path: root/media/libcubeb/src/cubeb_alsa.c

/*
 * Copyright © 2011 Mozilla Foundation
 *
 * This program is made available under an ISC-style license.  See the
 * accompanying file LICENSE for details.
 */
#undef NDEBUG
#define _DEFAULT_SOURCE
#define _BSD_SOURCE
#define _XOPEN_SOURCE 500
#include <pthread.h>
#include <sys/time.h>
#include <assert.h>
#include <limits.h>
#include <poll.h>
#include <unistd.h>
#include <alsa/asoundlib.h>
#include "cubeb/cubeb.h"
#include "cubeb-internal.h"

#define CUBEB_STREAM_MAX 16
#define CUBEB_WATCHDOG_MS 10000

#define CUBEB_ALSA_PCM_NAME "default"

#define ALSA_PA_PLUGIN "ALSA <-> PulseAudio PCM I/O Plugin"

/* ALSA is not thread-safe.  snd_pcm_t instances are individually protected
   by the owning cubeb_stream's mutex.  snd_pcm_t creation and destruction
   is not thread-safe until ALSA 1.0.24 (see alsa-lib.git commit 91c9c8f1),
   so those calls must be wrapped in the following mutex. */
static pthread_mutex_t cubeb_alsa_mutex = PTHREAD_MUTEX_INITIALIZER;
static int cubeb_alsa_error_handler_set = 0;

static struct cubeb_ops const alsa_ops;

struct cubeb {
  struct cubeb_ops const * ops;

  pthread_t thread;

  /* Mutex for streams array, must not be held while blocked in poll(2). */
  pthread_mutex_t mutex;

  /* Sparse array of streams managed by this context. */
  cubeb_stream * streams[CUBEB_STREAM_MAX];

  /* fds and nfds are only updated by alsa_run when rebuild is set. */
  struct pollfd * fds;
  nfds_t nfds;
  int rebuild;

  int shutdown;

  /* Control pipe for forcing poll to wake and rebuild fds or recalculate the timeout. */
  int control_fd_read;
  int control_fd_write;

  /* Track number of active streams.  This is limited to CUBEB_STREAM_MAX
     due to resource contraints. */
  unsigned int active_streams;

  /* Local configuration with handle_underrun workaround set for PulseAudio
     ALSA plugin.  Will be NULL if the PA ALSA plugin is not in use or the
     workaround is not required. */
  snd_config_t * local_config;
  int is_pa;
};

enum stream_state {
  INACTIVE,
  RUNNING,
  DRAINING,
  PROCESSING,
  ERROR
};

struct cubeb_stream {
  cubeb * context;
  pthread_mutex_t mutex;
  snd_pcm_t * pcm;
  cubeb_data_callback data_callback;
  cubeb_state_callback state_callback;
  void * user_ptr;
  snd_pcm_uframes_t write_position;
  snd_pcm_uframes_t last_position;
  snd_pcm_uframes_t buffer_size;
  cubeb_stream_params params;

  /* Every member after this comment is protected by the owning context's
     mutex rather than the stream's mutex, or is only used on the context's
     run thread. */
  pthread_cond_t cond; /* Signaled when the stream's state is changed. */

  enum stream_state state;

  struct pollfd * saved_fds; /* A copy of the pollfds passed in at init time. */
  struct pollfd * fds; /* Pointer to this waitable's pollfds within struct cubeb's fds. */
  nfds_t nfds;

  struct timeval drain_timeout;

  /* XXX: Horrible hack -- if an active stream has been idle for
     CUBEB_WATCHDOG_MS it will be disabled and the error callback will be
     called.  This works around a bug seen with older versions of ALSA and
     PulseAudio where streams would stop requesting new data despite still
     being logically active and playing. */
  struct timeval last_activity;
  float volume;
};

static int
any_revents(struct pollfd * fds, nfds_t nfds)
{
  nfds_t i;

  for (i = 0; i < nfds; ++i) {
    if (fds[i].revents) {
      return 1;
    }
  }

  return 0;
}

static int
cmp_timeval(struct timeval * a, struct timeval * b)
{
  if (a->tv_sec == b->tv_sec) {
    if (a->tv_usec == b->tv_usec) {
      return 0;
    }
    return a->tv_usec > b->tv_usec ? 1 : -1;
  }
  return a->tv_sec > b->tv_sec ? 1 : -1;
}

