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authorTakashi Iwai <tiwai@suse.de>2016-11-10 11:06:55 +0100
committerTakashi Iwai <tiwai@suse.de>2016-11-10 18:09:30 +0100
commit20a1d0f44d9447a68c387a2f561db4720302fb7c (patch)
treebc60ed6a51537a95c0df0d5d85ca1f97aa338c3d /Documentation/sound
parente9df12c3bac69e2cdfd76d7717bed92dee7cd617 (diff)
ALSA: doc: ReSTize timestamping document
A simple conversion from a plain text file. Put to designs subdirectory. Signed-off-by: Takashi Iwai <tiwai@suse.de>
Diffstat (limited to 'Documentation/sound')
-rw-r--r--Documentation/sound/designs/index.rst1
-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