SchOS/kernel/dev/audio.c

// AudioEngine: the audio class, and /dev/dsp above it.
//
// See include/dev/audio.h for what this is and what it deliberately is not. In
// short: a program opens /dev/dsp, says what its samples are, writes them, and
// stops. Everything here is either the class vtable's thin wrapper or the part
// of "play this" that is not specific to a chip.

#include "core/types.h"
#include "core/defs.h"
#include "core/param.h"
#include "core/spinlock.h"
#include "core/sleeplock.h"
#include "fs/fs.h"
#include "fs/file.h"
#include "dev/device.h"
#include "dev/audio.h"
#include "core/proc.h"
#include "core/uaccess.h"
#include "uapi/errno.h"

static struct spinlock audio_lock;
static int             audio_lock_init;

// The format the device is currently in. Kept here rather than asked of the
// driver on every write, because a caller that set 44100 and got 48000 needs
// to be told once and then have it stay told.
static struct audio_format current_fmt = { 48000, 2, AUDIO_FMT_S16LE };

static void
lock_once(void)
{
  if (!audio_lock_init) {
    initlock(&audio_lock, "audio");
    audio_lock_init = 1;
  }
}

// --- the class -------------------------------------------------------------

struct device *
audio_get(void)
{
  return device_find_class(DEVCLASS_AUDIO);
}

static const struct audio_ops *
ops_of(struct device *d)
{
  return d && d->ops ? (const struct audio_ops *)d->ops : 0;
}

int
audio_configure(struct audio_format *f)
{
  struct device *d = audio_get();
  const struct audio_ops *o = ops_of(d);
  int r;

  if (!o || !o->configure)
    return -ENODEV;
  if (f->bits != AUDIO_FMT_S16LE)
    return -EINVAL;
  if (f->channels != 1 && f->channels != 2)
    return -EINVAL;
  if (f->rate < 4000 || f->rate > 192000)
    return -EINVAL;

  r = o->configure(d, f);
  if (r < 0)
    return r;

  lock_once();
  acquire(&audio_lock);
  current_fmt = *f;          // what the driver settled on, not what was asked
  release(&audio_lock);
  return 0;
}

// Queue bytes, blocking while the hardware is full.
//
// The blocking is here rather than in a driver because it is the same in every
// driver: there is a ring, it has room or it does not, and a writer that is
// ahead of the speaker has nothing useful to do but wait. Sleeping on the
// device and being woken by its interrupt is the whole of it.
int
audio_write(const void *pcm, int bytes)
{
  struct device *d = audio_get();
  const struct audio_ops *o = ops_of(d);
  const char *p = pcm;
  int done = 0;

  if (!o || !o->queue)
    return -ENODEV;
  if (bytes <= 0)
    return 0;

  while (done < bytes) {
    int n = o->queue(d, p + done, bytes - done);

    if (n < 0)
      return done ? done : n;

    if (n == 0) {
      // Full. Start what is already queued -- a writer that fills the ring
      // and then waits for space would otherwise wait for a stream nobody
      // started -- and give the CPU up until some of it has been played.
      if (o->start)
        o->start(d);
      if (myproc() && myproc()->killed)
        return done ? done : -EINTR;
      audio_wait(d);
      continue;
    }

    done += n;
  }

  if (o->start)
    o->start(d);
  return done;
}

// Wait for the hardware to make room. A driver signals this from its
// interrupt handler with audio_wakeup().
//
// The timeout is not a nicety: a driver whose interrupt never arrives -- a
// misrouted line, a chip that needs a kick -- would otherwise hang the writer
// for ever with no way to tell what happened. Waking up anyway and asking the
// hardware again turns that into audio that stutters, which is a bug report
// somebody can make.
void
audio_wait(struct device *d)
{
  uint t0;