static int
timeval_to_relative_ms(struct timeval * tv)
{
  struct timeval now;
  struct timeval dt;
  long long t;
  int r;

  gettimeofday(&now, NULL);
  r = cmp_timeval(tv, &now);
  if (r >= 0) {
    timersub(tv, &now, &dt);
  } else {
    timersub(&now, tv, &dt);
  }
  t = dt.tv_sec;
  t *= 1000;
  t += (dt.tv_usec + 500) / 1000;

  if (t > INT_MAX) {
    t = INT_MAX;
  } else if (t < INT_MIN) {
    t = INT_MIN;
  }

  return r >= 0 ? t : -t;
}

static int
ms_until(struct timeval * tv)
{
  return timeval_to_relative_ms(tv);
}

static int
ms_since(struct timeval * tv)
{
  return -timeval_to_relative_ms(tv);
}

static void
rebuild(cubeb * ctx)
{
  nfds_t nfds;
  int i;
  nfds_t j;
  cubeb_stream * stm;

  assert(ctx->rebuild);

  /* Always count context's control pipe fd. */
  nfds = 1;
  for (i = 0; i < CUBEB_STREAM_MAX; ++i) {
    stm = ctx->streams[i];
    if (stm) {
      stm->fds = NULL;
      if (stm->state == RUNNING) {
        nfds += stm->nfds;
      }
    }
  }

  free(ctx->fds);
  ctx->fds = calloc(nfds, sizeof(struct pollfd));
  assert(ctx->fds);
  ctx->nfds = nfds;

  /* Include context's control pipe fd. */
  ctx->fds[0].fd = ctx->control_fd_read;
  ctx->fds[0].events = POLLIN | POLLERR;

  for (i = 0, j = 1; i < CUBEB_STREAM_MAX; ++i) {
    stm = ctx->streams[i];
    if (stm && stm->state == RUNNING) {
      memcpy(&ctx->fds[j], stm->saved_fds, stm->nfds * sizeof(struct pollfd));
      stm->fds = &ctx->fds[j];
      j += stm->nfds;
    }
  }

  ctx->rebuild = 0;
}

static void
poll_wake(cubeb * ctx)
{
  if (write(ctx->control_fd_write, "x", 1) < 0) {
    /* ignore write error */
  }
}

static void
set_timeout(struct timeval * timeout, unsigned int ms)
{
  gettimeofday(timeout, NULL);
  timeout->tv_sec += ms / 1000;
  timeout->tv_usec += (ms % 1000) * 1000;
}

static void
alsa_set_stream_state(cubeb_stream * stm, enum stream_state state)
{
  cubeb * ctx;
  int r;

  ctx = stm->context;
  stm->state = state;
  r = pthread_cond_broadcast(&stm->cond);
  assert(r == 0);
  ctx->rebuild = 1;
  poll_wake(ctx);
}

static enum stream_state
alsa_refill_stream(cubeb_stream * stm)
{
  snd_pcm_sframes_t avail;
  long got;
  void * p;
  int draining;

  draining = 0;

  pthread_mutex_lock(&stm->mutex);

  avail = snd_pcm_avail_update(stm->pcm);
  if (avail < 0) {
    snd_pcm_recover(stm->pcm, avail, 1);
    avail = snd_pcm_avail_update(stm->pcm);
  }

  /* Failed to recover from an xrun, this stream must be broken. */
  if (avail < 0) {
    pthread_mutex_unlock(&stm->mutex);
    stm->state_callback(stm, stm->user_ptr, CUBEB_STATE_ERROR);
    return ERROR;
  }

  /* This should never happen. */
  if ((unsigned int) avail > stm->buffer_size) {
    avail = stm->buffer_size;
  }

  /* poll(2) claims this stream is active, so there should be some space
     available to write.  If avail is still zero here, the stream must be in
     a funky state, bail and wait for another wakeup. */
  if (avail == 0) {
    pthread_mutex_unlock(&stm->mutex);
    return RUNNING;
  }

  p = calloc(1, snd_pcm_frames_to_bytes(stm->pcm, avail));
  assert(p);

  pthread_mutex_unlock(&stm->mutex);
  got = stm->data_callback(stm, stm->user_ptr, NULL, p, avail);
  pthread_mutex_lock(&stm->mutex);
  if (got < 0) {
    pthread_mutex_unlock(&stm->mutex);
    stm->state_callback(stm, stm->user_ptr, CUBEB_STATE_ERROR);
    free(p);
    return ERROR;
  }
  if (got > 0) {
    snd_pcm_sframes_t wrote;

