mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-12-15 20:36:49 +07:00
20a1d0f44d
A simple conversion from a plain text file. Put to designs subdirectory. Signed-off-by: Takashi Iwai <tiwai@suse.de>
216 lines
11 KiB
ReStructuredText
216 lines
11 KiB
ReStructuredText
=====================
|
|
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.
|
|
|
|
- tstamp is the current system timestamp updated during the last
|
|
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
|
|
timestamps allow for applications to keep track of the 'fullness' of
|
|
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.
|
|
|
|
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
|
|
dynamically with sw_params
|
|
|
|
|
|
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---->|
|
|
|
|
|
|
The analog time is taken at the last stage of the playback, as close
|
|
as possible to the actual transducer
|
|
|
|
The link time is taken at the output of the SoC/chipset as the samples
|
|
are pushed on a link. The link time can be directly measured if
|
|
supported in hardware by sample counters or wallclocks (e.g. with
|
|
HDAudio 24MHz or PTP clock for networked solutions) or indirectly
|
|
estimated (e.g. with the frame counter in USB).
|
|
|
|
The DMA time is measured using counters - typically the least reliable
|
|
of all measurements due to the bursty nature of DMA transfers.
|
|
|
|
The app time corresponds to the time tracked by an application after
|
|
writing in the ring buffer.
|
|
|
|
The application can query the hardware capabilities, define which
|
|
audio time it wants reported by selecting the relevant settings in
|
|
audio_tstamp_config fields, thus get an estimate of the timestamp
|
|
accuracy. It can also request the delay-to-analog be included in the
|
|
measurement. Direct access to the link time is very interesting on
|
|
platforms that provide an embedded DSP; measuring directly the link
|
|
time with dedicated hardware, possibly synchronized with system time,
|
|
removes the need to keep track of internal DSP processing times and
|
|
latency.
|
|
|
|
In case the application requests an audio tstamp that is not supported
|
|
in hardware/low-level driver, the type is overridden as DEFAULT and the
|
|
timestamp will report the DMA time based on the hw_pointer value.
|
|
|
|
For backwards compatibility with previous implementations that did not
|
|
provide timestamp selection, with a zero-valued COMPAT timestamp type
|
|
the results will default to the HDAudio wall clock for playback
|
|
streams and to the DMA time (hw_ptr) in all other cases.
|
|
|
|
The audio timestamp accuracy can be returned to user-space, so that
|
|
appropriate decisions are made:
|
|
|
|
- for dma time (default), the granularity of the transfers can be
|
|
inferred from the steps between updates and in turn provide
|
|
information on how much the application pointer can be rewound
|
|
safely.
|
|
|
|
- the link time can be used to track long-term drifts between audio
|
|
and system time using the (tstamp-trigger_tstamp)/audio_tstamp
|
|
ratio, the precision helps define how much smoothing/low-pass
|
|
filtering is required. The link time can be either reset on startup
|
|
or reported as is (the latter being useful to compare progress of
|
|
different streams - but may require the wallclock to be always
|
|
running and not wrap-around during idle periods). If supported in
|
|
hardware, the absolute link time could also be used to define a
|
|
precise start time (patches WIP)
|
|
|
|
- including the delay in the audio timestamp may
|
|
counter-intuitively not increase the precision of timestamps, e.g. if a
|
|
codec includes variable-latency DSP processing or a chain of
|
|
hardware components the delay is typically not known with precision.
|
|
|
|
The accuracy is reported in nanosecond units (using an unsigned 32-bit
|
|
word), which gives a max precision of 4.29s, more than enough for
|
|
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
|
|
any application selection. To work around this limitation without
|
|
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``.
|
|
|
|
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
|
|
timestamps from hardware registers or from IPC takes time, the more
|
|
timestamps are read the more imprecise the combined measurements
|
|
are. To avoid any interpretation issues, a single (system, audio)
|
|
timestamp is reported. Applications that need different timestamps
|
|
will be required to issue multiple queries and perform an
|
|
interpolation of the results
|
|
|
|
In some hardware-specific configuration, the system timestamp is
|
|
latched by a low-level audio subsystem, and the information provided
|
|
back to the driver. Due to potential delays in the communication with
|
|
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
|
|
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
|
|
|
|
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
|
|
|
|
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
|
|
|
|
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
|
|
measurement can help define rewind safeguards). Compensating for the
|
|
DMA-link delay in example 2 helps remove the hardware buffering but
|
|
the information is still very jittery, with up to one sample of
|
|
error. In example 3 where the timestamps are measured with the link
|
|
wallclock, the timestamps show a monotonic behavior and a lower
|
|
dispersion.
|
|
|
|
Example 3 and 4 are with USB audio class. Example 3 shows a high
|
|
offset between audio time and system time due to buffering. Example 4
|
|
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
|
|
|
|
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
|