[alsa-devel] DMA circular buffers - little help wanted

[Petr Kulhavy]

Hi,

I'm trying to write a sound card driver and need a bit of help 
understanding the logic behind the DMA circular buffers.

I can understand there is a buffer of runtime->buffer_size frames, with 
two pointers: runtime->control->appl_ptr and runtime->status->hw_ptr 
pointing into it (they are actually offsets from the buffer start).
One pointer is the producer, the other consumer. So far a standard 
circular buffer as I know it. Please correct me if I'm wrong.

But what is runtime->boundary good for? It seems the arithmetics is not 
module buffer_size but modulo boundary, which is pretty confusing me.
According to the code and comments some values (like buffer "pointers", 
available space, free, space, etc.)  are supposed to be within 0 ... 
boundary - 1 and some within 0 ... buffer_size - 1. Not clear to me what 
is what and why.

Could someone please help me to understand the logic behind?

The other thing I would like to understand, how is the data positioned 
in time in terms of synchronisation? I mean where in the buffer, 
relative to the hw_ptr, is the frame that is supposed to play "now", for 
an arbitrary "now".

Thanks
Petr