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 --- axfer/axfer-transfer.1 | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+)
diff --git a/axfer/axfer-transfer.1 b/axfer/axfer-transfer.1 index fbe0747..4ff6082 100644 --- a/axfer/axfer-transfer.1 +++ b/axfer/axfer-transfer.1 @@ -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
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