轉:https://blog.csdn.net/crycheng/article/details/7095899
CODEC :音頻芯片的控制,比如靜音、打開(關閉)ADC(DAC)、設置ADC(DAC)的增益、耳機模式的檢測等操作。
I2S :數字音頻接口,用於CPU和Codec之間的數字音頻流raw data的傳輸。每當有playback或record操作時,snd_soc_dai_ops.prepare()會被調用,啟動I2S總線。
PCM :我不知道為什么會取這個模塊名,它其實是定義DMA操作的,用於將音頻數據通過DMA傳到I2S控制器的FIFO中。
這里的PCM實際是就是更新和管理音頻數據流的地址,分配DMA等等,將RAM中存放的音頻數據的地址傳給I2S,不是PCM協議。
音頻數據流向:
| DMA | | I2S/PCM/AC97 |
RAM --------> I2SControllerFIFO -----------------> CODEC ----> SPK/Headset
PCM模塊初始化:
- struct snd_soc_platform rk29_soc_platform = {
- .name = "rockchip-audio",
- .pcm_ops = &rockchip_pcm_ops,
- .pcm_new = rockchip_pcm_new,
- .pcm_free = rockchip_pcm_free_dma_buffers,
- };
- EXPORT_SYMBOL_GPL(rk29_soc_platform);
- static int __init rockchip_soc_platform_init(void)
- {
- DBG("Enter::%s, %d\n", __FUNCTION__, __LINE__);
- return snd_soc_register_platform(&rk29_soc_platform);
- }
- module_init(rockchip_soc_platform_init);
- static void __exit rockchip_soc_platform_exit(void)
- {
- snd_soc_unregister_platform(&rk29_soc_platform);
- }
調用snd_soc_register_platform()向ALSA core注冊一個snd_soc_platform結構體。
成員pcm_new需要調用dma_alloc_writecombine()給DMA分配一塊write-combining的內存空間,並把這塊緩沖區的相關信息保存到substream->dma_buffer中,相當於構造函數。pcm_free則相反。這些成員函數都還算簡單,看看代碼即可以理解其流程。
snd_pcm_ops
接着我們看一下snd_pcm_ops結構體,該結構體的操作函數集的實現是本模塊的主體。
- struct snd_pcm_ops {
- int (*open)(struct snd_pcm_substream *substream);
- int (*close)(struct snd_pcm_substream *substream);
- int (*ioctl)(struct snd_pcm_substream * substream,
- unsigned int cmd, void *arg);
- int (*hw_params)(struct snd_pcm_substream *substream,
- struct snd_pcm_hw_params *params);
- int (*hw_free)(struct snd_pcm_substream *substream);
- int (*prepare)(struct snd_pcm_substream *substream);
- int (*trigger)(struct snd_pcm_substream *substream, int cmd);
- snd_pcm_uframes_t (*pointer)(struct snd_pcm_substream *substream);
- int (*copy)(struct snd_pcm_substream *substream, int channel,
- snd_pcm_uframes_t pos,
- void __user *buf, snd_pcm_uframes_t count);
- int (*silence)(struct snd_pcm_substream *substream, int channel,
- snd_pcm_uframes_t pos, snd_pcm_uframes_t count);
- struct page *(*page)(struct snd_pcm_substream *substream,
- unsigned long offset);
- int (*mmap)(struct snd_pcm_substream *substream, struct vm_area_struct *vma);
- int (*ack)(struct snd_pcm_substream *substream);
- };
我們主要實現open、close、hw_params、hw_free、prepare和trigger接口。
open
open函數為PCM模塊設定支持的傳輸模式、數據格式、通道數、period等參數,並為playback/capture stream分配相應的DMA通道。其一般實現如下:
- static int rockchip_pcm_open(struct snd_pcm_substream *substream)
- {
- struct snd_pcm_runtime *runtime = substream->runtime;
- struct rockchip_runtime_data *prtd;
- DBG("Enter::%s----%d\n",__FUNCTION__,__LINE__);
- snd_soc_set_runtime_hwparams(substream, &rockchip_pcm_hardware);
- prtd = kzalloc(sizeof(struct rockchip_runtime_data), GFP_KERNEL);
- if (prtd == NULL)
- return -ENOMEM;
- spin_lock_init(&prtd->lock);
- runtime->private_data = prtd;
- return 0;
- }
其中硬件參數要根據芯片的數據手冊來定義,如:
- int snd_soc_set_runtime_hwparams(struct snd_pcm_substream *substream,
- const struct snd_pcm_hardware *hw)
- {
- struct snd_pcm_runtime *runtime = substream->runtime;
- runtime->hw.info = hw->info;
- runtime->hw.formats = hw->formats;
- runtime->hw.period_bytes_min = hw->period_bytes_min;
- runtime->hw.period_bytes_max = hw->period_bytes_max;
- runtime->hw.periods_min = hw->periods_min;
- runtime->hw.periods_max = hw->periods_max;
- runtime->hw.buffer_bytes_max = hw->buffer_bytes_max;
- runtime->hw.fifo_size = hw->fifo_size;
- return 0;
- }
關於peroid的概念有這樣的描述:The “period” is a term that corresponds to a fragment in the OSS world. The period defines the size at which a PCM interrupt is generated. peroid的概念很重要,建議去alsa官網找相關詳細說明了解一下。
上層ALSA lib可以通過接口來獲得這些參數的,如snd_pcm_hw_params_get_buffer_size_max()來取得buffer_bytes_max。
hw_free是hw_params的相反操作,調用snd_pcm_set_runtime_buffer(substream, NULL)即可。
