Based on kernel version 4.9. Page generated on 2016-12-21 14:37 EST.
1 The ALSA API can provide two different system timestamps: 2 3 - Trigger_tstamp is the system time snapshot taken when the .trigger 4 callback is invoked. This snapshot is taken by the ALSA core in the 5 general case, but specific hardware may have synchronization 6 capabilities or conversely may only be able to provide a correct 7 estimate with a delay. In the latter two cases, the low-level driver 8 is responsible for updating the trigger_tstamp at the most appropriate 9 and precise moment. Applications should not rely solely on the first 10 trigger_tstamp but update their internal calculations if the driver 11 provides a refined estimate with a delay. 12 13 - tstamp is the current system timestamp updated during the last 14 event or application query. 15 The difference (tstamp - trigger_tstamp) defines the elapsed time. 16 17 The ALSA API provides two basic pieces of information, avail 18 and delay, which combined with the trigger and current system 19 timestamps allow for applications to keep track of the 'fullness' of 20 the ring buffer and the amount of queued samples. 21 22 The use of these different pointers and time information depends on 23 the application needs: 24 25 - 'avail' reports how much can be written in the ring buffer 26 - 'delay' reports the time it will take to hear a new sample after all 27 queued samples have been played out. 28 29 When timestamps are enabled, the avail/delay information is reported 30 along with a snapshot of system time. Applications can select from 31 CLOCK_REALTIME (NTP corrections including going backwards), 32 CLOCK_MONOTONIC (NTP corrections but never going backwards), 33 CLOCK_MONOTIC_RAW (without NTP corrections) and change the mode 34 dynamically with sw_params 35 36 37 The ALSA API also provide an audio_tstamp which reflects the passage 38 of time as measured by different components of audio hardware. In 39 ascii-art, this could be represented as follows (for the playback 40 case): 41 42 43 --------------------------------------------------------------> time 44 ^ ^ ^ ^ ^ 45 | | | | | 46 analog link dma app FullBuffer 47 time time time time time 48 | | | | | 49 |< codec delay >|<--hw delay-->|<queued samples>|<---avail->| 50 |<----------------- delay---------------------->| | 51 |<----ring buffer length---->| 52 53 The analog time is taken at the last stage of the playback, as close 54 as possible to the actual transducer 55 56 The link time is taken at the output of the SoC/chipset as the samples 57 are pushed on a link. The link time can be directly measured if 58 supported in hardware by sample counters or wallclocks (e.g. with 59 HDAudio 24MHz or PTP clock for networked solutions) or indirectly 60 estimated (e.g. with the frame counter in USB). 61 62 The DMA time is measured using counters - typically the least reliable 63 of all measurements due to the bursty nature of DMA transfers. 64 65 The app time corresponds to the time tracked by an application after 66 writing in the ring buffer. 67 68 The application can query the hardware capabilities, define which 69 audio time it wants reported by selecting the relevant settings in 70 audio_tstamp_config fields, thus get an estimate of the timestamp 71 accuracy. It can also request the delay-to-analog be included in the 72 measurement. Direct access to the link time is very interesting on 73 platforms that provide an embedded DSP; measuring directly the link 74 time with dedicated hardware, possibly synchronized with system time, 75 removes the need to keep track of internal DSP processing times and 76 latency. 77 78 In case the application requests an audio tstamp that is not supported 79 in hardware/low-level driver, the type is overridden as DEFAULT and the 80 timestamp will report the DMA time based on the hw_pointer value. 81 82 For backwards compatibility with previous implementations that did not 83 provide timestamp selection, with a zero-valued COMPAT timestamp type 84 the results will default to the HDAudio wall clock for playback 85 streams and to the DMA time (hw_ptr) in all other cases. 86 87 The audio timestamp accuracy can be returned to user-space, so that 88 appropriate decisions are made: 89 90 - for dma time (default), the granularity of the transfers can be 91 inferred from the steps between updates and in turn provide 92 information on how much the application pointer can be rewound 93 safely. 94 95 - the link time can be used to track long-term drifts between audio 96 and system time using the (tstamp-trigger_tstamp)/audio_tstamp 97 ratio, the precision helps define how much smoothing/low-pass 98 filtering is required. The link time can be either reset on startup 99 or reported as is (the latter being useful to compare progress of 100 different streams - but may require the wallclock to be always 101 running and not wrap-around during idle periods). If supported in 102 hardware, the absolute link time could also be used to define a 103 precise start time (patches WIP) 104 105 - including the delay in the audio timestamp may 106 counter-intuitively not increase the precision of timestamps, e.g. if a 107 codec includes variable-latency DSP processing or a chain of 108 hardware components the delay is typically not known with precision. 109 110 The accuracy is reported in nanosecond units (using an unsigned 32-bit 111 word), which gives a max precision of 4.29s, more than enough for 112 audio applications... 