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axfer: add an explanation about Timer-based scheduling model

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This commit fulfills a subsection titled as 'Timer-based scheduling model'.

This scheduling model is introduced in a recent decade. In this model,
applications should take care of its timing to operate sampled data
according to any timer. This is an optional behaviour of runtime of
PCM substream.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
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Takashi Sakamoto 2019-01-05 17:42:09 +09:00 committed by Takashi Iwai
parent a01d54d4e2
commit 87ed959f9b
1 changed files with 45 additions and 0 deletions
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@ -710,6 +710,51 @@ option with
.I irq
value.
.SS Timer\-based scheduling model
The
.I no\-period\-wakeup
mode is an optional mode of runtime of PCM substream. The mode assumes a
specific feature of hardware and assist of in\-kernel driver and PCM
applications. In this mode, in\-kernel drivers don't operate hardware to
generate periodical notification for transmission of audio data frame.
The hardware should automatically continue transmission of audio data frame
without periodical operation of the drivers; e.g. according to auto\-triggered
DMA transmission, a chain of registered descriptors.
In this mode, nothing wakes up blocked processes, therefore PCM applications
should be programmed without any blocking operation. For this reason, this mode
is enabled when the PCM applications explicitly configure hardware parameter to
runtime of PCM substream, to prevent disorder of existing applications.
Additionally, nothing maintains timing for transmission of audio data frame,
therefore the PCM applications should voluntarily handle any timer to queue
audio data frame in buffer of the PCM substream for lapse of time. Furthermore,
instead of driver, the PCM application should call a helper function of ALSA
PCM core to update a position to head of hardware transmission and to check
XRUN.
In
.I axfer
, this is called
.I timer\-based
scheduling model and available as long as hardware/driver assists
.I no\-period\-wakeup
runtime. Users should explicitly set this mode by usage of
.I \-\-sched\-model
option with
.I timer
value.
In the scheduling model, PCM applications need to care of available space on
PCM buffer by lapse of time, typically by yielding CPU and wait for
rescheduling. For the yielding, timeout is calculated for preferable amount of
PCM frames to process. This is convenient to a kind of applications, like sound
servers. when an I/O thread of the server wait for the timeout, the other
threads can process audio data frames for server clients. Furthermore, with
usage of rewinding/forwarding, applications can achieve low latency between
transmission position and handling position even if they uses large size of
PCM buffers.
.SH COMPATIBILITY TO APLAY
The