[alsa-devel] hw_ptr_interrupt removal broke interrupt pointer updates

Clemens Ladisch clemens at ladisch.de
Tue Jan 26 14:39:39 CET 2010

Commit "cleanup & merge hw_ptr update functions" says:
> The main change is hw_ptr_interrupt variable removal to simplify code
> logic. This variable can be computed directly from hw_ptr.

The hw_ptr_interrupt variable was needed to differentiate between the
position at the last normal pointer update and the position of the last
signaled period boundary.

	if (in_interrupt) {
		/* we know that one period was processed */
		/* delta = "expected next hw_ptr" for in_interrupt != 0 */
		delta = old_hw_ptr - (old_hw_ptr % runtime->period_size)
			+ runtime->period_size;
		if (delta > new_hw_ptr) {
			hw_base += runtime->buffer_size;

It is possible for the status/delay ioctls to be called when the sound
card's pointer register alreay shows a position at the beginning of the
new period, but immediately before the interrupt is actually executed.
(This happens regularly on a SMP machine with mplayer.)  When that
happens, the code thinks that the position must be at least one period
ahead of the current position and drops an entire buffer of data.

Best regards,

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