Linux 内核音频数据传递主要流程
Linux 用户空间应用程序通过声卡驱动程序（一般牵涉到多个设备驱动程序）和 Linux 内核 ALSA 框架导出的 PCM 设备文件，如 /dev/snd/pcmC0D0c 和 /dev/snd/pcmC0D0p 等，与 Linux 内核音频设备驱动程序和音频硬件进行数据传递。PCM 设备文件的文件操作定义 (位于 sound/core/pcm_native.c) 如下：
const struct file_operations snd_pcm_f_ops[2] = {
{
.owner = THIS_MODULE,
.write = snd_pcm_write,
.write_iter = snd_pcm_writev,
.open = snd_pcm_playback_open,
.release = snd_pcm_release,
.llseek = no_llseek,
.poll = snd_pcm_poll,
.unlocked_ioctl = snd_pcm_ioctl,
.compat_ioctl = snd_pcm_ioctl_compat,
.mmap = snd_pcm_mmap,
.fasync = snd_pcm_fasync,
.get_unmapped_area = snd_pcm_get_unmapped_area,
},
{
.owner = THIS_MODULE,
.read = snd_pcm_read,
.read_iter = snd_pcm_readv,
.open = snd_pcm_capture_open,
.release = snd_pcm_release,
.llseek = no_llseek,
.poll = snd_pcm_poll,
.unlocked_ioctl = snd_pcm_ioctl,
.compat_ioctl = snd_pcm_ioctl_compat,
.mmap = snd_pcm_mmap,
.fasync = snd_pcm_fasync,
.get_unmapped_area = snd_pcm_get_unmapped_area,
}
};
大多数情况下，音频设备会同时提供播放和录制功能，用于播放和录制的 PCM 设备文件是一起导出的，播放和录制的 PCM 设备文件的文件操作也是一起定义的，其中索引为 SNDRV_PCM_STREAM_PLAYBACK，也就是 0 的文件操作用于播放，索引为 SNDRV_PCM_STREAM_CAPTURE，也就是 1 的文件操作用于录制。
Linux 用户空间有 alsa-lib 和 tinyalsa 等库可用于与 Linux 内核 ALSA 框架交互。alsa-lib 和 tinyalsa 等库主要通过 ioctl 命令与 Linux 内核 ALSA 框架交互。PCM 设备文件的 ioctl 操作 snd_pcm_ioctl() 函数定义 (位于 sound/core/pcm_native.c) 如下：
static int snd_pcm_common_ioctl(struct file *file,
struct snd_pcm_substream *substream,
unsigned int cmd, void __user *arg)
{
struct snd_pcm_file *pcm_file = file->private_data;
int res;
if (PCM_RUNTIME_CHECK(substream))
return -ENXIO;
res = snd_power_wait(substream->pcm->card, SNDRV_CTL_POWER_D0);
if (res < 0)
return res;
switch (cmd) {
case SNDRV_PCM_IOCTL_PVERSION:
return put_user(SNDRV_PCM_VERSION, (int __user *)arg) ? -EFAULT : 0;
case SNDRV_PCM_IOCTL_INFO:
return snd_pcm_info_user(substream, arg);
case SNDRV_PCM_IOCTL_TSTAMP: /* just for compatibility */
return 0;
case SNDRV_PCM_IOCTL_TTSTAMP:
return snd_pcm_tstamp(substream, arg);
case SNDRV_PCM_IOCTL_USER_PVERSION:
if (get_user(pcm_file->user_pversion,
(unsigned int __user *)arg))
return -EFAULT;
return 0;
case SNDRV_PCM_IOCTL_HW_REFINE:
return snd_pcm_hw_refine_user(substream, arg);
case SNDRV_PCM_IOCTL_HW_PARAMS:
return snd_pcm_hw_params_user(substream, arg);
case SNDRV_PCM_IOCTL_HW_FREE:
return snd_pcm_hw_free(substream);
case SNDRV_PCM_IOCTL_SW_PARAMS:
return snd_pcm_sw_params_user(substream, arg);
case SNDRV_PCM_IOCTL_STATUS32:
return snd_pcm_status_user32(substream, arg, false);
case SNDRV_PCM_IOCTL_STATUS_EXT32:
return snd_pcm_status_user32(substream, arg, true);
case SNDRV_PCM_IOCTL_STATUS64:
return snd_pcm_status_user64(substream, arg, false);
case SNDRV_PCM_IOCTL_STATUS_EXT64:
return snd_pcm_status_user64(substream, arg, true);
case SNDRV_PCM_IOCTL_CHANNEL_INFO:
return snd_pcm_channel_info_user(substream, arg);
case SNDRV_PCM_IOCTL_PREPARE:
