Sound through ALSA, a 2D game, and a stack walk that cannot panic

SDL 3 has no OSS backend and SchOS has no ALSA kernel side, so sound now
goes SDL -> alsa-lib -> alsa-plugins' OSS plugin -> /dev/dsp. That path
found two bugs underneath it:

- ac97: resetting the PCM-out box also clears the descriptor list address,
  and nothing set it again. Every stream after the first stop played a
  "list" at physical address 0 -- silence -- and ALSA resets before each
  stream. If the engine halts at the end of the queue, the run bit is set
  again.
- /dev/dsp: SETFRAGMENT, GETFMTS, GETCAPS, SETTRIGGER, GETOPTR and GETODELAY
  for the plugin's ring-and-pointer model, and poll(), writable once a whole
  fragment fits (ALSA spun at 76% CPU otherwise). speaker-test now plays in
  real time at 0% CPU, verified in a recording of QEMU's AC'97.
- /etc/asound.conf: the default device is a plug to the OSS device at 16-bit
  stereo 48 kHz, what every SchOS audio driver plays.

Also:

- getstackpcs: each frame link must go up the same stack by less than
  KSTACKSIZE. The chain ends at the syscall entry with the user's rbp,
  which lbreakouthd uses as a register (-1024); the walk dereferenced it
  under the syscall tracer and the kernel panicked inside acquire().
- fb: FBIOGET_VSCREENINFO reports nominal timings, as Linux's efifb does,
  instead of zeros, which Xorg's fbdev turned into a 0 Hz RandR mode.
- procdump (Ctrl-P) shows where runnable threads are in user space.
- World: lbreakouthd instead of lbreakout2 (its stk.c passes vsnprintf a
  size of 511 plus the prefix length, and Alpine's fortified headers trap on
  that), plus curl, alsa-lib and alsa-plugins.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/fs/file.c

@@ -427,6 +427,11 @@ file_poll(struct file *f, int events)
        (f->ip->major == PTM_MAJOR || f->ip->major == PTS_MAJOR))
       return pty_poll(f->ip, events, f->readable, f->writable);
 
+    // /dev/dsp is writable when the driver has room. ALSA's OSS plugin waits
+    // in poll() for exactly this between periods.
+    if(f->ip && f->ip->type == T_DEV && f->ip->major == AUDIO_MAJOR)
+      return (events & POLLOUT) && f->writable && dsp_writable() ? POLLOUT : 0;
+
     if(f->ip && f->ip->type == T_DEV && f->ip->major == CONSOLE){
       int rev = 0;
       if((events & POLLIN) && f->readable && console_input_ready(f->ip ? f->ip->minor : 1))