[alsa-devel] ALSA calling pcm_pointer excessively?

[Russell King - ARM Linux]

I'm wondering what is causing ALSA to behave as follows - I have a 64KiB
buffer, which is divided up into 4 16KiB periods.  The log below is from
my soc-dmaengine.c code which supports audio on the Assabet (as we don't
have cyclic DMA support there yet.)

I'm just trying to sort out the DMA engine tx_status residue code, and
am using this as proof that its working.

However, it seems that ALSA for whatever reason likes to hammer away at
the pcm_pointer callback from time to time:

soc_dmaengine_pointer: index 3 stat 1 residue 0018 cookie 3cfe pos 10000 bytes ffe8
soc_dmaengine_pointer: index 3 stat 1 residue 0008 cookie 3cfe pos 10000 bytes fff8
soc_dmaengine_callback: complete
soc_dmaengine_pointer: index 0 stat 1 residue 3ff0 cookie 3cff pos 4000 bytes 0010
soc_dmaengine_submit: submitting index 3 size 4000 cookie 3d02
soc_dmaengine_pointer: index 0 stat 1 residue 3fd0 cookie 3cff pos 4000 bytes 0030
soc_dmaengine_pointer: index 0 stat 1 residue 17b8 cookie 3cff pos 4000 bytes 2848
soc_dmaengine_pointer: index 0 stat 1 residue 1770 cookie 3cff pos 4000 bytes 2890
soc_dmaengine_pointer: index 0 stat 1 residue 1768 cookie 3cff pos 4000 bytes 2898
soc_dmaengine_pointer: index 0 stat 1 residue 1758 cookie 3cff pos 4000 bytes 28a8
soc_dmaengine_pointer: index 0 stat 1 residue 1748 cookie 3cff pos 4000 bytes 28b8
soc_dmaengine_pointer: index 0 stat 1 residue 1740 cookie 3cff pos 4000 bytes 28c0
soc_dmaengine_pointer: index 0 stat 1 residue 1730 cookie 3cff pos 4000 bytes 28d0
soc_dmaengine_pointer: index 0 stat 1 residue 1720 cookie 3cff pos 4000 bytes 28e0
soc_dmaengine_pointer: index 0 stat 1 residue 1718 cookie 3cff pos 4000 bytes 28e8
...
soc_dmaengine_pointer: index 0 stat 1 residue 0018 cookie 3cff pos 4000 bytes 3fe8
soc_dmaengine_pointer: index 0 stat 1 residue 0008 cookie 3cff pos 4000 bytes 3ff8
soc_dmaengine_callback: complete
soc_dmaengine_pointer: index 1 stat 1 residue 3ff0 cookie 3d00 pos 8000 bytes 4010
soc_dmaengine_submit: submitting index 0 size 4000 cookie 3d03
soc_dmaengine_pointer: index 1 stat 1 residue 3fd0 cookie 3d00 pos 8000 bytes 4030
soc_dmaengine_pointer: index 1 stat 1 residue 3b28 cookie 3d00 pos 8000 bytes 44d8
soc_dmaengine_pointer: index 1 stat 1 residue 3b10 cookie 3d00 pos 8000 bytes 44f0
soc_dmaengine_pointer: index 1 stat 1 residue 3b00 cookie 3d00 pos 8000 bytes 4500

'pos' is the end of the current DMA buffer ((index + 1) * period_byte_size)
'residue' is the value returned from tx_status
'bytes' is the value passed into bytes_to_frames() to return from my
pcm_pointer function.

The question is - what would cause ALSA to hammer away at the buffer
like that - checking it every 16 or 8 DMA bytes transferred?  I'm also
seeing around 75% system CPU time, which really isn't good.

The above was captured while aplay was running, with no apparant audio
corruption.

Any ideas?