On Fri, Aug 26, 2011 at 1:29 AM, Olivier Guillion - Myriad < olivier@myriad-online.com> wrote:
Hi,
Raymond Toy wrote:
I'm working on an application where I'd like to have relatively fixed
timing
between calls to snd_pcm_writei.
Why?
It's a bit complicated. A different process generates the audio and
places the
data in a shared memory area. My process sends signals the other process
to
generate more data, I read the shared memory area (for the previously
generated
data). Sometime later, the other process fills the shared area with new samples. Thus, if the timing between calls is not fairly regular, I'll
either
reread the same data or miss the new data. (I'm not in control of how
this
works. I just have to deal with what I'm given.)
IMO, it's not a good idea to try to synchronize perfectly two processes by way of an interrupt. If the generating process has to remain untouched, you should probably bufferize its data in your own process before sending it to writei. It would add some latency, but make the data flow safer. Here is how it could work: 1- Your process writes a "magic" pattern at the end of the shared area 2- It sends a signal to the other process to make it generate data 3- It checks whether the magic pattern is still here 4- When changed, it means the data have been calculated. It saves them to an internal circular buffer
A nice idea, but I'm not sure it will work in my scenario. The "magic" pattern could potentially be real data (the buffer contains audio samples), so that would cause funny, hard-to-reproduce glitches. Plus, I think that when I request new data, something is always written.
Clemens idea of a counter at the beginning of the buffer would work nicely, but requires changes in the generating process.
Thanks,
Ray