    if (stm->params.format == CUBEB_SAMPLE_FLOAT32NE) {
      float * b = (float *) p;
      for (uint32_t i = 0; i < got * stm->params.channels; i++) {
        b[i] *= stm->volume;
      }
    } else {
      short * b = (short *) p;
      for (uint32_t i = 0; i < got * stm->params.channels; i++) {
        b[i] *= stm->volume;
      }
    }
    wrote = snd_pcm_writei(stm->pcm, p, got);
    if (wrote < 0) {
      snd_pcm_recover(stm->pcm, wrote, 1);
      wrote = snd_pcm_writei(stm->pcm, p, got);
    }
    if (wrote < 0 || wrote != got) {
      /* Recovery failed, somehow. */
      pthread_mutex_unlock(&stm->mutex);
      stm->state_callback(stm, stm->user_ptr, CUBEB_STATE_ERROR);
      return ERROR;
    }
    stm->write_position += wrote;
    gettimeofday(&stm->last_activity, NULL);
  }
  if (got != avail) {
    long buffer_fill = stm->buffer_size - (avail - got);
    double buffer_time = (double) buffer_fill / stm->params.rate;

    /* Fill the remaining buffer with silence to guarantee one full period
       has been written. */
    snd_pcm_writei(stm->pcm, (char *) p + got, avail - got);

    set_timeout(&stm->drain_timeout, buffer_time * 1000);

    draining = 1;
  }

  free(p);
  pthread_mutex_unlock(&stm->mutex);
  return draining ? DRAINING : RUNNING;
}

static int
alsa_run(cubeb * ctx)
{
  int r;
  int timeout;
  int i;
  char dummy;
  cubeb_stream * stm;
  enum stream_state state;

  pthread_mutex_lock(&ctx->mutex);

  if (ctx->rebuild) {
    rebuild(ctx);
  }

  /* Wake up at least once per second for the watchdog. */
  timeout = 1000;
  for (i = 0; i < CUBEB_STREAM_MAX; ++i) {
    stm = ctx->streams[i];
    if (stm && stm->state == DRAINING) {
      r = ms_until(&stm->drain_timeout);
      if (r >= 0 && timeout > r) {
        timeout = r;
      }
    }
  }

  pthread_mutex_unlock(&ctx->mutex);
  r = poll(ctx->fds, ctx->nfds, timeout);
  pthread_mutex_lock(&ctx->mutex);

  if (r > 0) {
    if (ctx->fds[0].revents & POLLIN) {
      if (read(ctx->control_fd_read, &dummy, 1) < 0) {
        /* ignore read error */
      }

      if (ctx->shutdown) {
        pthread_mutex_unlock(&ctx->mutex);
        return -1;
      }
    }

    for (i = 0; i < CUBEB_STREAM_MAX; ++i) {
      stm = ctx->streams[i];
      /* We can't use snd_pcm_poll_descriptors_revents here because of
         https://github.com/kinetiknz/cubeb/issues/135. */
      if (stm && stm->state == RUNNING && stm->fds && any_revents(stm->fds, stm->nfds)) {
        alsa_set_stream_state(stm, PROCESSING);
        pthread_mutex_unlock(&ctx->mutex);
        state = alsa_refill_stream(stm);
        pthread_mutex_lock(&ctx->mutex);
        alsa_set_stream_state(stm, state);
      }
    }
  } else if (r == 0) {
    for (i = 0; i < CUBEB_STREAM_MAX; ++i) {
      stm = ctx->streams[i];
      if (stm) {
        if (stm->state == DRAINING && ms_since(&stm->drain_timeout) >= 0) {
          alsa_set_stream_state(stm, INACTIVE);
          stm->state_callback(stm, stm->user_ptr, CUBEB_STATE_DRAINED);
        } else if (stm->state == RUNNING && ms_since(&stm->last_activity) > CUBEB_WATCHDOG_MS) {
          alsa_set_stream_state(stm, ERROR);
          stm->state_callback(stm, stm->user_ptr, CUBEB_STATE_ERROR);
        }
      }
    }
  }

  pthread_mutex_unlock(&ctx->mutex);

  return 0;
}

static void *
alsa_run_thread(void * context)
{
  cubeb * ctx = context;
  int r;

