[alsa-devel] ### Question on HW Pointer
clemens at ladisch.de
Wed Oct 6 08:46:51 CEST 2010
Steve Spano wrote:
> I think the flow should be as follows (lets consider the dac part)
> Step #1 ) interrupt occurs
> Step #2) dma/transfer begins as kicked off in interrupt routine
So I assume the driver is responsible for telling the hardware to begin
transferring the next period. Doesn't it do this automatically?
> Step #3) we advance the buffer pointer for the alsa buffer
An interrupt (that is, for ALSA, the call to snd_pcm_period_elapsed)
signals the end of the previous period.
> Step #4) we exit the interrupt
> . then the hardware should start advancing its pointer as it is reading the
> new samples
> .. then sometime later, the alsa layer calls the "hw_pointer" function
The pointer callback can be called at any time. It will definitely be
called from snd_pcm_period_elapsed, so the buffer pointer must already
have been increased before that call.
> And then we return the current value of the hardware pointer from the board?
Drop the word "then"; the pointer callback _always_ must return the
current value. :)
> So alsa knows how many samples the board has processed from the interrupt?
Yes; ALSA (or the application) wants to know how many samples the board
has processed. If your DMA controller does not have a register for this,
the pointer must be calculated and updated in the interrupt handler (as
> Can someone explain how this pointer/etc is supposed to flow and what alsa
> may be using it for? Maybe some latency calculation in a an upper layer?
This, and to know which part of the buffer can be written to (the
hardware pointer indicates which part of the buffer has been processed
by the hardware; the samples after the pointer will be read now or
More information about the Alsa-devel