probably this question is asked many times.

Anyway, I hope the experts are willing to help me here. I simply don't know how to avoid the DMA overrun that I'm facing when playing a tone.

Here's the thing.

I wrote a (new) ALSA driver (for AD1938 CODEC).

I test it with the ALSA-LIB pcm.c example program and it always results in a DMA underrun. I really wonder whether this is due to the driver (me doing something stupid) or a CPU shortage or something else beyond my control. (I cannot really imagine it's due to CPU shortage or anything).

I put may printk's in the kernel (in my driver and PCM middle layer). Here's the full test output/log.

root:/> /mnt/tprogs/pcm -c 2 -r 48000 -o S32_LE -v Playback device is hw:0,0 Stream parameters are 48000Hz, S32_LE, 2 channels Sine wave rate is 440.0000Hz Using transfer method: write setting start_threshold to (buffer_size=8192 / period_size=1024) * period_size=1024 = 8192 setting avail_min to 1024 Hardware PCM card 0 'ADI ad1938' device 0 subdevice 0 Its setup is: stream : PLAYBACK access : RW_INTERLEAVED format : S32_LE subformat : STD channels : 2 rate : 48000 exact rate : 48000 (48000/1) msbits : 32 buffer_size : 8192 period_size : 1024 period_time : 21333 tick_time : 4000 tstamp_mode : NONE period_step : 1 sleep_min : 0 avail_min : 1024 xfer_align : 1024 start_threshold : 8192 stop_threshold : 8192 silence_threshold: 0 silence_size : 0 boundary : 1073741824