return snd_pcm_prepare(substream, file);
case SNDRV_PCM_IOCTL_RESET:
return snd_pcm_reset(substream);
case SNDRV_PCM_IOCTL_START:
return snd_pcm_start_lock_irq(substream);
case SNDRV_PCM_IOCTL_LINK:
return snd_pcm_link(substream, (int)(unsigned long) arg);
case SNDRV_PCM_IOCTL_UNLINK:
return snd_pcm_unlink(substream);
case SNDRV_PCM_IOCTL_RESUME:
return snd_pcm_resume(substream);
case SNDRV_PCM_IOCTL_XRUN:
return snd_pcm_xrun(substream);
case SNDRV_PCM_IOCTL_HWSYNC:
return snd_pcm_hwsync(substream);
case SNDRV_PCM_IOCTL_DELAY:
{
snd_pcm_sframes_t delay;
snd_pcm_sframes_t __user *res = arg;
int err;
err = snd_pcm_delay(substream, &delay);
if (err)
return err;
if (put_user(delay, res))
return -EFAULT;
return 0;
}
case __SNDRV_PCM_IOCTL_SYNC_PTR32:
return snd_pcm_ioctl_sync_ptr_compat(substream, arg);
case __SNDRV_PCM_IOCTL_SYNC_PTR64:
return snd_pcm_sync_ptr(substream, arg);
#ifdef CONFIG_SND_SUPPORT_OLD_API
case SNDRV_PCM_IOCTL_HW_REFINE_OLD:
return snd_pcm_hw_refine_old_user(substream, arg);
case SNDRV_PCM_IOCTL_HW_PARAMS_OLD:
return snd_pcm_hw_params_old_user(substream, arg);
#endif
case SNDRV_PCM_IOCTL_DRAIN:
return snd_pcm_drain(substream, file);
case SNDRV_PCM_IOCTL_DROP:
return snd_pcm_drop(substream);
case SNDRV_PCM_IOCTL_PAUSE:
return snd_pcm_pause_lock_irq(substream, (unsigned long)arg);
case SNDRV_PCM_IOCTL_WRITEI_FRAMES:
case SNDRV_PCM_IOCTL_READI_FRAMES:
return snd_pcm_xferi_frames_ioctl(substream, arg);
case SNDRV_PCM_IOCTL_WRITEN_FRAMES:
case SNDRV_PCM_IOCTL_READN_FRAMES:
return snd_pcm_xfern_frames_ioctl(substream, arg);
case SNDRV_PCM_IOCTL_REWIND:
return snd_pcm_rewind_ioctl(substream, arg);
case SNDRV_PCM_IOCTL_FORWARD:
return snd_pcm_forward_ioctl(substream, arg);
}
pcm_dbg(substream->pcm, "unknown ioctl = 0x%x\n", cmd);
return -ENOTTY;
}
static long snd_pcm_ioctl(struct file *file, unsigned int cmd,
unsigned long arg)
{
struct snd_pcm_file *pcm_file;
pcm_file = file->private_data;
if (((cmd >> 8) & 0xff) != 'A')
return -ENOTTY;
return snd_pcm_common_ioctl(file, pcm_file->substream, cmd,
(void __user *)arg);
}
alsa-lib 和 tinyalsa 等库主要通过 SNDRV_PCM_IOCTL_WRITEI_FRAMES、SNDRV_PCM_IOCTL_READI_FRAMES、SNDRV_PCM_IOCTL_WRITEN_FRAMES 和 SNDRV_PCM_IOCTL_READN_FRAMES 等四个 ioctl 命令与 Linux 内核 ALSA 框架交换数据。这几个 ioctl 命令主要的区别在于，传递的数据的格式不同，SNDRV_PCM_IOCTL_READI_FRAMES 和 SNDRV_PCM_IOCTL_WRITEI_FRAMES 命令用于读写 interleaved 格式的音频数据，SNDRV_PCM_IOCTL_READN_FRAMES 和 SNDRV_PCM_IOCTL_WRITEN_FRAMES 命令则用于读写 noninterleaved 格式的音频数据。
SNDRV_PCM_IOCTL_READI_FRAMES 和 SNDRV_PCM_IOCTL_WRITEI_FRAMES 命令由 snd_pcm_xferi_frames_ioctl() 函数处理，SNDRV_PCM_IOCTL_READN_FRAMES 和 SNDRV_PCM_IOCTL_WRITEN_FRAMES 命令由 snd_pcm_xfern_frames_ioctl() 函数处理，这两个函数定义 (位于 sound/core/pcm_native.c) 如下：
static int snd_pcm_xferi_frames_ioctl(struct snd_pcm_substream *substream,
struct snd_xferi __user *_xferi)
{
struct snd_xferi xferi;
