alsaaudio: Set try-poll to false by default

Quoting Volker Rümelin: "try-poll=on tells the ALSA backend to try to
use an event loop instead of the audio timer. This works most of the
time. But the poll event handler in the ALSA backend has a bug. For
example, if the guest can't provide enough audio frames in time, the
ALSA buffer is only partly full and the event handler will be called
again and again on every iteration of the main loop. This increases
the processor load and the guest has less processor time to provide
new audio frames in time. I have two examples where a guest can't
recover from this situation and the guest seems to hang."

One reproducer I've found is booting MorphOS demo iso on
qemu-system-ppc -machine pegasos2 -audio alsa which should play a
startup sound but instead it freezes. Even when it does not hang it
plays choppy sound. Volker suggested using command line to set
try-poll=off saying: "The try-poll=off arguments are typically
necessary, because the alsa backend has a design issue with
try-poll=on. If the guest can't provide enough audio frames, it's
really unhelpful to ask for new audio frames on every main loop
iteration until the guest can provide enough audio frames. Timer based
playback doesn't have that problem."

But users cannot easily find this option and having a non-working
default is really unhelpful so to make life easier just set it to
false by default which works until the issue with the alsa backend can
be fixed.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
[ Marc-André - Updated QAPI and CLI doc ]
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20250316002046.D066A4E6004@zero.eik.bme.hu>

diff --git a/audio/alsaaudio.c b/audio/alsaaudio.c
index cacae1ea59ce82db43884a185f476c6749d4d5fc..9b6c01c0ef574451fcce8fc23eb24a937b38d3ac 100644 (file)
--- a/audio/alsaaudio.c
+++ b/audio/alsaaudio.c
@@ -899,7 +899,7 @@ static void alsa_enable_in(HWVoiceIn *hw, bool enable)
 static void alsa_init_per_direction(AudiodevAlsaPerDirectionOptions *apdo)
 {
     if (!apdo->has_try_poll) {
-        apdo->try_poll = true;
+        apdo->try_poll = false;
         apdo->has_try_poll = true;
     }
 }

diff --git a/qapi/audio.json b/qapi/audio.json
index dd5a58d13e682d3b373d1a2b6a2fa06788b89cbc..49633cf317c959e5eae07d6e127b5767e94669cc 100644 (file)
--- a/qapi/audio.json
+++ b/qapi/audio.json
@@ -96,7 +96,7 @@
 # @period-length: the period length in microseconds
 #
 # @try-poll: attempt to use poll mode, falling back to non-polling
-#     access on failure (default true)
+#     access on failure (default false)
 #
 # Since: 4.0
 ##

diff --git a/qemu-options.hx b/qemu-options.hx
index aab53bcfe8f572552fd31fba1ab8afbcf05ff539..7eb8e02b4b936050b7dffbf8b9d315e30d7dcd1f 100644 (file)
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -965,7 +965,7 @@ SRST
         Sets the period length in microseconds.
 
     ``in|out.try-poll=on|off``
-        Attempt to use poll mode with the device. Default is on.
+        Attempt to use poll mode with the device. Default is off.
 
     ``threshold=threshold``
         Threshold (in microseconds) when playback starts. Default is 0.
@@ -1002,7 +1002,7 @@ SRST
     ``in|out.buffer-count=count``
         Sets the count of the buffers.
 
-    ``in|out.try-poll=on|of``
+    ``in|out.try-poll=on|off``
         Attempt to use poll mode with the device. Default is on.
 
     ``try-mmap=on|off``