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

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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
This commit is contained in:
Nexoniarz 2026-10-25 05:39:16 +01:00
commit c77841ce3d
11 changed files with 237 additions and 12 deletions
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13
kernel/drivers/audio/ac97.c
View file

@ -234,6 +234,12 @@ ac97_queue(struct device *d, const void *pcm, int bytes)
// just written is what makes this a queue rather than a one-shot.
outb(a->nabm + PO_LVI, (uint8)idx);
// An engine that ran off the end of the queue halts. QEMU resumes it when
// LVI moves; the datasheet asks for the run bit again, so if it is still
// halted after the write above, give it that.
if (a->running && (inw(a->nabm + PO_SR) & SR_DCH))
outb(a->nabm + PO_CR, CR_RPBM | CR_IOCE);
release(&a->lock);
return take;
}
@ -263,6 +269,13 @@ ac97_stop(struct device *d)
acquire(&a->lock);
outb(a->nabm + PO_CR, 0);
outb(a->nabm + PO_CR, CR_RR); // reset the box: CIV, LVI, PICB to 0
for (int spin = 0; spin < 100000 && (inb(a->nabm + PO_CR) & CR_RR); spin++)
;
// The reset clears the descriptor list's address too, and the engine then
// walks a "list" at physical address 0: every write after the first stop
// played zeros. ALSA resets the device before each stream, so that was all
// of them. Linux's intel8x0 rewrites it after every reset for this reason.
outl(a->nabm + PO_BDBAR, a->bdl_phys);
for (int i = 0; i < NDESC; i++)
a->filled[i] = 0;
a->head = 0;