struct snd_pcm_runtime *runtime = substream->runtime;
snd_pcm_sframes_t result;
if (runtime->status->state == SNDRV_PCM_STATE_OPEN)
return -EBADFD;
if (put_user(0, &_xferi->result))
return -EFAULT;
if (copy_from_user(&xferi, _xferi, sizeof(xferi)))
return -EFAULT;
if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
result = snd_pcm_lib_write(substream, xferi.buf, xferi.frames);
else
result = snd_pcm_lib_read(substream, xferi.buf, xferi.frames);
if (put_user(result, &_xferi->result))
return -EFAULT;
return result < 0 ? result : 0;
}
static int snd_pcm_xfern_frames_ioctl(struct snd_pcm_substream *substream,
struct snd_xfern __user *_xfern)
{
struct snd_xfern xfern;
struct snd_pcm_runtime *runtime = substream->runtime;
void *bufs;
snd_pcm_sframes_t result;
if (runtime->status->state == SNDRV_PCM_STATE_OPEN)
return -EBADFD;
if (runtime->channels > 128)
return -EINVAL;
if (put_user(0, &_xfern->result))
return -EFAULT;
if (copy_from_user(&xfern, _xfern, sizeof(xfern)))
return -EFAULT;
bufs = memdup_user(xfern.bufs, sizeof(void *) * runtime->channels);
if (IS_ERR(bufs))
return PTR_ERR(bufs);
if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
result = snd_pcm_lib_writev(substream, bufs, xfern.frames);
else
result = snd_pcm_lib_readv(substream, bufs, xfern.frames);
kfree(bufs);
if (put_user(result, &_xfern->result))
return -EFAULT;
return result < 0 ? result : 0;
}
snd_pcm_xferi_frames_ioctl() 和 snd_pcm_xfern_frames_ioctl() 函数将参数复制到内核空间栈上，并通过 snd_pcm_lib_write()、snd_pcm_lib_read()、snd_pcm_lib_writev() 和 snd_pcm_lib_readv() 四个函数执行读写操作。这四个函数定义 (位于 include/sound/pcm.h) 如下：
static inline snd_pcm_sframes_t
snd_pcm_lib_write(struct snd_pcm_substream *substream,
const void __user *buf, snd_pcm_uframes_t frames)
{
return __snd_pcm_lib_xfer(substream, (void __force *)buf, true, frames, false);
}
static inline snd_pcm_sframes_t
snd_pcm_lib_read(struct snd_pcm_substream *substream,
void __user *buf, snd_pcm_uframes_t frames)
{
return __snd_pcm_lib_xfer(substream, (void __force *)buf, true, frames, false);
}
static inline snd_pcm_sframes_t
snd_pcm_lib_writev(struct snd_pcm_substream *substream,
void __user **bufs, snd_pcm_uframes_t frames)
{
return __snd_pcm_lib_xfer(substream, (void *)bufs, false, frames, false);
}
static inline snd_pcm_sframes_t
snd_pcm_lib_readv(struct snd_pcm_substream *substream,
void __user **bufs, snd_pcm_uframes_t frames)
{
return __snd_pcm_lib_xfer(substream, (void *)bufs, false, frames, false);
}
所有的音频数据传递操作最终都由 __snd_pcm_lib_xfer() 函数完成。__snd_pcm_lib_xfer() 函数定义 (位于 sound/core/pcm_lib.c) 如下：
typedef int (*pcm_transfer_f)(struct snd_pcm_substream *substream,
int channel, unsigned long hwoff,
void *buf, unsigned long bytes);
typedef int (*pcm_copy_f)(struct snd_pcm_substream *, snd_pcm_uframes_t, void *,
snd_pcm_uframes_t, snd_pcm_uframes_t, pcm_transfer_f);
. . . . . .
