[Qemu-devel] [PATCH] pulse-audio: fix bug on updating rpos

[Wu Fengguang]

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 <address@hidden>
Signed-off-by: Wu Fengguang <address@hidden>
---
 audio/paaudio.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- qemu-kvm.orig/audio/paaudio.c       2010-09-29 10:28:25.000000000 +0800
+++ qemu-kvm/audio/paaudio.c    2010-09-29 12:11:45.000000000 +0800
@@ -111,8 +111,8 @@ static void *qpa_thread_out (void *arg)
             return NULL;
         }
 
+        pa->live = 0;
         pa->rpos = rpos;
-        pa->live -= decr;
         pa->decr += decr;
     }