root:/proc/asound/card0> root:/proc/asound/card0> snd_pcm_lib_write1: (still) to write 1024 frames snd_pcm_playback_avail(nr=0x0000): avail = (hwptr=0 + bz=8192) - apptr=0) = 8192 snd_pcm_lib_write1: actually wrote 0 frames, err=1024, offset=0, appl_ptr=0, appl_ofs=0 snd_pcm_playback_avail(nr=0x0100): avail = (hwptr=0 + bz=8192) - apptr=1024) = 7168 snd_pcm_lib_write1: (still) to write 1024 frames snd_pcm_playback_avail(nr=0x0000): avail = (hwptr=0 + bz=8192) - apptr=1024) = 7168 snd_pcm_lib_write1: actually wrote 0 frames, err=1024, offset=0, appl_ptr=1024, appl_ofs=1024 snd_pcm_playback_avail(nr=0x0100): avail = (hwptr=0 + bz=8192) - apptr=2048) = 6144 snd_pcm_lib_write1: (still) to write 1024 frames snd_pcm_playback_avail(nr=0x0000): avail = (hwptr=0 + bz=8192) - apptr=2048) = 6144 snd_pcm_lib_write1: actually wrote 0 frames, err=1024, offset=0, appl_ptr=2048, appl_ofs=2048 snd_pcm_playback_avail(nr=0x0100): avail = (hwptr=0 + bz=8192) - apptr=3072) = 5120 snd_pcm_lib_write1: (still) to write 1024 frames snd_pcm_playback_avail(nr=0x0000): avail = (hwptr=0 + bz=8192) - apptr=3072) = 5120 snd_pcm_lib_write1: actually wrote 0 frames, err=1024, offset=0, appl_ptr=3072, appl_ofs=3072 snd_pcm_playback_avail(nr=0x0100): avail = (hwptr=0 + bz=8192) - apptr=4096) = 4096 snd_pcm_lib_write1: (still) to write 1024 frames snd_pcm_playback_avail(nr=0x0000): avail = (hwptr=0 + bz=8192) - apptr=4096) = 4096 snd_pcm_lib_write1: actually wrote 0 frames, err=1024, offset=0, appl_ptr=4096, appl_ofs=4096 snd_pcm_playback_avail(nr=0x0100): avail = (hwptr=0 + bz=8192) - apptr=5120) = 3072 snd_pcm_lib_write1: (still) to write 1024 frames snd_pcm_playback_avail(nr=0x0000): avail = (hwptr=0 + bz=8192) - apptr=5120) = 3072 snd_pcm_lib_write1: actually wrote 0 frames, err=1024, offset=0, appl_ptr=5120, appl_ofs=5120 snd_pcm_playback_avail(nr=0x0100): avail = (hwptr=0 + bz=8192) - apptr=6144) = 2048 snd_pcm_lib_write1: (still) to write 1024 frames snd_pcm_playback_avail(nr=0x0000): avail = (hwptr=0 + bz=8192) - apptr=6144) = 2048 snd_pcm_lib_write1: actually wrote 0 frames, err=1024, offset=0, appl_ptr=6144, appl_ofs=6144 snd_pcm_playback_avail(nr=0x0100): avail = (hwptr=0 + bz=8192) - apptr=7168) = 1024 snd_pcm_lib_write1: (still) to write 1024 frames snd_pcm_playback_avail(nr=0x0000): avail = (hwptr=0 + bz=8192) - apptr=7168) = 1024 snd_pcm_lib_write1: actually wrote 0 frames, err=1024, offset=0, appl_ptr=7168, appl_ofs=7168 snd_pcm_playback_avail(nr=0x0100): avail = (hwptr=0 + bz=8192) - apptr=8192) = 0 snd_pcm_playback_avail(nr=0x0300): avail = (hwptr=0 + bz=8192) - apptr=8192) = 0 snd_pcm_lib_write1: (still) to write 1024 frames snd_ad1938_playback_pointer: bytes_per_frame=32, diff=24352, frames=761, dma_offset=0x0, dma_last_pos=0x0, dma_pos_base=0x0 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=761 + bz=8192) - apptr=8192) = 761 snd_pcm_playback_avail(nr=0x0000): avail = (hwptr=761 + bz=8192) - apptr=8192) = 761 snd_pcm_lib_write1: WAIT snd_ad1938_playback_pointer: bytes_per_frame=32, diff=45840, frames=1432, dma_offset=0x0, dma_last_pos=0x2f9, dma_pos_base=0x0 snd_pcm_update_hw_ptr_interrupt: pos=1432, new_hw_ptr=(hwpb=0 + pos)=1432, hwpi=1024, pz=1024, delta=-408, bz=8192 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=1432 + bz=8192) - apptr=8192) = 1432 snd_pcm_playback_avail(nr=0x0001): avail = (hwptr=1432 + bz=8192) - apptr=8192) = 1432 snd_pcm_lib_write1: WAKE-UP snd_pcm_lib_write1: actually wrote 0 frames, err=1024, offset=0, appl_ptr=8192, appl_ofs=0 snd_ad1938_playback_pointer: bytes_per_frame=32, diff=81920, frames=2560, dma_offset=0x0, dma_last_pos=0x598, dma_pos_base=0x0 snd_pcm_update_hw_ptr_interrupt: pos=2560, new_hw_ptr=(hwpb=0 + pos)=2560, hwpi=2048, pz=1024, delta=-512, bz=8192 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=2560 + bz=8192) - apptr=9216) = 1536 snd_ad1938_playback_pointer: bytes_per_frame=32, diff=114688, frames=3584, dma_offset=0x0, dma_last_pos=0xa00, dma_pos_base=0x0 snd_pcm_update_hw_ptr_interrupt: pos=3584, new_hw_ptr=(hwpb=0 + pos)=3584, hwpi=3072, pz=1024, delta=-512, bz=8192 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=3584 + bz=8192) - apptr=9216) = 2560 snd_pcm_lib_write1: (still) to write 1024 frames snd_ad1938_playback_pointer: bytes_per_frame=32, diff=150368, frames=4699, dma_offset=0x0, dma_last_pos=0xe00, dma_pos_base=0x0 