DMA传输类型：
enum dma_transaction_type {
DMA_MEMCPY, // 内存拷贝
DMA_XOR, // 异或操作
DMA_PQ, // 乘方操作
DMA_XOR_VAL, // 异或值操作
DMA_PQ_VAL, // 乘方值操作
DMA_INTERRUPT, // DMA中断？
DMA_SG, // 散射-聚集（Scatter-Gather）传输
DMA_PRIVATE, // 私有的DMA传输
DMA_ASYNC_TX, // 异步传输
DMA_SLAVE, // DMA控制器设备功能，必有
DMA_CYCLIC, // 循环DMA传输
DMA_INTERLEAVE, // 交错DMA传输
/* last transaction type for creation of the capabilities mask */
DMA_TX_TYPE_END,
};
此节以audio使用DMA_CYCLIC举例，说明下如何使用：
static int dmaengine_pcm_prepare_and_submit(struct snd_pcm_substream *substream)
{
struct dmaengine_pcm_runtime_data *prtd = substream_to_prtd(substream);
struct dma_chan *chan = prtd->dma_chan;
struct dma_async_tx_descriptor *desc;
enum dma_transfer_direction direction;
unsigned long flags = DMA_CTRL_ACK;
direction = snd_pcm_substream_to_dma_direction(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); // 进行dmac cyclic传输
if (!desc)
return -ENOMEM;
desc->callback = dmaengine_pcm_dma_complete; // 传输完成时的回调函数
desc->callback_param = substream; // 传输完成时回调函数的参数
prtd->cookie = dmaengine_submit(desc); // 提交一次传输
return 0;
}
调用到dmaengine的核心层：
static inline struct dma_async_tx_descriptor *dmaengine_prep_dma_cyclic(
struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
size_t period_len, enum dma_transfer_direction dir,
unsigned long flags)
{
if (!chan || !chan->device || !chan->device->device_prep_dma_cyclic)
return NULL;
return chan->device->device_prep_dma_cyclic(chan, buf_addr, buf_len,
period_len, dir, flags); // dmac具体实现
}
struct dma_async_tx_descriptor {
dma_cookie_t cookie; // 一个整型数，用于追踪本次传输
enum dma_ctrl_flags flags; /* not a 'long' to pack with cookie */
dma_addr_t phys;
struct dma_chan *chan;
dma_cookie_t (*tx_submit)(struct dma_async_tx_descriptor *tx);
int (*desc_free)(struct dma_async_tx_descriptor *tx);
dma_async_tx_callback callback; // 传输完成时的回调函数
dma_async_tx_callback_result callback_result;
void *callback_param; // 传输完成时回调函数的参数
struct dmaengine_unmap_data *unmap;
enum dma_desc_metadata_mode desc_metadata_mode;
struct dma_descriptor_metadata_ops *metadata_ops;
#ifdef CONFIG_ASYNC_TX_ENABLE_CHANNEL_SWITCH
struct dma_async_tx_descriptor *next;
struct dma_async_tx_descriptor *parent;
spinlock_t lock;
#endif
};
调用到dma控制器的dma_device.device_prep_dma_cyclic具体实现，device_prep_dma_cyclic会对dmac进行配置，返回dma_async_tx_descriptor描述符 在dmac实现中，通常会对每一个vchan进行vchan_init操作，例如：
... ...
for (i = 0; i < sdc->cfg->nr_max_vchans; i++) {
struct sun6i_vchan *vchan = &sdc->vchans[i];
INIT_LIST_HEAD(&vchan->node);
vchan->vc.desc_free = sun6i_dma_free_desc;
vchan_init(&vchan->vc, &sdc->slave);
}
... ...
而在vchan_init函数中，会定义一个tasklet任务，如下：
void vchan_init(struct virt_dma_chan *vc, struct dma_device *dmadev)
{
dma_cookie_init(&vc->chan);
spin_lock_init(&vc->lock);
INIT_LIST_HEAD(&vc->desc_submitted);
INIT_LIST_HEAD(&vc->desc_issued);
INIT_LIST_HEAD(&vc->desc_completed);
tasklet_init(&vc->task, vchan_complete, (unsigned long)vc); // 初始化一个tasklet任务vchan_complete
vc->chan.device = dmadev;
list_add_tail(&vc->chan.device_node, &dmadev->channels);
}
这个tasklet在一次描述符传输完成时被调度，例如：
static void sun6i_dma_issue_pending(struct dma_chan *chan)
{
struct sun6i_dma_dev *sdev = to_sun6i_dma_dev(chan->device);
struct sun6i_vchan *vchan = to_sun6i_vchan(chan);
unsigned long flags;
spin_lock_irqsave(&vchan->vc.lock, flags);
if (vchan_issue_pending(&vchan->vc)) {
spin_lock(&sdev->lock);
if (!vchan->phy && list_empty(&vchan->node)) {
list_add_tail(&vchan->node, &sdev->pending);
tasklet_schedule(&sdev->task); // 调度vchan_init初始化的tasklet，也就是调度vchan_complete
dev_dbg(chan2dev(chan), "vchan %p: issued\n",
&vchan->vc);
}
... ...
