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229d0430 PLB |
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 reports 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 natured 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 what the hardware supports, define which | |
69 | audio time it wants reported by selecting the relevant settings in | |
70 | audio_tstamp_config fields, 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 subsytem, 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 abut | |
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 |