hw/audio/lm4549: Don't try to open a zero-frequency audio voice

If the guest incorrectly programs the lm4549 audio chip with a zero
frequency, we will pass this to AUD_open_out(), which will complain:

   A bug was just triggered in AUD_open_out
   Save all your work and restart without audio
   I am sorry
   Context:
   audio: frequency=0 nchannels=2 fmt=S16 endianness=little

The datasheet doesn't say what we should do here, only that the valid
range for the freqency is 4000 to 48000 Hz; we choose to log the
guest error and ignore an attempt to change the DAC rate to something
outside the valid range.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/410
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20251107154116.1396769-1-peter.maydell@linaro.org

hw/audio/lm4549.c

@@ -15,6 +15,7 @@
 
 #include "qemu/osdep.h"
 #include "hw/hw.h"
+#include "qemu/log.h"
 #include "qemu/audio.h"
 #include "lm4549.h"
 #include "migration/vmstate.h"
@@ -179,9 +180,23 @@ void lm4549_write(lm4549_state *s,
         break;
 
     case LM4549_PCM_Front_DAC_Rate:
-        regfile[LM4549_PCM_Front_DAC_Rate] = value;
         DPRINTF("DAC rate change = %i\n", value);
 
+        /*
+         * Valid sample rates are 4kHz to 48kHz.
+         * The datasheet doesn't say what happens if you try to
+         * set the frequency to zero. AUD_open_out() will print
+         * a bug message if we pass it a zero frequency, so just
+         * ignore attempts to set the DAC frequency to zero.
+         */
+        if (value < 4000 || value > 48000) {
+            qemu_log_mask(LOG_GUEST_ERROR,
+                          "%s: DAC sample rate %d Hz is invalid, ignoring it\n",
+                          __func__, value);
+            break;
+        }
+        regfile[LM4549_PCM_Front_DAC_Rate] = value;
+
         /* Re-open a voice with the new sample rate */
         struct audsettings as;
         as.freq = value;