113 114 Due to the varied nature of timestamping needs, even for a single 115 application, the audio_tstamp_config can be changed dynamically. In 116 the STATUS ioctl, the parameters are read-only and do not allow for 117 any application selection. To work around this limitation without 118 impacting legacy applications, a new STATUS_EXT ioctl is introduced 119 with read/write parameters. ALSA-lib will be modified to make use of 120 STATUS_EXT and effectively deprecate STATUS. 121 122 The ALSA API only allows for a single audio timestamp to be reported 123 at a time. This is a conscious design decision, reading the audio 124 timestamps from hardware registers or from IPC takes time, the more 125 timestamps are read the more imprecise the combined measurements 126 are. To avoid any interpretation issues, a single (system, audio) 127 timestamp is reported. Applications that need different timestamps 128 will be required to issue multiple queries and perform an 129 interpolation of the results 130 131 In some hardware-specific configuration, the system timestamp is 132 latched by a low-level audio subsystem, and the information provided 133 back to the driver. Due to potential delays in the communication with 134 the hardware, there is a risk of misalignment with the avail and delay 135 information. To make sure applications are not confused, a 136 driver_timestamp field is added in the snd_pcm_status structure; this 137 timestamp shows when the information is put together by the driver 138 before returning from the STATUS and STATUS_EXT ioctl. in most cases 139 this driver_timestamp will be identical to the regular system tstamp. 140 141 Examples of typestamping with HDaudio: 142 143 1. DMA timestamp, no compensation for DMA+analog delay 144 $ ./audio_time -p --ts_type=1 145 playback: systime: 341121338 nsec, audio time 342000000 nsec, systime delta -878662 146 playback: systime: 426236663 nsec, audio time 427187500 nsec, systime delta -950837 147 playback: systime: 597080580 nsec, audio time 598000000 nsec, systime delta -919420 148 playback: systime: 682059782 nsec, audio time 683020833 nsec, systime delta -961051 149 playback: systime: 852896415 nsec, audio time 853854166 nsec, systime delta -957751 150 playback: systime: 937903344 nsec, audio time 938854166 nsec, systime delta -950822 151 152 2. DMA timestamp, compensation for DMA+analog delay 153 $ ./audio_time -p --ts_type=1 -d 154 playback: systime: 341053347 nsec, audio time 341062500 nsec, systime delta -9153 155 playback: systime: 426072447 nsec, audio time 426062500 nsec, systime delta 9947 156 playback: systime: 596899518 nsec, audio time 596895833 nsec, systime delta 3685 157 playback: systime: 681915317 nsec, audio time 681916666 nsec, systime delta -1349 158 playback: systime: 852741306 nsec, audio time 852750000 nsec, systime delta -8694 159 160 3. link timestamp, compensation for DMA+analog delay 161 $ ./audio_time -p --ts_type=2 -d 162 playback: systime: 341060004 nsec, audio time 341062791 nsec, systime delta -2787 163 playback: systime: 426242074 nsec, audio time 426244875 nsec, systime delta -2801 164 playback: systime: 597080992 nsec, audio time 597084583 nsec, systime delta -3591 165 playback: systime: 682084512 nsec, audio time 682088291 nsec, systime delta -3779 166 playback: systime: 852936229 nsec, audio time 852940916 nsec, systime delta -4687 167 playback: systime: 938107562 nsec, audio time 938112708 nsec, systime delta -5146 168 169 Example 1 shows that the timestamp at the DMA level is close to 1ms 170 ahead of the actual playback time (as a side time this sort of 171 measurement can help define rewind safeguards). Compensating for the 172 DMA-link delay in example 2 helps remove the hardware buffering but 173 the information is still very jittery, with up to one sample of 174 error. In example 3 where the timestamps are measured with the link 175 wallclock, the timestamps show a monotonic behavior and a lower 176 dispersion. 177 178 Example 3 and 4 are with USB audio class. Example 3 shows a high 179 offset between audio time and system time due to buffering. Example 4 180 shows how compensating for the delay exposes a 1ms accuracy (due to 181 the use of the frame counter by the driver) 182 183 Example 3: DMA timestamp, no compensation for delay, delta of ~5ms 184 $ ./audio_time -p -Dhw:1 -t1 185 playback: systime: 120174019 nsec, audio time 125000000 nsec, systime delta -4825981 186 playback: systime: 245041136 nsec, audio time 250000000 nsec, systime delta -4958864 187 playback: systime: 370106088 nsec, audio time 375000000 nsec, systime delta -4893912 188 playback: systime: 495040065 nsec, audio time 500000000 nsec, systime delta -4959935 189 playback: systime: 620038179 nsec, audio time 625000000 nsec, systime delta -4961821 190 playback: systime: 745087741 nsec, audio time 750000000 nsec, systime delta -4912259 191 playback: systime: 870037336 nsec, audio time 875000000 nsec, systime delta -4962664 192 193 Example 4: DMA timestamp, compensation for delay, delay of ~1ms 194 $ ./audio_time -p -Dhw:1 -t1 -d 195 playback: systime: 120190520 nsec, audio time 120000000 nsec, systime delta 190520 196 playback: systime: 245036740 nsec, audio time 244000000 nsec, systime delta 1036740 197 playback: systime: 370034081 nsec, audio time 369000000 nsec, systime delta 1034081 198 playback: systime: 495159907 nsec, audio time 494000000 nsec, systime delta 1159907 199 playback: systime: 620098824 nsec, audio time 619000000 nsec, systime delta 1098824 200 playback: systime: 745031847 nsec, audio time 744000000 nsec, systime delta 1031847