注:代碼中的dma_buffer是DMA緩沖區,它通過4個字段定義:dma_area、dma_addr、dma_bytes和dma_private。其中dma_area是緩沖區邏輯地址,dma_addr是緩沖區的物理地址,dma_bytes是緩沖區的大小,dma_private是ALSA的DMA管理用到的。dma_buffer是在pcm_new()中初始化的;當然也可以把分配dma緩沖區的工作放到這部分來實現,但考慮到減少碎片,故還是在pcm_new中以最大size(即buffer_bytes_max)來分配。
關於DMA的中斷處理
另外留意open函數中的audio_dma_request(&s[0], audio_dma_callback);中的audio_dma_callback,這是dma的中斷函數,這里以callback的形式存在,其實到dma的底層還是這樣的形式:static irqreturn_t dma_irq_handler(int irq, void *dev_id),在DMA中斷處理dma_irq_handler()中調用callback。這些跟具體硬件平台的DMA實現相關,如果沒有類似的機制,那么還是要在pcm模塊中實現這個中斷。
- void rockchip_pcm_dma_irq(s32 ch, void *data)
- {
- struct snd_pcm_substream *substream = data;
- struct rockchip_runtime_data *prtd;
- unsigned long flags;
- DBG("Enter::%s----%d\n",__FUNCTION__,__LINE__);
- prtd = substream->runtime->private_data;
- if (substream)
- snd_pcm_period_elapsed(substream);
- spin_lock(&prtd->lock);
- prtd->dma_loaded--;
- if (prtd->state & ST_RUNNING) {
- rockchip_pcm_enqueue(substream);
- }
- spin_unlock(&prtd->lock);
- local_irq_save(flags);
- if (prtd->state & ST_RUNNING) {
- if (prtd->dma_loaded) {
- if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
- audio_start_dma(substream, DMA_MODE_WRITE);
- else
- audio_start_dma(substream, DMA_MODE_READ);
- }
- }
- local_irq_restore(flags);
- }
prepare
當pcm“准備好了”調用該函數。在這里根據channels、buffer_bytes等來設定DMA傳輸參數,跟具體硬件平台相關。注:每次調用snd_pcm_prepare()的時候均會調用prepare函數。
trigger
當pcm開始、停止、暫停的時候都會調用trigger函數。
- static int rockchip_pcm_trigger(struct snd_pcm_substream *substream, int cmd)
- {
- struct rockchip_runtime_data *prtd = substream->runtime->private_data;
- int ret = 0;
- /**************add by qiuen for volume*****/
- struct snd_soc_pcm_runtime *rtd = substream->private_data;
- struct snd_soc_dai *pCodec_dai = rtd->dai->codec_dai;
- int vol = 0;
- int streamType = 0;
- DBG("Enter::%s----%d\n",__FUNCTION__,__LINE__);
- if(cmd==SNDRV_PCM_TRIGGER_VOLUME){
- vol = substream->number % 100;
- streamType = (substream->number / 100) % 100;
- DBG("enter:vol=%d,streamType=%d\n",vol,streamType);
- if(pCodec_dai->ops->set_volume)
- pCodec_dai->ops->set_volume(streamType, vol);
- }
- /****************************************************/
- spin_lock(&prtd->lock);
- switch (cmd) {
- case SNDRV_PCM_TRIGGER_START:
- DBG(" START \n");
- prtd->state |= ST_RUNNING;
- rk29_dma_ctrl(prtd->params->channel, RK29_DMAOP_START);
- /*
- if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
- audio_start_dma(substream, DMA_MODE_WRITE);
- } else {
- audio_start_dma(substream, DMA_MODE_READ);
- }
- */
- #ifdef CONFIG_ANDROID_POWER
- android_lock_suspend(&audio_lock);
- DBG("%s::start audio , lock system suspend\n" , __func__ );
- #endif
- break;
- case SNDRV_PCM_TRIGGER_RESUME:
- DBG(" RESUME \n");
- break;
- case SNDRV_PCM_TRIGGER_PAUSE_RELEASE:
- DBG(" RESTART \n");
- break;
- case SNDRV_PCM_TRIGGER_STOP:
- case SNDRV_PCM_TRIGGER_SUSPEND:
- case SNDRV_PCM_TRIGGER_PAUSE_PUSH:
- DBG(" STOPS \n");
- prtd->state &= ~ST_RUNNING;
- rk29_dma_ctrl(prtd->params->channel, RK29_DMAOP_STOP);
- //disable_dma(prtd->params->channel);
- #ifdef CONFIG_ANDROID_POWER
- android_unlock_suspend(&audio_lock );
- DBG("%s::stop audio , unlock system suspend\n" , __func__ );
- #endif
- break;
- default:
- ret = -EINVAL;
- break;
- }
- spin_unlock(&prtd->lock);
- return ret;
- }
Trigger函數里面的操作應該是原子的,不要在調用這些操作時進入睡眠,trigger函數應盡量小,甚至僅僅是觸發DMA。
pointer
static snd_pcm_uframes_t wmt_pcm_pointer(struct snd_pcm_substream *substream)
PCM中間層通過調用這個函數來獲取緩沖區的位置。一般情況下,在中斷函數中調用snd_pcm_period_elapsed()或在pcm中間層更新buffer的時候調用它。然后pcm中間層會更新指針位置和計算緩沖區可用空間,喚醒那些在等待的線程。這個函數也是原子的。
snd_pcm_runtime
我們會留意到ops各成員函數均需要取得一個snd_pcm_runtime結構體指針,這個指針可以通過substream->runtime來獲得。snd_pcm_runtime是pcm運行時的信息。當打開一個pcm子流時,pcm運行時實例就會分配給這個子流。它擁有很多多種信息:hw_params和sw_params配置拷貝,緩沖區指針,mmap記錄,自旋鎖等。snd_pcm_runtime對於驅動程序操作集函數是只讀的,僅pcm中間層可以改變或更新這些信息。