[alsa-devel] Need help diagnosing "hw_ptr skipping" message

[Clemens Ladisch]

Mike Looijmans wrote:
> If I enable the "jiffies check", what is my driver doing wrong if I
> get this complaint on the kernel log while capturing:
>
> PCM: hw_ptr skipping! (pos=13677, delta=876, period=6400, jdelta=0/17/0, hw_ptr=1241601/1241601)

"hw_ptr skipping" means that the hw_ptr is skipping.  Er, the value
returned by the .pointer callback moves too far in a too short amount
of time.

> hwptr log: pcmC6D0c:0 [Q]: j=19310, pos=12801/6400/25600, hwptr=1236601/1228800
> hwptr log: pcmC6D0c:0 [ ]: j=19310, pos=13677/6400/25600, hwptr=1241601/1228800

The interrupt and the userspace check happen at almost the same time
(same jiffies value), but there is suddenly a jump of 876 frames.

How does your driver compute the .pointer return value?  How does this
value change?


Regards,
Clemens