/* sanity-check for read/write methods */
static int pcm_sanity_check(struct snd_pcm_substream *substream)
{
struct snd_pcm_runtime *runtime;
if (PCM_RUNTIME_CHECK(substream))
return -ENXIO;
runtime = substream->runtime;
if (snd_BUG_ON(!substream->ops->copy_user && !runtime->dma_area))
return -EINVAL;
if (runtime->status->state == SNDRV_PCM_STATE_OPEN)
return -EBADFD;
return 0;
}
static int pcm_accessible_state(struct snd_pcm_runtime *runtime)
{
switch (runtime->status->state) {
case SNDRV_PCM_STATE_PREPARED:
case SNDRV_PCM_STATE_RUNNING:
case SNDRV_PCM_STATE_PAUSED:
return 0;
case SNDRV_PCM_STATE_XRUN:
return -EPIPE;
case SNDRV_PCM_STATE_SUSPENDED:
return -ESTRPIPE;
default:
return -EBADFD;
}
}
/* update to the given appl_ptr and call ack callback if needed;
* when an error is returned, take back to the original value
*/
int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream,
snd_pcm_uframes_t appl_ptr)
{
struct snd_pcm_runtime *runtime = substream->runtime;
snd_pcm_uframes_t old_appl_ptr = runtime->control->appl_ptr;
int ret;
if (old_appl_ptr == appl_ptr)
return 0;
runtime->control->appl_ptr = appl_ptr;
if (substream->ops->ack) {
ret = substream->ops->ack(substream);
if (ret < 0) {
runtime->control->appl_ptr = old_appl_ptr;
return ret;
}
}
trace_applptr(substream, old_appl_ptr, appl_ptr);
return 0;
}
/* the common loop for read/write data */
snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
void *data, bool interleaved,
snd_pcm_uframes_t size, bool in_kernel)
{
struct snd_pcm_runtime *runtime = substream->runtime;
snd_pcm_uframes_t xfer = 0;
snd_pcm_uframes_t offset = 0;
snd_pcm_uframes_t avail;
pcm_copy_f writer;
pcm_transfer_f transfer;
bool nonblock;
bool is_playback;
int err;
err = pcm_sanity_check(substream);
if (err < 0)
return err;
is_playback = substream->stream == SNDRV_PCM_STREAM_PLAYBACK;
if (interleaved) {
if (runtime->access != SNDRV_PCM_ACCESS_RW_INTERLEAVED &&
runtime->channels > 1)
return -EINVAL;
writer = interleaved_copy;
} else {
if (runtime->access != SNDRV_PCM_ACCESS_RW_NONINTERLEAVED)
return -EINVAL;
writer = noninterleaved_copy;
}
if (!data) {
if (is_playback)
transfer = fill_silence;
else
return -EINVAL;
} else if (in_kernel) {
if (substream->ops->copy_kernel)
transfer = substream->ops->copy_kernel;
else
transfer = is_playback ?
default_write_copy_kernel : default_read_copy_kernel;
} else {
if (substream->ops->copy_user)
transfer = (pcm_transfer_f)substream->ops->copy_user;
else
transfer = is_playback ?