  do {
    r = alsa_run(ctx);
  } while (r >= 0);

  return NULL;
}

static snd_config_t *
get_slave_pcm_node(snd_config_t * lconf, snd_config_t * root_pcm)
{
  int r;
  snd_config_t * slave_pcm;
  snd_config_t * slave_def;
  snd_config_t * pcm;
  char const * string;
  char node_name[64];

  slave_def = NULL;

  r = snd_config_search(root_pcm, "slave", &slave_pcm);
  if (r < 0) {
    return NULL;
  }

  r = snd_config_get_string(slave_pcm, &string);
  if (r >= 0) {
    r = snd_config_search_definition(lconf, "pcm_slave", string, &slave_def);
    if (r < 0) {
      return NULL;
    }
  }

  do {
    r = snd_config_search(slave_def ? slave_def : slave_pcm, "pcm", &pcm);
    if (r < 0) {
      break;
    }

    r = snd_config_get_string(slave_def ? slave_def : slave_pcm, &string);
    if (r < 0) {
      break;
    }

    r = snprintf(node_name, sizeof(node_name), "pcm.%s", string);
    if (r < 0 || r > (int) sizeof(node_name)) {
      break;
    }
    r = snd_config_search(lconf, node_name, &pcm);
    if (r < 0) {
      break;
    }

    return pcm;
  } while (0);

  if (slave_def) {
    snd_config_delete(slave_def);
  }

  return NULL;
}

/* Work around PulseAudio ALSA plugin bug where the PA server forces a
   higher than requested latency, but the plugin does not update its (and
   ALSA's) internal state to reflect that, leading to an immediate underrun
   situation.  Inspired by WINE's make_handle_underrun_config.
   Reference: http://mailman.alsa-project.org/pipermail/alsa-devel/2012-July/05 */
static snd_config_t *
init_local_config_with_workaround(char const * pcm_name)
{
  int r;
  snd_config_t * lconf;
  snd_config_t * pcm_node;
  snd_config_t * node;
  char const * string;
  char node_name[64];

  lconf = NULL;

  if (snd_config == NULL) {
    return NULL;
  }

  r = snd_config_copy(&lconf, snd_config);
  if (r < 0) {
    return NULL;
  }

  do {
    r = snd_config_search_definition(lconf, "pcm", pcm_name, &pcm_node);
    if (r < 0) {
      break;
    }

    r = snd_config_get_id(pcm_node, &string);
    if (r < 0) {
      break;
    }

    r = snprintf(node_name, sizeof(node_name), "pcm.%s", string);
    if (r < 0 || r > (int) sizeof(node_name)) {
      break;
    }
    r = snd_config_search(lconf, node_name, &pcm_node);
    if (r < 0) {
      break;
    }

    /* If this PCM has a slave, walk the slave configurations until we reach the bottom. */
    while ((node = get_slave_pcm_node(lconf, pcm_node)) != NULL) {
      pcm_node = node;
    }

    /* Fetch the PCM node's type, and bail out if it's not the PulseAudio plugin. */
    r = snd_config_search(pcm_node, "type", &node);
    if (r < 0) {
      break;
    }

    r = snd_config_get_string(node, &string);
    if (r < 0) {
      break;
    }

    if (strcmp(string, "pulse") != 0) {
      break;
    }

    /* Don't clobber an explicit existing handle_underrun value, set it only
       if it doesn't already exist. */
    r = snd_config_search(pcm_node, "handle_underrun", &node);
    if (r != -ENOENT) {
      break;
    }

    /* Disable pcm_pulse's asynchronous underrun handling. */
    r = snd_config_imake_integer(&node, "handle_underrun", 0);
    if (r < 0) {
      break;
    }

    r = snd_config_add(pcm_node, node);
    if (r < 0) {
      break;
    }

    return lconf;
  } while (0);

  snd_config_delete(lconf);

  return NULL;
}

static int
alsa_locked_pcm_open(snd_pcm_t ** pcm, snd_pcm_stream_t stream, snd_config_t * local_config)
{
  int r;

  pthread_mutex_lock(&cubeb_alsa_mutex);
  if (local_config) {
    r = snd_pcm_open_lconf(pcm, CUBEB_ALSA_PCM_NAME, stream, SND_PCM_NONBLOCK, local_config);
  } else {
    r = snd_pcm_open(pcm, CUBEB_ALSA_PCM_NAME, stream, SND_PCM_NONBLOCK);
  }
  pthread_mutex_unlock(&cubeb_alsa_mutex);

