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axfer: add an explanation about IRQ-based scheduling model

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This commit adds a section titled as 'SCHEDULING MODEL' and fulfill a
subsection titled as 'IRQ-based scheduling model'.

This scheduling model is for a typical applications to operate a kind of
data sampled against actual time. In this model, ALSA PCM core maintains
timing of the applications with notification of hardware by
blocking/waking up their processes. This is a default behaviour of
runtime of PCM substream.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
Takashi Sakamoto 2019-01-05 17:42:08 +09:00 committed by Takashi Iwai
parent 1c2315010c
commit a01d54d4e2
1 changed files with 44 additions and 1 deletions
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@ -665,7 +665,50 @@ named \(aqchannels\-1.au\(aq and \(aqchannels\-2.au\(aq.
.SH SCHEDULING MODEL .SH SCHEDULING MODEL
(placeholder) In a design of ALSA PCM core, runtime of PCM substream supports two modes;
.I period\-wakeup
and
.I no\-period\-wakeup.
These two modes are for different scheduling models.
.SS IRQ\-based scheduling model
As a default,
.I period\-wakeup
mode is used. In this mode, in\-kernel drivers should operate hardware to
generate periodical notification for transmission of audio data frame. The
interval of notification is equivalent to the same amount of audio data frame
as one period of buffer, against actual time.
In a handler assigned to the notification, a helper function of ALSA PCM core
is called to update a position to head of hardware transmission, then compare
it with a position to head of application operation to judge overrun/underrun
(XRUN) and to wake up blocked processes.
For this purpose, hardware IRQ of controller for serial audio bus such as
Inter\-IC sound is typically used. In this case, the controller generates the
IRQ according to transmission on the serial audio bus. In the handler assigned
to the IRQ, direct media access (DMA) transmission is requested between
dedicated host memory and device memory.
If target hardware doesn't support this kind of mechanism, the periodical
notification should be emulated by any timer; e.g. hrtimer, kernel timer.
External PCM plugins generated by PCM plugin SDK in alsa\-lib should also
emulate the above behaviour.
In this mode, PCM applications are programmed according to typical way of I/O
operations. They execute blocking system calls to read/write audio data frame
in buffer of PCM substream, or blocking system calls to wait until any audio
data frame is available. In
.I axfer
, this is called
.I IRQ\-based
scheduling model and a default behaviour. Users can explicitly configure this
mode by usage of
.I \-\-sched\-model
option with
.I irq
value.
.SH COMPATIBILITY TO APLAY .SH COMPATIBILITY TO APLAY