I've been chasing a capture dropout in the old OSS cs46xx driver (drivers/sound/cs46xx.c) for the past two months (evenings and weekends), and I finally nailed it. I looked over at the ALSA driver, and I think I see the same problem there as well. I traced the peek/poke(PD1_CBA) from the update_ptr routine to the read routine, so I'm pretty confident the bug is there.
Briefly, I'm doing 8-channel recording, using 4 TB Santa Cruz cards. Occasionally, one of the cards loses a chunk of data. I traced the problem to the 4k dma buffer size. The IRQ updates the capture pointer, but it only has a 1/44th second real-time latency before it loses data. If the read doesn't get reposted by the application within that time, data will be lost. All it really takes is a busy system - either an IRQ from another card, or another thread getting in the way.
I fixed the problem in the OSS driver by copying the 4k ping-pong into a larger buffer, which is then drained by read(). Essentially, I implemented the Scatter/Gather in the ISR. It's not pretty, but it works. (Incidentally, if anyone has found the Cirrus documentation on cs46xx capture S/G, I'd be most appreciative. The only document I found in the ALSA documentation archive glibly said that S/G is "complicated" and that 1/44th second should be OK - yeah, right.)
This problem is likely to affect *any* sound card with a 4k dma ping-pong. I'm not sure if the cs46xx is the only pathological case out there, but I wouldn't be surprised if there are others. The really disturbing thought is that ALSA itself has real-time issues because 4k is a common dma size (say it isn't so!)
I'm a couple weeks away from porting my application to ALSA, so if someone else wants to load up a bunch of sound cards, feel free. I'm finding the bug injecting a sine wave into the four cards, then looking for large deltas from one sample to the next.
Comments?
Perry Scott
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