Hi all,
I am writing a PCM audio driver for a piece of (embedded) hardware that has no DMA, only a FIFO capable of holding 2048 samples. I was trying to use a kthread with high realtime (SCHED_FIFO) priority to keep the FIFO filled, and sleep with schedule_timeout(x), where "x" is depending on the current FIFO fill level. The same thread also calls snd_pcm_period_elapsed() on every completed period. It seems to work, but I get sporadic audio skips and I am trying to figure out where they come from. Most of the time schedule_timeout(1) takes almost exactly 1ms (1 jiffy at HZ=1000) to complete, but sometimes it takes up to 30ms. It seems related to file-IO (via NFS) happening on the system, but the effect is far bigger if the program that is playing the audio (mplayer) itself is producing file-IO. When using a large cache parameter on mplayer (effectively preloading the entire MP3 file into RAM), the problem is almost gone. Before deciding whether I should debug the network driver to see if it could produce such tremendous amounts of latency, is there a better way to solve this problem? Are there any other DMA-less audio drivers I could look at as an example? Any suggestion is welcome.
Best regards,