default_write_copy : default_read_copy;
}
if (size == 0)
return 0;
nonblock = !!(substream->f_flags & O_NONBLOCK);
snd_pcm_stream_lock_irq(substream);
err = pcm_accessible_state(runtime);
if (err < 0)
goto _end_unlock;
runtime->twake = runtime->control->avail_min ? : 1;
if (runtime->status->state == SNDRV_PCM_STATE_RUNNING)
snd_pcm_update_hw_ptr(substream);
/*
* If size < start_threshold, wait indefinitely. Another
* thread may start capture
*/
if (!is_playback &&
runtime->status->state == SNDRV_PCM_STATE_PREPARED &&
size >= runtime->start_threshold) {
err = snd_pcm_start(substream);
if (err < 0)
goto _end_unlock;
}
avail = snd_pcm_avail(substream);
while (size > 0) {
snd_pcm_uframes_t frames, appl_ptr, appl_ofs;
snd_pcm_uframes_t cont;
if (!avail) {
if (!is_playback &&
runtime->status->state == SNDRV_PCM_STATE_DRAINING) {
snd_pcm_stop(substream, SNDRV_PCM_STATE_SETUP);
goto _end_unlock;
}
if (nonblock) {
err = -EAGAIN;
goto _end_unlock;
}
runtime->twake = min_t(snd_pcm_uframes_t, size,
runtime->control->avail_min ? : 1);
err = wait_for_avail(substream, &avail);
if (err < 0)
goto _end_unlock;
if (!avail)
continue; /* draining */
}
frames = size > avail ? avail : size;
appl_ptr = READ_ONCE(runtime->control->appl_ptr);
appl_ofs = appl_ptr % runtime->buffer_size;
cont = runtime->buffer_size - appl_ofs;
if (frames > cont)
frames = cont;
if (snd_BUG_ON(!frames)) {
err = -EINVAL;
goto _end_unlock;
}
if (!atomic_inc_unless_negative(&runtime->buffer_accessing)) {
err = -EBUSY;
goto _end_unlock;
}
snd_pcm_stream_unlock_irq(substream);
err = writer(substream, appl_ofs, data, offset, frames,
transfer);
snd_pcm_stream_lock_irq(substream);
atomic_dec(&runtime->buffer_accessing);
if (err < 0)
goto _end_unlock;
err = pcm_accessible_state(runtime);
if (err < 0)
goto _end_unlock;
appl_ptr += frames;
if (appl_ptr >= runtime->boundary)
appl_ptr -= runtime->boundary;
err = pcm_lib_apply_appl_ptr(substream, appl_ptr);
if (err < 0)
goto _end_unlock;
offset += frames;
size -= frames;
xfer += frames;
avail -= frames;
if (is_playback &&
runtime->status->state == SNDRV_PCM_STATE_PREPARED &&
snd_pcm_playback_hw_avail(runtime) >= (snd_pcm_sframes_t)runtime->start_threshold) {
err = snd_pcm_start(substream);
if (err < 0)
goto _end_unlock;
}
}
_end_unlock:
runtime->twake = 0;
if (xfer > 0 && err >= 0)
snd_pcm_update_state(substream, runtime);
snd_pcm_stream_unlock_irq(substream);
return xfer > 0 ? (snd_pcm_sframes_t)xfer : err;
}
EXPORT_SYMBOL(__snd_pcm_lib_xfer);
__snd_pcm_lib_xfer() 函数的执行过程如下：
-
调用
pcm_sanity_check()函数执行合理性检查，pcm_sanity_check()函数检查 DMA buffer 和状态。音频设备驱动程序需要保证，在__snd_pcm_lib_xfer()函数执行时，DMA buffer 已经准备好。 -
根据传入的参数，即数据格式是 interleaved 还是 noninterleaved，播放还是录制，数据缓冲区是否为空，操作是由内核空间代码发起还是由用户空间代码发起等，选择
writer和transfer操作。writer和transfer操作主要用于在传入的数据缓冲区和 DMA buffer 之间传递数据。 -
检查
runtime的状态，当状态不为SNDRV_PCM_STATE_PREPARED、SNDRV_PCM_STATE_RUNNING和SNDRV_PCM_STATE_PAUSED时，报错并返回。 -
当
runtime状态为SNDRV_PCM_STATE_RUNNING时，调用snd_pcm_update_hw_ptr(substream)函数更新 hw_ptr。 -
对于音频录制，如果
runtime状态为SNDRV_PCM_STATE_PREPARED，且请求录制的帧数超过发起阈值，则调用snd_pcm_start(substream)函数触发录制。 -
调用
snd_pcm_avail(substream)函数获得 DMA buffer 中以帧为单位的可用空间大小。
用户空间应用程序和 Linux 内核设备驱动程序对 DMA buffer 的访问是典型的读者-写者模型。对于播放来说，用户空间应用程序向 DMA buffer 写入数据，Linux 内核设备驱动程序从 DMA buffer 读取数据并发送给硬件设备。对于录制来说，Linux 内核设备驱动程序从硬件设备获得数据并写入 DMA buffer，用户空间应用程序从 DMA buffer 读取数据并作进一步处理。
这里的可用空间大小是站在用户空间应用程序的视角来说的，即对于播放来说，可用空间大小指还可以向 DMA buffer 写入的数据量，对于录制来说，则指可以从 DMA buffer 读取的数据量。 -
通过一个循环，处理所有的数据传递请求。
(1). 当 DMA buffer 的可用空间为 0 时，则等待直到可用空间大于 0。
(2). 计算拷贝的数据量大小，拷贝的目的内存在 DMA buffer 中的位置等。
