On Mon, 18 May 2020 06:30:38 +0200, Brent Lu wrote: > > There is a corner case that ALSA keeps increasing the hw_ptr but DMA > already stop working/updating the position for a long time. > > In following log we can see the position returned from DMA driver does > not move at all but the hw_ptr got increased at some point of time so > snd_pcm_avail() will return a large number which seems to be a buffer > underrun event from user space program point of view. The program > thinks there is space in the buffer and fill more data. > > [ 418.510086] sound pcmC0D5p: pos 96 hw_ptr 96 appl_ptr 4096 avail 12368 > [ 418.510149] sound pcmC0D5p: pos 96 hw_ptr 96 appl_ptr 6910 avail 9554 > ... > [ 418.681052] sound pcmC0D5p: pos 96 hw_ptr 96 appl_ptr 15102 avail 1362 > [ 418.681130] sound pcmC0D5p: pos 96 hw_ptr 96 appl_ptr 16464 avail 0 > [ 418.726515] sound pcmC0D5p: pos 96 hw_ptr 16464 appl_ptr 16464 avail 16368 > > This is because the hw_base will be increased by runtime->buffer_size > frames unconditionally if the hw_ptr is not updated for over half of > buffer time. As the hw_base increases, so does the hw_ptr increased > by the same number. > > The avail value returned from snd_pcm_avail() could exceed the limit > (buffer_size) easily becase the hw_ptr itself got increased by same > buffer_size samples when the corner case happens. In following log, > the buffer_size is 16368 samples but the avail is 21810 samples so > CRAS server complains about it. > > [ 418.851755] sound pcmC0D5p: pos 96 hw_ptr 16464 appl_ptr 27390 avail 5442 > [ 418.926491] sound pcmC0D5p: pos 96 hw_ptr 32832 appl_ptr 27390 avail 21810 > > cras_server[1907]: pcm_avail returned frames larger than buf_size: > sof-glkda7219max: :0,5: 21810 > 16368 > > By updating runtime->hw_ptr_jiffies each time the HWSYNC is called, > the hw_base will keep the same when buffer stall happens at long as > the interval between each HWSYNC call is shorter than half of buffer > time. > > Following is a log captured by a patched kernel. The hw_base/hw_ptr > value is fixed in this corner case and user space program should be > aware of the buffer stall and handle it. > > [ 293.525543] sound pcmC0D5p: pos 96 hw_ptr 96 appl_ptr 4096 avail 12368 > [ 293.525606] sound pcmC0D5p: pos 96 hw_ptr 96 appl_ptr 6880 avail 9584 > [ 293.525975] sound pcmC0D5p: pos 96 hw_ptr 96 appl_ptr 10976 avail 5488 > [ 293.611178] sound pcmC0D5p: pos 96 hw_ptr 96 appl_ptr 15072 avail 1392 > [ 293.696429] sound pcmC0D5p: pos 96 hw_ptr 96 appl_ptr 16464 avail 0 > ... > [ 381.139517] sound pcmC0D5p: pos 96 hw_ptr 96 appl_ptr 16464 avail 0 > > Signed-off-by: Brent Lu <brent.lu@xxxxxxxxx> Thanks, applied now with Reviewed-by tag from Jaroslav. I also put Cc to stable, as it can fix the actual issues. Takashi