}
或调用vchan_cyclic_callback函数调度
static inline void vchan_cyclic_callback(struct virt_dma_desc *vd)
{
struct virt_dma_chan *vc = to_virt_chan(vd->tx.chan);
vc->cyclic = vd;
tasklet_schedule(&vc->task);
}
在vchan_complete函数中，
static void vchan_complete(struct tasklet_struct *t)
{
struct virt_dma_chan *vc = from_tasklet(vc, t, task);
struct virt_dma_desc *vd, *_vd;
struct dmaengine_desc_callback cb;
LIST_HEAD(head);
spin_lock_irq(&vc->lock);
list_splice_tail_init(&vc->desc_completed, &head);
vd = vc->cyclic;
if (vd) {
vc->cyclic = NULL;
dmaengine_desc_get_callback(&vd->tx, &cb); // 获取pcm设置的函数
} else {
memset(&cb, 0, sizeof(cb));
}
spin_unlock_irq(&vc->lock);
dmaengine_desc_callback_invoke(&cb, &vd->tx_result); // 进行回调
list_for_each_entry_safe(vd, _vd, &head, node) {
dmaengine_desc_get_callback(&vd->tx, &cb);
list_del(&vd->node);
dmaengine_desc_callback_invoke(&cb, &vd->tx_result);
vchan_vdesc_fini(vd);
}
}
dmaengine_desc_get_callback和dmaengine_desc_callback_invoke定义如下：
static inline void
dmaengine_desc_get_callback(struct dma_async_tx_descriptor *tx,
struct dmaengine_desc_callback *cb)
{
cb->callback = tx->callback;
cb->callback_result = tx->callback_result;
cb->callback_param = tx->callback_param;
}
... ...
static inline void
dmaengine_desc_callback_invoke(struct dmaengine_desc_callback *cb,
const struct dmaengine_result *result)
{
struct dmaengine_result dummy_result = {
.result = DMA_TRANS_NOERROR,
.residue = 0
};
if (cb->callback_result) { // 没设置callback_result就调用callback
if (!result)
result = &dummy_result;
cb->callback_result(cb->callback_param, result);
} else if (cb->callback) {
cb->callback(cb->callback_param);
}
}
自此回来分析的起点dmaengine_pcm_prepare_and_submit，在这个函数中我们定义了callback函数，该callback函数定义如下：
static void dmaengine_pcm_dma_complete(void *arg)
{
struct snd_pcm_substream *substream = arg;
struct dmaengine_pcm_runtime_data *prtd = substream_to_prtd(substream);
prtd->pos += snd_pcm_lib_period_bytes(substream);
if (prtd->pos >= snd_pcm_lib_buffer_bytes(substream))
prtd->pos = 0;
// 通知 ALSA 子系统一个音频周期已经结束，并且 ALSA 子系统会自动进行后续处理，包括更新 PCM 子流的状态以及准备处理下一个周期的音频数据。
snd_pcm_period_elapsed(substream);
}
... ...
void snd_pcm_period_elapsed(struct snd_pcm_substream *substream)
{
struct snd_pcm_runtime *runtime;
unsigned long flags;
if (snd_BUG_ON(!substream))
return;
snd_pcm_stream_lock_irqsave(substream, flags);
if (PCM_RUNTIME_CHECK(substream))
goto _unlock;
runtime = substream->runtime;
if (!snd_pcm_running(substream) ||
snd_pcm_update_hw_ptr0(substream, 1) < 0) // ALSA 中用于更新 PCM（Pulse Code Modulation）音频子流的硬件指针（hardware pointer）的函数
goto _end;
#ifdef CONFIG_SND_PCM_TIMER
if (substream->timer_running)
snd_timer_interrupt(substream->timer, 1);
#endif
_end:
kill_fasync(&runtime->fasync, SIGIO, POLL_IN);
_unlock:
snd_pcm_stream_unlock_irqrestore(substream, flags);
}
EXPORT_SYMBOL(snd_pcm_period_elapsed);
分析完毕
标签：DMA,vchan,struct,dma,CYCLIC,callback,substream,使用,desc
From： https://www.cnblogs.com/lethe1203/p/18109160