(3). 调用writer和transfer操作在 DMA buffer 和传入的缓冲区之间传递数据，appl_ofs作为hwoff参数传给writer操作。
(4). 检查runtime的状态。
(5). 更新并调用pcm_lib_apply_appl_ptr()函数应用新的 appl_ptr。
(6). 根据传递的帧数，更新偏移量、要传递的帧数、已经传递的帧数和 DMA buffer 中可用的空间大小。
(7). 对于音频播放，如果runtime状态为SNDRV_PCM_STATE_PREPARED，且 DMA buffer 中可以播放的数据量超过发起阈值，则调用snd_pcm_start(substream)函数触发播放。
不同情况下，用于在传入的数据缓冲区和 DMA buffer 之间传递数据的 writer 和 transfer 操作定义 (位于 sound/core/pcm_lib.c) 如下：
/* calculate the target DMA-buffer position to be written/read */
static void *get_dma_ptr(struct snd_pcm_runtime *runtime,
int channel, unsigned long hwoff)
{
return runtime->dma_area + hwoff +
channel * (runtime->dma_bytes / runtime->channels);
}
/* default copy_user ops for write; used for both interleaved and non- modes */
static int default_write_copy(struct snd_pcm_substream *substream,
int channel, unsigned long hwoff,
void *buf, unsigned long bytes)
{
if (copy_from_user(get_dma_ptr(substream->runtime, channel, hwoff),
(void __user *)buf, bytes))
return -EFAULT;
return 0;
}
/* default copy_kernel ops for write */
static int default_write_copy_kernel(struct snd_pcm_substream *substream,
int channel, unsigned long hwoff,
void *buf, unsigned long bytes)
{
memcpy(get_dma_ptr(substream->runtime, channel, hwoff), buf, bytes);
return 0;
}
/* fill silence instead of copy data; called as a transfer helper
* from __snd_pcm_lib_write() or directly from noninterleaved_copy() when
* a NULL buffer is passed
*/
static int fill_silence(struct snd_pcm_substream *substream, int channel,
unsigned long hwoff, void *buf, unsigned long bytes)
{
struct snd_pcm_runtime *runtime = substream->runtime;
if (substream->stream != SNDRV_PCM_STREAM_PLAYBACK)
return 0;
if (substream->ops->fill_silence)
return substream->ops->fill_silence(substream, channel,
hwoff, bytes);
snd_pcm_format_set_silence(runtime->format,
get_dma_ptr(runtime, channel, hwoff),
bytes_to_samples(runtime, bytes));
return 0;
}
/* default copy_user ops for read; used for both interleaved and non- modes */
static int default_read_copy(struct snd_pcm_substream *substream,
int channel, unsigned long hwoff,
void *buf, unsigned long bytes)
{
if (copy_to_user((void __user *)buf,
get_dma_ptr(substream->runtime, channel, hwoff),
bytes))
return -EFAULT;
return 0;
}
/* default copy_kernel ops for read */
static int default_read_copy_kernel(struct snd_pcm_substream *substream,
int channel, unsigned long hwoff,
void *buf, unsigned long bytes)
{
memcpy(buf, get_dma_ptr(substream->runtime, channel, hwoff), bytes);
return 0;
}
/* call transfer function with the converted pointers and sizes;
* for interleaved mode, it's one shot for all samples
*/
static int interleaved_copy(struct snd_pcm_substream *substream,
snd_pcm_uframes_t hwoff, void *data,
snd_pcm_uframes_t off,
snd_pcm_uframes_t frames,
pcm_transfer_f transfer)
{
struct snd_pcm_runtime *runtime = substream->runtime;
/* convert to bytes */
hwoff = frames_to_bytes(runtime, hwoff);
off = frames_to_bytes(runtime, off);
frames = frames_to_bytes(runtime, frames);
return transfer(substream, 0, hwoff, data + off, frames);
}
/* call transfer function with the converted pointers and sizes for each
* non-interleaved channel; when buffer is NULL, silencing instead of copying
*/
static int noninterleaved_copy(struct snd_pcm_substream *substream,
snd_pcm_uframes_t hwoff, void *data,
snd_pcm_uframes_t off,
snd_pcm_uframes_t frames,
pcm_transfer_f transfer)
{
struct snd_pcm_runtime *runtime = substream->runtime;
int channels = runtime->channels;
void **bufs = data;
int c, err;
/* convert to bytes; note that it's not frames_to_bytes() here.
