping needs, even for a single application, the audio_tstamp_config can be changed dynamically. In -the STATUS ioctl, the parameters are read-only and do not allow for +the ``STATUS`` ioctl, the parameters are read-only and do not allow for any application selection. To work around this limitation without -impacting legacy applications, a new STATUS_EXT ioctl is introduced +impacting legacy applications, a new ``STATUS_EXT`` ioctl is introduced with read/write parameters. ALSA-lib will be modified to make use of -STATUS_EXT and effectively deprecate STATUS. +``STATUS_EXT`` and effectively deprecate ``STATUS``. The ALSA API only allows for a single audio timestamp to be reported at a time. This is a conscious design decision, reading the audio @@ -135,36 +140,42 @@ the hardware, there is a risk of misalignment with the avail and delay information. To make sure applications are not confused, a driver_timestamp field is added in the snd_pcm_status structure; this timestamp shows when the information is put together by the driver -before returning from the STATUS and STATUS_EXT ioctl. in most cases +before returning from the ``STATUS`` and ``STATUS_EXT`` ioctl. in most cases this driver_timestamp will be identical to the regular system tstamp. Examples of typestamping with HDaudio: 1. DMA timestamp, no compensation for DMA+analog delay -$ ./audio_time -p --ts_type=1 -playback: systime: 341121338 nsec, audio time 342000000 nsec, systime delta -878662 -playback: systime: 426236663 nsec, audio time 427187500 nsec, systime delta -950837 -playback: systime: 597080580 nsec, audio time 598000000 nsec, systime delta -919420 -playback: systime: 682059782 nsec, audio time 683020833 nsec, systime delta -961051 -playback: systime: 852896415 nsec, audio time 853854166 nsec, systime delta -957751 -playback: systime: 937903344 nsec, audio time 938854166 nsec, systime delta -950822 +:: + + $ ./audio_time -p --ts_type=1 + playback: systime: 341121338 nsec, audio time 342000000 nsec, systime delta -878662 + playback: systime: 426236663 nsec, audio time 427187500 nsec, systime delta -950837 + playback: systime: 597080580 nsec, audio time 598000000 nsec, systime delta -919420 + playback: systime: 682059782 nsec, audio time 683020833 nsec, systime delta -961051 + playback: systime: 852896415 nsec, audio time 853854166 nsec, systime delta -957751 + playback: systime: 937903344 nsec, audio time 938854166 nsec, systime delta -950822 2. DMA timestamp, compensation for DMA+analog delay -$ ./audio_time -p --ts_type=1 -d -playback: systime: 341053347 nsec, audio time 341062500 nsec, systime delta -9153 -playback: systime: 426072447 nsec, audio time 426062500 nsec, systime delta 9947 -playback: systime: 596899518 nsec, audio time 596895833 nsec, systime delta 3685 -playback: systime: 681915317 nsec, audio time 681916666 nsec, systime delta -1349 -playback: systime: 852741306 nsec, audio time 852750000 nsec, systime delta -8694 +:: + + $ ./audio_time -p --ts_type=1 -d + playback: systime: 341053347 nsec, audio time 341062500 nsec, systime delta -9153 + playback: systime: 426072447 nsec, audio time 426062500 nsec, systime delta 9947 + playback: systime: 596899518 nsec, audio time 596895833 nsec, systime delta 3685 + playback: systime: 681915317 nsec, audio time 681916666 nsec, systime delta -1349 + playback: systime: 852741306 nsec, audio time 852750000 nsec, systime delta -8694 3. link timestamp, compensation for DMA+analog delay -$ ./audio_time -p --ts_type=2 -d -playback: systime: 341060004 nsec, audio time 341062791 nsec, systime delta -2787 -playback: systime: 426242074 nsec, audio time 426244875 nsec, systime delta -2801 -playback: systime: 597080992 nsec, audio time 597084583 nsec, systime delta -3591 -playback: systime: 682084512 nsec, audio time 682088291 nsec, systime delta -3779 -playback: systime: 852936229 nsec, audio time 852940916 nsec, systime delta -4687 -playback: systime: 938107562 nsec, audio time 938112708 nsec, systime delta -5146 +:: + + $ ./audio_time -p --ts_type=2 -d + playback: systime: 341060004 nsec, audio time 341062791 nsec, systime delta -2787 + playback: systime: 426242074 nsec, audio time 426244875 nsec, systime delta -2801 + playback: systime: 597080992 nsec, audio time 597084583 nsec, systime delta -3591 + playback: systime: 682084512 nsec, audio time 682088291 nsec, systime delta -3779 + playback: systime: 852936229 nsec, audio time 852940916 nsec, systime delta -4687 + playback: systime: 938107562 nsec, audio time 938112708 nsec, systime delta -5146 Example 1 shows that the timestamp at the DMA level is close to 1ms ahead of the actual playback time (as a side time this sort of @@ -181,20 +192,24 @@ shows how compensating for the delay exposes a 1ms accuracy (due to the use of the frame counter by the driver) Example 3: DMA timestamp, no compensation for delay, delta of ~5ms -$ ./audio_time -p -Dhw:1 -t1 -playback: systime: 120174019 nsec, audio time 125000000 nsec, systime delta -4825981 -playback: systime: 245041136 nsec, audio time 250000000 nsec, systime delta -4958864 -playback: systime: 370106088 nsec, audio time 375000000 nsec, systime delta -4893912 -playback: systime: 495040065 nsec, audio time 500000000 nsec, systime delta -4959935 -playback: systime: 620038179 nsec, audio time 625000000 nsec, systime delta -4961821 -playback: systime: 745087741 nsec, audio time 750000000 nsec, systime delta -4912259 -playback: systime: 870037336 nsec, audio time 875000000 nsec, systime delta -4962664 +:: + + $ ./audio_time -p -Dhw:1 -t1 + playback: systime: 120174019 nsec, audio time 125000000 nsec, systime delta -4825981 + playback: systime: 245041136 nsec, audio time 250000000 nsec, systime delta -4958864 + playback: systime: 370106088 nsec, audio time 375000000 nsec, systime delta -4893912 + playback: systime: 495040065 nsec, audio time 500000000 nsec, systime delta -4959935 + playback: systime: 620038179 nsec, audio time 625000000 nsec, systime delta -4961821 + playback: systime: 745087741 nsec, audio time 750000000 nsec, systime delta -4912259 + playback: systime: 870037336 nsec, audio time 875000000 nsec, systime delta -4962664 Example 4: DMA timestamp, compensation for delay, delay of ~1ms -$ ./audio_time -p -Dhw:1 -t1 -d -playback: systime: 120190520 nsec, audio time 120000000 nsec, systime delta 190520 -playback: systime: 245036740 nsec, audio time 244000000 nsec, systime delta 1036740 -playback: systime: 370034081 nsec, audio time 369000000 nsec, systime delta 1034081 -playback: systime: 495159907 nsec, audio time 494000000 nsec, systime delta 1159907 -playback: systime: 620098824 nsec, audio time 619000000 nsec, systime delta 1098824 -playback: systime: 745031847 nsec, audio time 744000000 nsec, systime delta 1031847 +:: + + $ ./audio_time -p -Dhw:1 -t1 -d + playback: systime: 120190520 nsec, audio time 120000000 nsec, systime delta 190520 + playback: systime: 245036740 nsec, audio time 244000000 nsec, systime delta 1036740 + playback: systime: 370034081 nsec, audio time 369000000 nsec, systime delta 1034081 + playback: systime: 495159907 nsec, audio time 494000000 nsec, systime delta 1159907 + playback: systime: 620098824 nsec, audio time 619000000 nsec, systime delta 1098824 + playback: systime: 745031847 nsec, audio time 744000000 nsec, systime delta 1031847 |