  (void)d;
  // One timer tick, the same way kernel/sys/sys_poll.c waits: there is no
  // timed sleep() in this kernel, and adding one for a single caller would be
  // a scheduler change to avoid a ten-line loop.
  //
  // Sleeping on `ticks` rather than on the device means a driver interrupt is
  // not strictly required to make progress -- which is the point. A ring that
  // drains is a ring that has room, and asking again in ten milliseconds
  // costs one wakeup per buffer.
  acquire(&tickslock);
  t0 = ticks;
  while (ticks == t0 && !(myproc() && myproc()->killed))
    sleep(&ticks, &tickslock);
  release(&tickslock);
}

void
audio_drain(void)
{
  struct device *d = audio_get();
  const struct audio_ops *o = ops_of(d);
  int spins;

  if (!o || !o->pending)
    return;

  // Bounded, for the same reason the wait above is. Ten seconds of audio is
  // longer than any buffer this engine will hold, so a `pending` that has not
  // reached zero by then is a driver that is not playing rather than one that
  // is busy.
  for (spins = 0; spins < 1000 && o->pending(d) > 0; spins++) {
    if (myproc() && myproc()->killed)
      return;
    audio_wait(d);
  }
}

void
audio_stop(void)
{
  struct device *d = audio_get();
  const struct audio_ops *o = ops_of(d);

  if (o && o->stop)
    o->stop(d);
}

// --- /dev/dsp ---------------------------------------------------------------

int
dspwrite(struct inode *ip, uint64 off, char *src, int n)
{
  (void)ip; (void)off;
  return audio_write(src, n);
}

// Recording is not implemented, and says so. Returning zero would look like
// end-of-file and a recorder would write an empty file and call it a success.
int
dspread(struct inode *ip, uint64 off, char *dst, int n)
{
  (void)ip; (void)off; (void)dst; (void)n;
  return -ENOSYS;
}

// The last descriptor is closing. Anything still queued is played out first:
// a program that writes a sound and exits expects to hear the end of it, and
// dropping the tail is the one behaviour that would make this device feel
// broken while technically working.
void
dspclose(struct inode *ip)
{
  (void)ip;
  audio_drain();
  audio_stop();
}

int
dsp_ioctl(int request, uint64 argp)
{
  struct device *d = audio_get();
  const struct audio_ops *o = ops_of(d);
  struct audio_format f;
  int val;

  if (!o)
    return -ENODEV;

  lock_once();
  acquire(&audio_lock);
  f = current_fmt;
  release(&audio_lock);

  switch (request) {
  case SNDCTL_DSP_SPEED:
  case SNDCTL_DSP_CHANNELS:
  case SNDCTL_DSP_SETFMT: {
    if (copy_from_user(&val, argp, sizeof(val)) < 0)
      return -EFAULT;

    if (request == SNDCTL_DSP_SPEED)         f.rate = (unsigned)val;
    else if (request == SNDCTL_DSP_CHANNELS) f.channels = (unsigned)val;
    else {
      if (val != AFMT_S16_LE)
        return -EINVAL;
      f.bits = AUDIO_FMT_S16LE;
    }

    int r = audio_configure(&f);
    if (r < 0)
      return r;

    // OSS writes back what was actually set, which is the entire reason these
    // are read/write ioctls: a program asks for 44100, is told 48000, and
    // resamples. One that ignores the answer gets a chipmunk, and that is its
    // bug rather than ours.
    val = (request == SNDCTL_DSP_SPEED)    ? (int)f.rate
        : (request == SNDCTL_DSP_CHANNELS) ? (int)f.channels
                                           : AFMT_S16_LE;
    if (copy_to_user(argp, &val, sizeof(val)) < 0)
      return -EFAULT;
    return 0;
  }

  case SNDCTL_DSP_SYNC:
    audio_drain();
    return 0;

  case SNDCTL_DSP_RESET:
    audio_stop();
    return 0;

  case SNDCTL_DSP_GETOSPACE: {
    struct audio_buf_info bi;
    int space = o->space ? o->space(d) : 0;

    bi.fragsize   = 4096;
    bi.bytes      = space;
    bi.fragments  = space / bi.fragsize;
    bi.fragstotal = 0;
    if (copy_to_user(argp, &bi, sizeof(bi)) < 0)
      return -EFAULT;
    return 0;
  }
  }

  return -ENOTTY;
}