Problem with SoundBlaster 16 Interrupt Flags (fix included)

[SpyderTL]

I stumbled across this issue while working on a SoundBlaster 16 driver for my personal OS.

I noticed that the Interrupt Flags, located at Mixer Register 0x82 (bits 0 and 1) are never cleared, even after an interrupt has been acknowledged.

I scanned through the devsb16.cpp file, and found this:

1164    case 0x0e:                  /* data available status | irq 8 ack */ 
1165        retval = (!s->out_data_len || s->highspeed) ? 0 : 0x80; 
1166        if (s->mixer_regs[0x82] & 1) { 
1167            ack = 1; 
1168            s->mixer_regs[0x82] &= 1;                 // PROBLEM!
1169#ifndef VBOX 
1170            qemu_irq_lower (s->pic[s->irq]); 
1171#else 
1172            PDMDevHlpISASetIrq(s->pDevIns, s->irq, 0); 
1173#endif 
1174        } 
1175        break; 
1176 
1177    case 0x0f:                  /* irq 16 ack */ 
1178        retval = 0xff; 
1179        if (s->mixer_regs[0x82] & 2) { 
1180            ack = 1; 
1181            s->mixer_regs[0x82] &= 2;              // PROBLEM!!
1182#ifndef VBOX 
1183            qemu_irq_lower (s->pic[s->irq]); 
1184#else 
1185            PDMDevHlpISASetIrq(s->pDevIns, s->irq, 0); 
1186#endif 
1187        } 
1188        break;

The problem appears to be on line 1168 and 1181. Those flags should be inverted before they are ANDed with the current value:

1168            s->mixer_regs[0x82] &= ~1;              // FIXED!
1181            s->mixer_regs[0x82] &= ~2;              // FIXED!

I would submit this patched code myself, but I am not set up on this machine to download and compile VBOX.

Thanks for your help, guys. Great product.

[Michael Thayer]

That certainly looks sensible, but none of us have looked at that code for a long time (if at all). Perhaps you could run your patch past the original Qemu author, Vasily Karpov<1>? Thanks.

<1> ​http://git.qemu.org/?p=qemu.git;a=commit;h=cf4dc461a4cfc3e056ee24edb26154f4d34a6278

[Frank Mehnert]

If you have a test case we would even consider applying the patch if the original Qemu author would not respond.

[SpyderTL]

Patch has been committed in QEMU git repository.

​http://git.qemu.org/?p=qemu.git;a=blob;f=hw/audio/sb16.c;h=444eb9e419f1babb75c831faca0d3ebfe9a6672d;hb=HEAD

[Frank Mehnert]

Thank you! The fix will be part of the next maintenance release.