  return r;
}

static int
alsa_locked_pcm_close(snd_pcm_t * pcm)
{
  int r;

  pthread_mutex_lock(&cubeb_alsa_mutex);
  r = snd_pcm_close(pcm);
  pthread_mutex_unlock(&cubeb_alsa_mutex);

  return r;
}

static int
alsa_register_stream(cubeb * ctx, cubeb_stream * stm)
{
  int i;

  pthread_mutex_lock(&ctx->mutex);
  for (i = 0; i < CUBEB_STREAM_MAX; ++i) {
    if (!ctx->streams[i]) {
      ctx->streams[i] = stm;
      break;
    }
  }
  pthread_mutex_unlock(&ctx->mutex);

  return i == CUBEB_STREAM_MAX;
}

static void
alsa_unregister_stream(cubeb_stream * stm)
{
  cubeb * ctx;
  int i;

  ctx = stm->context;

  pthread_mutex_lock(&ctx->mutex);
  for (i = 0; i < CUBEB_STREAM_MAX; ++i) {
    if (ctx->streams[i] == stm) {
      ctx->streams[i] = NULL;
      break;
    }
  }
  pthread_mutex_unlock(&ctx->mutex);
}

static void
silent_error_handler(char const * file, int line, char const * function,
                     int err, char const * fmt, ...)
{
  (void)file;
  (void)line;
  (void)function;
  (void)err;
  (void)fmt;
}

/*static*/ int
alsa_init(cubeb ** context, char const * context_name)
{
  (void)context_name;
  cubeb * ctx;
  int r;
  int i;
  int fd[2];
  pthread_attr_t attr;
  snd_pcm_t * dummy;

  assert(context);
  *context = NULL;

  pthread_mutex_lock(&cubeb_alsa_mutex);
  if (!cubeb_alsa_error_handler_set) {
    snd_lib_error_set_handler(silent_error_handler);
    cubeb_alsa_error_handler_set = 1;
  }
  pthread_mutex_unlock(&cubeb_alsa_mutex);

  ctx = calloc(1, sizeof(*ctx));
  assert(ctx);

  ctx->ops = &alsa_ops;

  r = pthread_mutex_init(&ctx->mutex, NULL);
  assert(r == 0);

  r = pipe(fd);
  assert(r == 0);

  for (i = 0; i < 2; ++i) {
    fcntl(fd[i], F_SETFD, fcntl(fd[i], F_GETFD) | FD_CLOEXEC);
    fcntl(fd[i], F_SETFL, fcntl(fd[i], F_GETFL) | O_NONBLOCK);
  }

  ctx->control_fd_read = fd[0];
  ctx->control_fd_write = fd[1];

  /* Force an early rebuild when alsa_run is first called to ensure fds and
     nfds have been initialized. */
  ctx->rebuild = 1;

  r = pthread_attr_init(&attr);
  assert(r == 0);

  r = pthread_attr_setstacksize(&attr, 256 * 1024);
  assert(r == 0);

  r = pthread_create(&ctx->thread, &attr, alsa_run_thread, ctx);
  assert(r == 0);

  r = pthread_attr_destroy(&attr);
  assert(r == 0);

  /* Open a dummy PCM to force the configuration space to be evaluated so that
     init_local_config_with_workaround can find and modify the default node. */
  r = alsa_locked_pcm_open(&dummy, SND_PCM_STREAM_PLAYBACK, NULL);
  if (r >= 0) {
    alsa_locked_pcm_close(dummy);
  }
  ctx->is_pa = 0;
  pthread_mutex_lock(&cubeb_alsa_mutex);
  ctx->local_config = init_local_config_with_workaround(CUBEB_ALSA_PCM_NAME);
  pthread_mutex_unlock(&cubeb_alsa_mutex);
  if (ctx->local_config) {
    ctx->is_pa = 1;
    r = alsa_locked_pcm_open(&dummy, SND_PCM_STREAM_PLAYBACK, ctx->local_config);
    /* If we got a local_config, we found a PA PCM.  If opening a PCM with that
       config fails with EINVAL, the PA PCM is too old for this workaround. */
    if (r == -EINVAL) {
      pthread_mutex_lock(&cubeb_alsa_mutex);
      snd_config_delete(ctx->local_config);
      pthread_mutex_unlock(&cubeb_alsa_mutex);
      ctx->local_config = NULL;
    } else if (r >= 0) {
      alsa_locked_pcm_close(dummy);
    }
  }

