pulse-audio: fix bug on updating rpos

pulse-audio: fix bug on updating rpos

Fix a rpos coordination bug between qpa_run_out() and qpa_thread_out(),
which shows up as playback noises.

	qpa_run_out()
			qpa_thread_out loop N critical section 1
	qpa_run_out()   qpa_thread_out loop N doing pa_simple_write()
	qpa_run_out()	qpa_thread_out loop N doing pa_simple_write()
			qpa_thread_out loop N critical section 2
			qpa_thread_out loop N+1 critical section 1
	qpa_run_out()	qpa_thread_out loop N+1 doing pa_simple_write()

In the above scheme, "qpa_thread_out loop N+1 critical section 1" will
get the same rpos as the one used by "qpa_thread_out loop N critical
section 1". So it will be reading dead samples from the old rpos.

The rpos can only be updated back to qpa_thread_out when there is a
qpa_run_out() run between two qpa_thread_out loops.

normal sequence:
	qpa_thread_out:
			hw->rpos (X0) => local rpos => pa->rpos (X1)
	qpa_run_out:
			pa->rpos (X1) => hw->rpos (X1)
	qpa_thread_out:
			hw->rpos (X1) => local rpos => pa->rpos (X2)

buggy sequence:
	qpa_thread_out:
			hw->rpos (X0) => local rpos => pa->rpos (X1)
	qpa_thread_out:
			hw->rpos (X0) => local rpos => pa->rpos (X1')

Obviously qpa_run_out() shall be called at least once between any two
qpa_thread_out loops (after pa->rpos is set), in order for the new
qpa_thread_out loop to see the updated rpos.

Setting pa->live to 0 does the trick. The next loop will have to wait
for one qpa_run_out() invocation in order to get a non-zero pa->live
and proceed.

Signed-off-by: malc <[email protected]>
Signed-off-by: Wu Fengguang <[email protected]>