zynq 音乐播放流程

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在《zynq audio pcm DMA》里提到了snd_pcm_writei（）这个函数，这个函数是alsa lib里的接口，其实现如下：

snd_pcm_sframes_t snd_pcm_writei(snd_pcm_t *pcm, const void *buffer, snd_pcm_uframes_t size){assert(pcm);assert(size == 0 || buffer);if (CHECK_SANITY(! pcm->setup)) {SNDMSG("PCM not set up");return -EIO;}if (pcm->access != SND_PCM_ACCESS_RW_INTERLEAVED) {SNDMSG("invalid access type %s", snd_pcm_access_name(pcm->access));return -EINVAL;}return _snd_pcm_writei(pcm, buffer, size);}

该函数调用了_snd_pcm_writei去实现具体的任务。

static inline snd_pcm_sframes_t _snd_pcm_writei(snd_pcm_t *pcm, const void *buffer, snd_pcm_uframes_t size){return pcm->fast_ops->writei(pcm->fast_op_arg, buffer, size);}

而_snd_pcm_writei实际上调用了一个函数指针操作集去实现写操作。该writei函数实际上是alsa库里的snd_pcm_hw_writei()。

static snd_pcm_sframes_t snd_pcm_hw_writei(snd_pcm_t *pcm, const void *buffer, snd_pcm_uframes_t size){int err;snd_pcm_hw_t *hw = pcm->private_data;int fd = hw->fd;struct snd_xferi xferi;xferi.buf = (char*) buffer;xferi.frames = size;xferi.result = 0; /* make valgrind happy */err = ioctl(fd, SNDRV_PCM_IOCTL_WRITEI_FRAMES, &xferi);err = err >= 0 ? sync_ptr(hw, SNDRV_PCM_SYNC_PTR_APPL) : -errno;#ifdef DEBUG_RWfprintf(stderr, "hw_writei: frames = %li, xferi.result = %li, err = %i\n", size, xferi.result, err);#endifif (err < 0)return snd_pcm_check_error(pcm, err);return xferi.result;}

这个函数实际上是调用了ioctl函数完成了相关的操作，实际上alsa lib是对pcm操作的封装，简化了alsa的用户空间编程。
ioctl系统调用是在linux 内核头文件syscalls.h里声明的。

<4>[<c03a9a2c>] (snd_pcm_playback_ioctl1) from [<c00d7af0>] (do_vfs_ioctl+0x564/0x688)<4>[<c00d7af0>] (do_vfs_ioctl) from [<c00d7c48>] (SyS_ioctl+0x34/0x5c)<4>[<c00d7c48>] (SyS_ioctl) from [<c000edc0>] (ret_fast_syscall+0x0/0x3c)

为什么对应到linux kernel对应里是snd_pcm_writei？这在注册时就确定的。

snd_soc_register_card-->snd_card_register--->snd_device_register_all-->__snd_device_register.part.0-->snd_pcm_dev_register--->snd_pcm_dev_register(){err = snd_register_device(devtype, pcm->card, pcm->device,  &snd_pcm_f_ops[cidx], pcm,  &pcm->streams[cidx].dev);}.unlocked_ioctl =snd_pcm_playback_ioctl,snd_pcm_playback_ioctl--->snd_pcm_playback_ioctl1

由于用户空间传递的ioctl命令是SNDRV_PCM_IOCTL_WRITEI_FRAMES，则执行相应的case语句。播放调用流程如下：

snd_pcm_playback_ioctl1-->snd_pcm_common_ioctl1-->snd_pcm_action_lock_irq-->snd_pcm_action-->snd_pcm_action_single-->snd_pcm_do_start-->soc_pcm_trigger

2807 static int snd_pcm_playback_ioctl1(struct file *file,2808                    struct snd_pcm_substream *substream,2809                    unsigned int cmd, void __user *arg)2810 {2811     if (snd_BUG_ON(!substream))2812         return -ENXIO;2813     if (snd_BUG_ON(substream->stream != SNDRV_PCM_STREAM_PLAYBACK))2814         return -EINVAL;2815 2816     switch (cmd) {2817     case SNDRV_PCM_IOCTL_WRITEI_FRAMES:2818     {2819         struct snd_xferi xferi;2820         struct snd_xferi __user *_xferi = arg;2821         struct snd_pcm_runtime *runtime = substream->runtime;2822         snd_pcm_sframes_t result;2823         if (runtime->status->state == SNDRV_PCM_STATE_OPEN)2824             return -EBADFD;2825         if (put_user(0, &_xferi->result))2826             return -EFAULT;2827         if (copy_from_user(&xferi, _xferi, sizeof(xferi)))2828             return -EFAULT;2829         result = snd_pcm_lib_write(substream, xferi.buf, xferi.frames);2830         __put_user(result, &_xferi->result);2831         return result < 0 ? result : 0;2832     }