  *context = ctx;

  return CUBEB_OK;
}

static char const *
alsa_get_backend_id(cubeb * ctx)
{
  (void)ctx;
  return "alsa";
}

static void
alsa_destroy(cubeb * ctx)
{
  int r;

  assert(ctx);

  pthread_mutex_lock(&ctx->mutex);
  ctx->shutdown = 1;
  poll_wake(ctx);
  pthread_mutex_unlock(&ctx->mutex);

  r = pthread_join(ctx->thread, NULL);
  assert(r == 0);

  close(ctx->control_fd_read);
  close(ctx->control_fd_write);
  pthread_mutex_destroy(&ctx->mutex);
  free(ctx->fds);

  if (ctx->local_config) {
    pthread_mutex_lock(&cubeb_alsa_mutex);
    snd_config_delete(ctx->local_config);
    pthread_mutex_unlock(&cubeb_alsa_mutex);
  }

  free(ctx);
}

static void alsa_stream_destroy(cubeb_stream * stm);

static int
alsa_stream_init(cubeb * ctx, cubeb_stream ** stream, char const * stream_name,
                 cubeb_devid input_device,
                 cubeb_stream_params * input_stream_params,
                 cubeb_devid output_device,
                 cubeb_stream_params * output_stream_params,
                 unsigned int latency_frames,
                 cubeb_data_callback data_callback, cubeb_state_callback state_callback,
                 void * user_ptr)
{
  (void)stream_name;
  cubeb_stream * stm;
  int r;
  snd_pcm_format_t format;
  snd_pcm_uframes_t period_size;
  int latency_us = 0;


  assert(ctx && stream);

  if (input_stream_params) {
    /* Capture support not yet implemented. */
    return CUBEB_ERROR_NOT_SUPPORTED;
  }

  if (input_device || output_device) {
    /* Device selection not yet implemented. */
    return CUBEB_ERROR_DEVICE_UNAVAILABLE;
  }

  *stream = NULL;

  switch (output_stream_params->format) {
  case CUBEB_SAMPLE_S16LE:
    format = SND_PCM_FORMAT_S16_LE;
    break;
  case CUBEB_SAMPLE_S16BE:
    format = SND_PCM_FORMAT_S16_BE;
    break;
  case CUBEB_SAMPLE_FLOAT32LE:
    format = SND_PCM_FORMAT_FLOAT_LE;
    break;
  case CUBEB_SAMPLE_FLOAT32BE:
    format = SND_PCM_FORMAT_FLOAT_BE;
    break;
  default:
    return CUBEB_ERROR_INVALID_FORMAT;
  }

  pthread_mutex_lock(&ctx->mutex);
  if (ctx->active_streams >= CUBEB_STREAM_MAX) {
    pthread_mutex_unlock(&ctx->mutex);
    return CUBEB_ERROR;
  }
  ctx->active_streams += 1;
  pthread_mutex_unlock(&ctx->mutex);

  stm = calloc(1, sizeof(*stm));
  assert(stm);

  stm->context = ctx;
  stm->data_callback = data_callback;
  stm->state_callback = state_callback;
  stm->user_ptr = user_ptr;
  stm->params = *output_stream_params;
  stm->state = INACTIVE;
  stm->volume = 1.0;

  r = pthread_mutex_init(&stm->mutex, NULL);
  assert(r == 0);

  r = alsa_locked_pcm_open(&stm->pcm, SND_PCM_STREAM_PLAYBACK, ctx->local_config);
  if (r < 0) {
    alsa_stream_destroy(stm);
    return CUBEB_ERROR;
  }

  r = snd_pcm_nonblock(stm->pcm, 1);
  assert(r == 0);

  latency_us = latency_frames * 1e6 / stm->params.rate;

