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axfer: add an explanation about advantages/issues of Timer-based scheduling model

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Timer-based scheduling model has some merits against IRQ-based
scheduling model. However, as of Linux kernel v4.21, ALSA PCM interface
between kernel/userspace has a lack of some features.

This commit adds a subsection to describe it.

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 17:42:10 +09:00 committed by Takashi Iwai
parent 87ed959f9b
commit 1ad002bb2e
1 changed files with 22 additions and 0 deletions
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axfer/axfer-transfer.1
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@ -755,6 +755,28 @@ usage of rewinding/forwarding, applications can achieve low latency between
transmission position and handling position even if they uses large size of
PCM buffers.
.SS Advantages and issues
Ideally, timer\-based scheduling model has some advantages than IRQ\-based
scheduling model. At first, no interrupt context runs for PCM substream. The
PCM substream is handled in any process context only. No need to care of race
conditions between IRQ and process contexts. This reduces some concerns for
some developers of drivers and applications. Secondary, CPU time is not used
for handlers on the interrupt context. The CPU time can be dedicated for the
other tasks. This is good in a point of Time Sharing System. Thirdly, hardware
is not configured to generate interrupts. This is good in a point of reduction
of overall power consumption possibly.
In either scheduling model, the hardware should allow drivers to read the
number of audio data frame transferred between the dedicated memory and the
device memory for audio serial bus. However, in timer\-based scheduling model,
fine granularity and accuracy of the value is important. Actually hardware
performs transmission between dedicated memory and device memory for a small
batch of audio data frames or bytes. In a view of PCM applications, the
granularity in current transmission is required to decide correct timeout for
each I/O operation. As of Linux kernel v4.21, ALSA PCM interface between
kernel/userspace has no feature to report it.
.SH COMPATIBILITY TO APLAY
The