* in non-interleaved mode, we copy for each channel, thus
* each copy is n_samples bytes x channels = whole frames.
*/
off = samples_to_bytes(runtime, off);
frames = samples_to_bytes(runtime, frames);
hwoff = samples_to_bytes(runtime, hwoff);
for (c = 0; c < channels; ++c, ++bufs) {
if (!data || !*bufs)
err = fill_silence(substream, c, hwoff, NULL, frames);
else
err = transfer(substream, c, hwoff, *bufs + off,
frames);
if (err < 0)
return err;
}
return 0;
}
transfer 操作用于执行数据拷贝。根据发起数据传递的来源是内核还是用户空间应用程序，以及是播放还是录制，transfer 操作有四个默认实现，分别为 default_write_copy_kernel()、default_read_copy_kernel()、default_write_copy() 和 default_read_copy()，这几个函数根据 channel 和 hwoff 通过 get_dma_ptr() 函数获得 DMA 指针，并通过内核提供的 copy_from_user()、memcpy() 和 copy_to_user() 等函数在传入的数据缓冲区和 DMA buffer 之间传递数据。
从 get_dma_ptr() 函数可以看到，当数据格式为 noninterleaved 时，在 DMA buffer 中，各个 channel 的数据的布局。
writer 操作用于为数据拷贝做准备，它将帧为单位的偏移量和大小等数据拷贝参数转为以字节为单位，并调用 transfer 操作执行数据拷贝。writer 操作有两个默认实现，分别为 interleaved_copy() 和 noninterleaved_copy()。当数据格式为 noninterleaved 时，writer 操作分别拷贝各个 channel 的数据，传入的音频数据保存在多个不同的缓冲区中，每个通道一个，音频数据通过指针数组的方式传递。
writer 和 transfer 操作中用到的指向 DMA buffer 的 hwoff 偏移量，由 runtime->control->appl_ptr 计算而来，在关于 DMA buffer 访问的读者-写者模型中，对于播放，runtime->control->appl_ptr 是写指针，对于录制，它是读指针。
snd_pcm_avail() 函数用于获得 DMA buffer 中，用户空间应用程序视角的，以帧为单位的可用空间大小，Linux 内核还提供了另一个函数 snd_pcm_hw_avail()，用于获得音频硬件设备驱动视角的，以帧为单位的可用空间大小，这两个函数定义 (位于 sound/core/pcm_local.h) 如下：
static inline snd_pcm_uframes_t
snd_pcm_avail(struct snd_pcm_substream *substream)
{
if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
return snd_pcm_playback_avail(substream->runtime);
else
return snd_pcm_capture_avail(substream->runtime);
}
static inline snd_pcm_uframes_t
snd_pcm_hw_avail(struct snd_pcm_substream *substream)
{
if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
return snd_pcm_playback_hw_avail(substream->runtime);
else
return snd_pcm_capture_hw_avail(substream->runtime);
}
这两个函数根据流的类型是播放还是录制，将操作分派给另外四个函数 snd_pcm_playback_avail()、snd_pcm_capture_avail()、snd_pcm_playback_hw_avail() 和 snd_pcm_capture_hw_avail()。snd_pcm_playback_avail() 等函数定义 (位于 include/sound/pcm.h) 如下：
/**
* snd_pcm_playback_avail - Get the available (writable) space for playback
* @runtime: PCM runtime instance
*
* Result is between 0 ... (boundary - 1)
*/
static inline snd_pcm_uframes_t snd_pcm_playback_avail(struct snd_pcm_runtime *runtime)
{
snd_pcm_sframes_t avail = runtime->status->hw_ptr + runtime->buffer_size - runtime->control->appl_ptr;
if (avail < 0)
avail += runtime->boundary;
else if ((snd_pcm_uframes_t) avail >= runtime->boundary)
avail -= runtime->boundary;
return avail;
}
/**
* snd_pcm_capture_avail - Get the available (readable) space for capture
* @runtime: PCM runtime instance
*
* Result is between 0 ... (boundary - 1)
*/
static inline snd_pcm_uframes_t snd_pcm_capture_avail(struct snd_pcm_runtime *runtime)
{