  /* Ugly hack: the PA ALSA plugin allows buffer configurations that can't
     possibly work.  See https://bugzilla.mozilla.org/show_bug.cgi?id=761274.
     Only resort to this hack if the handle_underrun workaround failed. */
  if (!ctx->local_config && ctx->is_pa) {
    const int min_latency = 5e5;
    latency_us = latency_us < min_latency ? min_latency: latency_us;
  }

  r = snd_pcm_set_params(stm->pcm, format, SND_PCM_ACCESS_RW_INTERLEAVED,
                         stm->params.channels, stm->params.rate, 1,
                         latency_us);
  if (r < 0) {
    alsa_stream_destroy(stm);
    return CUBEB_ERROR_INVALID_FORMAT;
  }

  r = snd_pcm_get_params(stm->pcm, &stm->buffer_size, &period_size);
  assert(r == 0);

  stm->nfds = snd_pcm_poll_descriptors_count(stm->pcm);
  assert(stm->nfds > 0);

  stm->saved_fds = calloc(stm->nfds, sizeof(struct pollfd));
  assert(stm->saved_fds);
  r = snd_pcm_poll_descriptors(stm->pcm, stm->saved_fds, stm->nfds);
  assert((nfds_t) r == stm->nfds);

  r = pthread_cond_init(&stm->cond, NULL);
  assert(r == 0);

  if (alsa_register_stream(ctx, stm) != 0) {
    alsa_stream_destroy(stm);
    return CUBEB_ERROR;
  }

  *stream = stm;

  return CUBEB_OK;
}

static void
alsa_stream_destroy(cubeb_stream * stm)
{
  int r;
  cubeb * ctx;

  assert(stm && (stm->state == INACTIVE ||
                 stm->state == ERROR ||
                 stm->state == DRAINING));

  ctx = stm->context;

  pthread_mutex_lock(&stm->mutex);
  if (stm->pcm) {
    if (stm->state == DRAINING) {
      snd_pcm_drain(stm->pcm);
    }
    alsa_locked_pcm_close(stm->pcm);
    stm->pcm = NULL;
  }
  free(stm->saved_fds);
  pthread_mutex_unlock(&stm->mutex);
  pthread_mutex_destroy(&stm->mutex);

  r = pthread_cond_destroy(&stm->cond);
  assert(r == 0);

  alsa_unregister_stream(stm);

  pthread_mutex_lock(&ctx->mutex);
  assert(ctx->active_streams >= 1);
  ctx->active_streams -= 1;
  pthread_mutex_unlock(&ctx->mutex);

  free(stm);
}

static int
alsa_get_max_channel_count(cubeb * ctx, uint32_t * max_channels)
{
  int r;
  cubeb_stream * stm;
  snd_pcm_hw_params_t* hw_params;
  cubeb_stream_params params;
  params.rate = 44100;
  params.format = CUBEB_SAMPLE_FLOAT32NE;
  params.channels = 2;

  snd_pcm_hw_params_alloca(&hw_params);

  assert(ctx);

  r = alsa_stream_init(ctx, &stm, "", NULL, NULL, NULL, &params, 100, NULL, NULL, NULL);
  if (r != CUBEB_OK) {
    return CUBEB_ERROR;
  }

  r = snd_pcm_hw_params_any(stm->pcm, hw_params);
  if (r < 0) {
    return CUBEB_ERROR;
  }

  r = snd_pcm_hw_params_get_channels_max(hw_params, max_channels);
  if (r < 0) {
    return CUBEB_ERROR;
  }

  alsa_stream_destroy(stm);

  return CUBEB_OK;
}

static int
alsa_get_preferred_sample_rate(cubeb * ctx, uint32_t * rate) {
  (void)ctx;
  int r, dir;
  snd_pcm_t * pcm;
  snd_pcm_hw_params_t * hw_params;

  snd_pcm_hw_params_alloca(&hw_params);

  /* get a pcm, disabling resampling, so we get a rate the
   * hardware/dmix/pulse/etc. supports. */
  r = snd_pcm_open(&pcm, CUBEB_ALSA_PCM_NAME, SND_PCM_STREAM_PLAYBACK, SND_PCM_NO_AUTO_RESAMPLE);
  if (r < 0) {
    return CUBEB_ERROR;
  }

  r = snd_pcm_hw_params_any(pcm, hw_params);
  if (r < 0) {
    snd_pcm_close(pcm);
    return CUBEB_ERROR;
  }

  r = snd_pcm_hw_params_get_rate(hw_params, rate, &dir);
  if (r >= 0) {
    /* There is a default rate: use it. */
    snd_pcm_close(pcm);
    return CUBEB_OK;
  }

