软件工程师证书报考网站,dede系统做的网站如何调用cu3er官网flash 3d焦点图,手机上做整蛊网站,苍南县规划建设局网站Linux 用户空间应用程序通过声卡驱动程序#xff08;一般牵涉到多个设备驱动程序#xff09;和 Linux 内核 ALSA 框架导出的 PCM 设备文件#xff0c;如 /dev/snd/pcmC0D0c 和 /dev/snd/pcmC0D0p 等#xff0c;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…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_APIcase 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);
#endifcase 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);elseresult 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);elseresult 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;elsereturn -EINVAL;} else if (in_kernel) {if (substream-ops-copy_kernel)transfer substream-ops-copy_kernel;elsetransfer is_playback ?default_write_copy_kernel : default_read_copy_kernel;} else {if (substream-ops-copy_user)transfer (pcm_transfer_f)substream-ops-copy_user;elsetransfer 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, its 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 its 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);elseerr 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);elsereturn 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);elsereturn 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。
