[alsa-devel] what is exactly pcm.boundary?

[Pierre-Louis Bossart]

On 1/6/17 10:08 AM, Ricardo Biehl wrote:
> 2017-01-06 6:09 GMT-02:00, Clemens Ladisch <clemens at ladisch.de>:
>>
>> Are you talking about code in your driver (where you are responsible for
>> asking the hardware), or user space code (where you must not directly
>> read hw_ptr)?  What is the actual problem you're trying to solve?
>>
>
> Hello!
> I'm developing a "live microphone system over local network" which are user
> space programs (clients => server) transferring audio nearly real-time
> (human imperceptible latency).
> I need to elaborate a good solution of implementation to have everything working
> as expected -- networking, scheduling, IO management etc -- However I need at
> first make sure I can get control over audio.
>
> I decided to use ioctl() level of communication with ALSA in kernel, and what I
> need is synchronization :-)
> I must know which sample the hardware is currently processing to prevent a lot
> of losses or latency increasing.

I would hate to sound pedantic but sample-accurate control (10us at 
48kHz) is far from trivial. The hw_ptr points to the ring buffer, but 
you may have additional samples queued up in DMA transfers, FIFOS, 
codecs and last some analog delays. On a network-based solution the 
different components typically don't use the same audio clock and you 
will have to deal with drifts between source and sink.
Best of luck.