  /* Use a common rate, alsa may adjust it based on hw/etc. capabilities. */
  *rate = 44100;

  r = snd_pcm_hw_params_set_rate_near(pcm, hw_params, rate, NULL);
  if (r < 0) {
    snd_pcm_close(pcm);
    return CUBEB_ERROR;
  }

  snd_pcm_close(pcm);

  return CUBEB_OK;
}

static int
alsa_get_min_latency(cubeb * ctx, cubeb_stream_params params, uint32_t * latency_frames)
{
  (void)ctx;
  /* 40ms is found to be an acceptable minimum, even on a super low-end
   * machine. */
  *latency_frames = 40 * params.rate / 1000;

  return CUBEB_OK;
}

static int
alsa_stream_start(cubeb_stream * stm)
{
  cubeb * ctx;

  assert(stm);
  ctx = stm->context;

  pthread_mutex_lock(&stm->mutex);
  snd_pcm_pause(stm->pcm, 0);
  gettimeofday(&stm->last_activity, NULL);
  pthread_mutex_unlock(&stm->mutex);

  pthread_mutex_lock(&ctx->mutex);
  if (stm->state != INACTIVE) {
    pthread_mutex_unlock(&ctx->mutex);
    return CUBEB_ERROR;
  }
  alsa_set_stream_state(stm, RUNNING);
  pthread_mutex_unlock(&ctx->mutex);

  return CUBEB_OK;
}

static int
alsa_stream_stop(cubeb_stream * stm)
{
  cubeb * ctx;
  int r;

  assert(stm);
  ctx = stm->context;

  pthread_mutex_lock(&ctx->mutex);
  while (stm->state == PROCESSING) {
    r = pthread_cond_wait(&stm->cond, &ctx->mutex);
    assert(r == 0);
  }

  alsa_set_stream_state(stm, INACTIVE);
  pthread_mutex_unlock(&ctx->mutex);

  pthread_mutex_lock(&stm->mutex);
  snd_pcm_pause(stm->pcm, 1);
  pthread_mutex_unlock(&stm->mutex);

  return CUBEB_OK;
}

static int
alsa_stream_get_position(cubeb_stream * stm, uint64_t * position)
{
  snd_pcm_sframes_t delay;

  assert(stm && position);

  pthread_mutex_lock(&stm->mutex);

  delay = -1;
  if (snd_pcm_state(stm->pcm) != SND_PCM_STATE_RUNNING ||
      snd_pcm_delay(stm->pcm, &delay) != 0) {
    *position = stm->last_position;
    pthread_mutex_unlock(&stm->mutex);
    return CUBEB_OK;
  }

  assert(delay >= 0);

  *position = 0;
  if (stm->write_position >= (snd_pcm_uframes_t) delay) {
    *position = stm->write_position - delay;
  }

  stm->last_position = *position;

  pthread_mutex_unlock(&stm->mutex);
  return CUBEB_OK;
}

static int
alsa_stream_get_latency(cubeb_stream * stm, uint32_t * latency)
{
  snd_pcm_sframes_t delay;
  /* This function returns the delay in frames until a frame written using
     snd_pcm_writei is sent to the DAC. The DAC delay should be < 1ms anyways. */
  if (snd_pcm_delay(stm->pcm, &delay)) {
    return CUBEB_ERROR;
  }

  *latency = delay;

  return CUBEB_OK;
}

static int
alsa_stream_set_volume(cubeb_stream * stm, float volume)
{
  /* setting the volume using an API call does not seem very stable/supported */
  pthread_mutex_lock(&stm->mutex);
  stm->volume = volume;
  pthread_mutex_unlock(&stm->mutex);

  return CUBEB_OK;
}

static struct cubeb_ops const alsa_ops = {
  .init = alsa_init,
  .get_backend_id = alsa_get_backend_id,
  .get_max_channel_count = alsa_get_max_channel_count,
  .get_min_latency = alsa_get_min_latency,
  .get_preferred_sample_rate = alsa_get_preferred_sample_rate,
  .enumerate_devices = NULL,
  .destroy = alsa_destroy,
  .stream_init = alsa_stream_init,
  .stream_destroy = alsa_stream_destroy,
  .stream_start = alsa_stream_start,
  .stream_stop = alsa_stream_stop,
  .stream_get_position = alsa_stream_get_position,
  .stream_get_latency = alsa_stream_get_latency,
  .stream_set_volume = alsa_stream_set_volume,
  .stream_set_panning = NULL,
  .stream_get_current_device = NULL,
  .stream_device_destroy = NULL,
  .stream_register_device_changed_callback = NULL,
  .register_device_collection_changed = NULL
};