DMA在ASOC声卡驱动中的使用

DMA在ASOC声卡驱动中的使用

Linux 中DMA 的使用方法：

1. dma_request_channel 申请一个chan
2. tx = dma_prep_xxxx 准备，tx结构体
3. tx->callback = tx_callback; 设置回调
4. dma_cookie = dmaengine_submit(tx); 提交
5. dma_async_issue_pending(chan); 启动硬件开始传输

注：dmaengine_slave_config 接口有时也要调用

下面看一下DMS 在ASOC 代码中的使用。
从I2S 驱动看起，因为ASOC 中主要是用到 I2S 接口来传输数据，DMA 是内存与 I2S 交换数据的工具。

static int sun4i_i2s_probe(struct platform_device *pdev)
{.../* 设置物理地址*/i2s->playback_dma_data.addr = res->start +i2s->variant->reg_offset_txdata;i2s->playback_dma_data.maxburst = 8;i2s->capture_dma_data.addr = res->start + SUN4I_I2S_FIFO_RX_REG;i2s->capture_dma_data.maxburst = 8;/*注册PCM*/ret = snd_dmaengine_pcm_register(&pdev->dev, NULL, 0);if (ret) {dev_err(&pdev->dev, "Could not register PCMn");goto err_suspend;}
}

带着问题看代码，既然是用到DMA 那第一步肯定是要 dma_request_channel ，
问题1：ASOC 中何时调用了 dma_request_channel ？

snd_dmaengine_pcm_registerdmaengine_pcm_request_chan_offor (i = SNDRV_PCM_STREAM_PLAYBACK; i <= SNDRV_PCM_STREAM_CAPTURE;i++) {...chan = dma_request_slave_channel_reason(dev, name);   // 这里面调用了dma_pcm->chan[i] = chan;...}

#define dma_request_slave_channel_reason(dev, name) dma_request_chan(dev, name)
struct dma_chan *dma_request_chan(struct device *dev, const char *name)
{struct dma_device *d, *_d;struct dma_chan *chan = NULL;/* If device-tree is present get slave info from here */if (dev->of_node)chan = of_dma_request_slave_channel(dev->of_node, name);   //这里往下就稍复杂了，先不跟吧，总之chan 在这就request 到手了if (chan) {/* Valid channel found or requester need to be deferred */if (!IS_ERR(chan) || PTR_ERR(chan) == -EPROBE_DEFER)return chan;}...

问题2：dma_prep_xxxx 什么地方调用？
问题3：dmaengine_submit 什么地方调用？

static int dmaengine_pcm_prepare_and_submit(struct snd_pcm_substream *substream)
{...if (!substream->runtime->no_period_wakeup)flags |= DMA_PREP_INTERRUPT;prtd->pos = 0;desc = dmaengine_prep_dma_cyclic(chan,substream->runtime->dma_addr,snd_pcm_lib_buffer_bytes(substream),snd_pcm_lib_period_bytes(substream), direction, flags);desc->callback = dmaengine_pcm_dma_complete;desc->callback_param = substream;prtd->cookie = dmaengine_submit(desc);return 0;
}

问题4：dma_async_issue_pending 在哪调用？

int snd_dmaengine_pcm_trigger(struct snd_pcm_substream *substream, int cmd)
{...	switch (cmd) {case SNDRV_PCM_TRIGGER_START:ret = dmaengine_pcm_prepare_and_submit(substream);if (ret)return ret;dma_async_issue_pending(prtd->dma_chan);  //在这里调用break;...
}

ASOC 框架虽然复杂，但DMA 的用法还是遵循基本法则的。