Sound XRUN on RT-Linux

Hello ALSA-users!
Hello RT-users!

I think, this mail might be interesting for both the lists, since it's dealing with ALSA and the usage of a realtime-kernel.

I've been playing around with ALSA for quite some weeks now and I am trying to get reading of sound done with small latencies. Therefore I am using the "hw:0:0"-device with SND_PCM_STREAM_CAPTURE and SND_PCM_NONBLOCK. My device is configured with a total buffersize of 882 frames (44100Hz in SND_PCM_FORMAT_S16 on 2 channels).

All I do, is waiting 2 milliseconds and then reading with snd_ocm_readi() as much as I can from the soundcard (well, I get the amount of frames available after 2ms, of course. This timing is quite stable).

I have the program running on an X86-board (500MHz Geode with CS5535-Chipset), I applied the realtime patches of Ingo Molnar, I did set my software to SCHED_FIFO and prio 90, the ICQ of the sounddevice is set to 98 (this is done with chrt -f -p 98 pidof "IRQ-11"), my /proc/interrupts is quite empty (see below) and I did set the latency of the PCI-devices in order to give prio ty the soundcard:

setpci -v -s '*:*' latency_timer=20
setpci -v -d '1022:2093' latency_timer=80

My uname:

# uname -a
Linux master 2.6.29.5-rt22-486-voyage #1 PREEMPT RT Wed Jun 24 09:10:02
GMT 2009 i586 GNU/Linux

Here goes the /proc/interrupts - stuff:

#cat /proc/interrupts

CPU0
0: 1020847 XT-PIC-XT
timer
1: 2 XT-PIC-XT
i8042
2: 0 XT-PIC-XT
cascade
4: 982 XT-PIC-XT
serial
10: 762598 XT-PIC-XT
wifi0
11: 1788972 XT-PIC-XT CS5535
Audio
12: 4 XT-PIC-XT
i8042
14: 873 XT-PIC-XT
ide0
NMI: 0 Non-maskable
interrupts
LOC: 0 Local timer
interrupts
SPU: 0 Spurious
interrupts
CNT: 0 Performance counter
interrupts
ERR:
0
MIS: 0

My problem now: roundabout 5 times per minute, I get a buffer outrun, which results in a "scratch" of the sound, that I am recording. It seems like I am running over- and underruns again and again. The output looks like this:

curdma=f3d8000 < f970000
bufaddr.
curdma=f3d8000 < f970000
bufaddr.
curdma=f3d8000 < f970000
bufaddr.
curdma=f3d8000 < f970000
bufaddr.
diff=dc8 >= dc8
buf_bytes.
diff=dc8 >= dc8
buf_bytes.
curdma=f3d8000 < f970000
bufaddr.
curdma=f3d8000 < f970000
bufaddr.
curdma=f3d8000 < f970000
bufaddr.
diff=dc8 >= dc8 buf_bytes.
(and so on...)

I found the driver module, that produces this output, it is

/usr/src/linux-2.6.29.5/sound/pci/cs5535audio/cs5535audio_pcm.c

static snd_pcm_uframes_t snd_cs5535audio_pcm_pointer

, which genreates this output and seems to be the reason for my crappy sound.
Is there anyone among the RT-list or the ALSA-list who has a slight idea, what my problem might be related to and how I can get rid of it?

Very kind regards,
Dennis Borgmann