这个函数调用的write，最终调用

<sound/core/pcm_lib.c>实现

static int soc_pcm_trigger(struct snd_pcm_substream *substream, int cmd){    struct snd_soc_pcm_runtime *rtd = substream->private_data;    struct snd_soc_platform *platform = rtd->platform;    struct snd_soc_dai *cpu_dai = rtd->cpu_dai;    struct snd_soc_dai *codec_dai;    int i, ret;    for (i = 0; i < rtd->num_codecs; i++) {        codec_dai = rtd->codec_dais[i];        if (codec_dai->driver->ops && codec_dai->driver->ops->trigger) {            ret = codec_dai->driver->ops->trigger(substream,                                  cmd, codec_dai);            if (ret < 0)                return ret;        }    }    if (platform->driver->ops && platform->driver->ops->trigger) {        ret = platform->driver->ops->trigger(substream, cmd);        if (ret < 0)            return ret;    }    if (cpu_dai->driver->ops && cpu_dai->driver->ops->trigger) {        ret = cpu_dai->driver->ops->trigger(substream, cmd, cpu_dai);        if (ret < 0)            return ret;    }    if (rtd->dai_link->ops && rtd->dai_link->ops->trigger) {        ret = rtd->dai_link->ops->trigger(substream, cmd);        if (ret < 0)            return ret;    }    return 0;}

这里调用了四个triger函数，每一个函数对应一个trigger。platform对应的trigger函数是snd_dmaengine_pcm_trigger(), cpu侧的trigger函数是axi_i2s_trigger()。而codec则没有实现trigger函数。axi-i2s_trigger()用于设置cpu侧dai的使能。真正和DMA相关的操作在

184 int snd_dmaengine_pcm_trigger(struct snd_pcm_substream *substream, int cmd)185 {186     struct dmaengine_pcm_runtime_data *prtd = substream_to_prtd(substream);187     struct snd_pcm_runtime *runtime = substream->runtime;188     int ret;189 190     switch (cmd) {191     case SNDRV_PCM_TRIGGER_START:192         ret = dmaengine_pcm_prepare_and_submit(substream);193         if (ret)194             return ret;195         dma_async_issue_pending(prtd->dma_chan);196         break;197     case SNDRV_PCM_TRIGGER_RESUME:198     case SNDRV_PCM_TRIGGER_PAUSE_RELEASE:199         dmaengine_resume(prtd->dma_chan);200         break;201     case SNDRV_PCM_TRIGGER_SUSPEND:202         if (runtime->info & SNDRV_PCM_INFO_PAUSE)203             dmaengine_pause(prtd->dma_chan);204         else205             dmaengine_terminate_all(prtd->dma_chan);206         break;207     case SNDRV_PCM_TRIGGER_PAUSE_PUSH:208         dmaengine_pause(prtd->dma_chan);209         break;210     case SNDRV_PCM_TRIGGER_STOP:211         dmaengine_terminate_all(prtd->dma_chan);212         break;213     default:214         return -EINVAL;215     }216 217     return 0;218 }219 EXPORT_SYMBOL_GPL(snd_dmaengine_pcm_trigger);

这个函数调用dmaengine_pcm_prepare_and_submit()进行发送操作。

145 static int dmaengine_pcm_prepare_and_submit(struct snd_pcm_substream *substream)146 {147     struct dmaengine_pcm_runtime_data *prtd = substream_to_prtd(substream);148     struct dma_chan *chan = prtd->dma_chan;149     struct dma_async_tx_descriptor *desc;150     enum dma_transfer_direction direction;151     unsigned long flags = DMA_CTRL_ACK;152 153     direction = snd_pcm_substream_to_dma_direction(substream);154 155     if (!substream->runtime->no_period_wakeup)156         flags |= DMA_PREP_INTERRUPT;157 158     prtd->pos = 0;159     desc = dmaengine_prep_dma_cyclic(chan,160         substream->runtime->dma_addr,161         snd_pcm_lib_buffer_bytes(substream),162         snd_pcm_lib_period_bytes(substream), direction, flags);163 164     if (!desc)165         return -ENOMEM;166 167     desc->callback = dmaengine_pcm_dma_complete;168     desc->callback_param = substream;169     prtd->cookie = dmaengine_submit(desc);170 171     return 0;172 }

上面这个函数159调用pl330_prep_dma_cyclic()函数，dmaengine_submit（）调用pl330_tx_submit()函数添加发送描述符。

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