snd_pcm_sframes_t avail = runtime->status->hw_ptr - runtime->control->appl_ptr;
if (avail < 0)
avail += runtime->boundary;
return avail;
}
/**
* snd_pcm_playback_hw_avail - Get the queued space for playback
* @runtime: PCM runtime instance
*/
static inline snd_pcm_sframes_t snd_pcm_playback_hw_avail(struct snd_pcm_runtime *runtime)
{
return runtime->buffer_size - snd_pcm_playback_avail(runtime);
}
/**
* snd_pcm_capture_hw_avail - Get the free space for capture
* @runtime: PCM runtime instance
*/
static inline snd_pcm_sframes_t snd_pcm_capture_hw_avail(struct snd_pcm_runtime *runtime)
{
return runtime->buffer_size - snd_pcm_capture_avail(runtime);
}
关于 DMA buffer 访问的读者-写者模型中，对于播放，runtime->status->hw_ptr 是读指针，(runtime->control->appl_ptr - runtime->status->hw_ptr) 是已经写入，但还未播放的数据量，对于录制，runtime->status->hw_ptr 是写指针。
__snd_pcm_lib_xfer() 函数调用 snd_pcm_start() 函数触发数据传递开始执行，这个函数定义 (位于 sound/core/pcm_native.c) 如下：
/*
* start callbacks
*/
static int snd_pcm_pre_start(struct snd_pcm_substream *substream,
snd_pcm_state_t state)
{
struct snd_pcm_runtime *runtime = substream->runtime;
if (runtime->status->state != SNDRV_PCM_STATE_PREPARED)
return -EBADFD;
if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
!snd_pcm_playback_data(substream))
return -EPIPE;
runtime->trigger_tstamp_latched = false;
runtime->trigger_master = substream;
return 0;
}
static int snd_pcm_do_start(struct snd_pcm_substream *substream,
snd_pcm_state_t state)
{
if (substream->runtime->trigger_master != substream)
return 0;
return substream->ops->trigger(substream, SNDRV_PCM_TRIGGER_START);
}
static void snd_pcm_undo_start(struct snd_pcm_substream *substream,
snd_pcm_state_t state)
{
if (substream->runtime->trigger_master == substream)
substream->ops->trigger(substream, SNDRV_PCM_TRIGGER_STOP);
}
static void snd_pcm_post_start(struct snd_pcm_substream *substream,
snd_pcm_state_t state)
{
struct snd_pcm_runtime *runtime = substream->runtime;
snd_pcm_trigger_tstamp(substream);
runtime->hw_ptr_jiffies = jiffies;
runtime->hw_ptr_buffer_jiffies = (runtime->buffer_size * HZ) /
runtime->rate;
runtime->status->state = state;
if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
runtime->silence_size > 0)
snd_pcm_playback_silence(substream, ULONG_MAX);
snd_pcm_timer_notify(substream, SNDRV_TIMER_EVENT_MSTART);
}
static const struct action_ops snd_pcm_action_start = {
.pre_action = snd_pcm_pre_start,
.do_action = snd_pcm_do_start,
.undo_action = snd_pcm_undo_start,
.post_action = snd_pcm_post_start
};
/**
* snd_pcm_start - start all linked streams
* @substream: the PCM substream instance
*
* Return: Zero if successful, or a negative error code.
* The stream lock must be acquired before calling this function.
*/
int snd_pcm_start(struct snd_pcm_substream *substream)
{
return snd_pcm_action(&snd_pcm_action_start, substream,
SNDRV_PCM_STATE_RUNNING);
}
这里调用音频硬件设备驱动程序的 trigger 操作，触发发起硬件的数据传输。