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=4699 + bz=8192) - apptr=9216) = 3675 snd_pcm_playback_avail(nr=0x0000): avail = (hwptr=4699 + bz=8192) - apptr=9216) = 3675 snd_ad1938_playback_pointer: bytes_per_frame=32, diff=170664, frames=5333, dma_offset=0x0, dma_last_pos=0x125b, dma_pos_base=0x0 snd_pcm_update_hw_ptr_interrupt: pos=5333, new_hw_ptr=(hwpb=0 + pos)=5333, hwpi=4096, pz=1024, delta=-1237, bz=8192 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=5333 + bz=8192) - apptr=9216) = 4309 snd_pcm_lib_write1: actually wrote 0 frames, err=1024, offset=0, appl_ptr=9216, appl_ofs=1024 snd_ad1938_playback_pointer: bytes_per_frame=32, diff=199248, frames=6226, dma_offset=0x0, dma_last_pos=0x14d5, dma_pos_base=0x0 snd_pcm_update_hw_ptr_interrupt: pos=6226, new_hw_ptr=(hwpb=0 + pos)=6226, hwpi=6144, pz=1024, delta=-82, bz=8192 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=6226 + bz=8192) - apptr=9216) = 5202 snd_ad1938_playback_pointer: bytes_per_frame=32, diff=229376, frames=7168, dma_offset=0x0, dma_last_pos=0x1852, dma_pos_base=0x0 snd_pcm_update_hw_ptr_interrupt: pos=7168, new_hw_ptr=(hwpb=0 + pos)=7168, hwpi=7168, pz=1024, delta=0, bz=8192 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=7168 + bz=8192) - apptr=10240) = 5120 snd_ad1938_playback_pointer: bytes_per_frame=32, diff=262144, frames=8192, dma_offset=0x0, dma_last_pos=0x1c00, dma_pos_base=0x0 snd_pcm_update_hw_ptr_interrupt: pos=0, new_hw_ptr=(hwpb=0 + pos)=0, hwpi=8192, pz=1024, delta=8192, bz=8192 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=8192 + bz=8192) - apptr=10240) = 6144 snd_pcm_lib_write1: (still) to write 1024 frames snd_ad1938_playback_pointer: bytes_per_frame=32, diff=27384, frames=855, dma_offset=0x0, dma_last_pos=0x0, dma_pos_base=0x2000 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=9047 + bz=8192) - apptr=10240) = 6999 snd_pcm_playback_avail(nr=0x0000): avail = (hwptr=9047 + bz=8192) - apptr=10240) = 6999 snd_ad1938_playback_pointer: bytes_per_frame=32, diff=47744, frames=1492, dma_offset=0x0, dma_last_pos=0x357, dma_pos_base=0x2000 snd_pcm_update_hw_ptr_interrupt: pos=1492, new_hw_ptr=(hwpb=8192 + pos)=9684, hwpi=9216, pz=1024, delta=-468, bz=8192 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=9684 + bz=8192) - apptr=10240) = 7636 snd_pcm_lib_write1: actually wrote 0 frames, err=1024, offset=0, appl_ptr=10240, appl_ofs=2048 snd_ad1938_playback_pointer: bytes_per_frame=32, diff=81920, frames=2560, dma_offset=0x0, dma_last_pos=0x5d4, dma_pos_base=0x2000 snd_pcm_update_hw_ptr_interrupt: pos=2560, new_hw_ptr=(hwpb=8192 + pos)=10752, hwpi=10240, pz=1024, delta=-512, bz=8192 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=10752 + bz=8192) - apptr=11264) = 7680 snd_ad1938_playback_pointer: bytes_per_frame=32, diff=114688, frames=3584, dma_offset=0x0, dma_last_pos=0xa00, dma_pos_base=0x2000 snd_pcm_update_hw_ptr_interrupt: pos=3584, new_hw_ptr=(hwpb=8192 + pos)=11776, hwpi=11264, pz=1024, delta=-512, bz=8192 snd_pcm_playback_avail(nr=0x0010): avail = (hwptr=11776 + bz=8192) - apptr=11264) = 8704 snd_pcm_stop: stopping stream, state=4 snd_pcm_do_stop: SNDRV_PCM_TRIGGER_STOP snd_pcm_lib_write1: (still) to write 1024 frames

snd_pcm_playback_avail(nr=0x0000) = snd_pcm_lib_write1 calling snd_pcm_playback_avail (1st time) snd_pcm_playback_avail(nr=0x0001) = snd_pcm_lib_write1 calling snd_pcm_playback_avail (2nd time) snd_pcm_playback_avail(nr=0x0010) = snd_pcm_update_hw_ptr_post calling snd_pcm_playback_avail snd_pcm_playback_avail(nr=0x0100) = snd_pcm_playback_hw_avail calling snd_pcm_playback_avail snd_pcm_playback_avail(nr=0x0300) = snd_pcm_playback_data calling snd_pcm_playback_avail

I hope you're still with me. so the APP fills the DMA TX buf with 8192 frames. Then DMA starts.

DMA stops because there's more available in the DMA buffer then stop_threshold. stop_threshold is max = 8192 frames. somehow DMA is running faster than the APP. Probably this is a classic example of a playback underrun.

Most of the times the underrun happens sooner, the APP then not even gets a chance of writing more frames (than 8192).

so I understand why DMA stops but I don't want it to stop.

Is my driver doing something wrong ? What can I do to avoid having this underrun ?

Btw. what exactly is the purpose of boundary ?

thanks for any advice, N. van Bolhuis --------------