Linux驱动修炼之道-DMA框架源码分析(下)

努力成为linux kernel hacker的人李万鹏原创作品，为梦而战。转载请标明出处

http://blog.csdn.net/woshixingaaa/archive/2011/06/02/6462149.aspx

DMA通道的使用：申请通道，申请中断，设置寄存器，安装回调函数，设置标志，将数据放入队列，最后就是调用static int s3c2410_dma_start(struct s3c2410_dma_chan *chan)来开始DMA的传输了。
首先看通道的申请：

int s3c2410_dma_request(unsigned int channel,
			struct s3c2410_dma_client *client,
			void *dev)
{
	struct s3c2410_dma_chan *chan;
	unsigned long flags;
	int err;

	pr_debug("dma%d: s3c2410_request_dma: client=%s, dev=%p\n",
		 channel, client->name, dev);

	local_irq_save(flags);                  //关中断
	/*找到一个有效的物理通道*/
	chan = s3c2410_dma_map_channel(channel);
	if (chan == NULL) {
		local_irq_restore(flags);
		return -EBUSY;
	}

	dbg_showchan(chan);
	/*设置通道的名字*/
	chan->client = client;
	/*设置通道的使用标志*/
	chan->in_use = 1;

	if (!chan->irq_claimed) {       //该中断没有被注册
		pr_debug("dma%d: %s : requesting irq %d\n",
			 channel, __func__, chan->irq);

		chan->irq_claimed = 1;   //标记该中断被注册
		local_irq_restore(flags); //开中断

		err = request_irq(chan->irq, s3c2410_dma_irq, IRQF_DISABLED,  //注册中断处理程序
				  client->name, (void *)chan);

		local_irq_save(flags);

		if (err) {
			chan->in_use = 0;
			chan->irq_claimed = 0;
			local_irq_restore(flags);

			printk(KERN_ERR "%s: cannot get IRQ %d for DMA %d\n",
			       client->name, chan->irq, chan->number);
			return err;
		}

		chan->irq_enabled = 1;
	}

	local_irq_restore(flags);

	/* need to setup */

	pr_debug("%s: channel initialised, %p\n", __func__, chan);

	return chan->number | DMACH_LOW_LEVEL;
}

下面的函数是找通道好，先在板子通道映射中找，再在芯片通道映射中找。

static struct s3c2410_dma_chan *s3c2410_dma_map_channel(int channel)
{
	struct s3c24xx_dma_order_ch *ord = NULL;
	struct s3c24xx_dma_map *ch_map;
	struct s3c2410_dma_chan *dmach;
	int ch;

	if (dma_sel.map == NULL || channel > dma_sel.map_size)
		return NULL;
	/*获得芯片的虚拟通道与真实通道映射的结构*/
	ch_map = dma_sel.map + channel;

	/* first, try the board mapping */
	/*如果有板子通道映射*/
	if (dma_order) {
		/*得到对应虚拟通道的所有真实通道的结构*/
		ord = &dma_order->channels[channel];
		/*找这个虚拟通道对应的每一个真实通道，看有没有有效并且未被使用的*/
		for (ch = 0; ch < dma_channels; ch++) {
			if (!is_channel_valid(ord->list[ch]))
				continue;

			if (s3c2410_chans[ord->list[ch]].in_use == 0) {
				ch = ord->list[ch] & ~DMA_CH_VALID;
				goto found;
			}
		}

		if (ord->flags & DMA_CH_NEVER)
			return NULL;
	}
	/*检查芯片虚拟通道与真实通道的映射，看有没有有效且未被使用的真实通道*/
	for (ch = 0; ch < dma_channels; ch++) {
		if (!is_channel_valid(ch_map->channels[ch]))
			continue;

		if (s3c2410_chans[ch].in_use == 0) {
			printk("mapped channel %d to %d\n", channel, ch);
			break;
		}
	}
	if (ch >= dma_channels)
		return NULL;
	/* update our channel mapping */

 found:
	/*将找到的通道保存在dmach中，并返回*/
	dmach = &s3c2410_chans[ch];
	dmach->map = ch_map;
	dma_chan_map[channel] = dmach;

	/* select the channel */
	/*调用选择通道的函数*/
	(dma_sel.select)(dmach, ch_map);

	return dmach;
}

设置寄存器，设置寄存器的工作由s3c2410_dma_devconfig和s3c2410_dma_config完成：

int s3c2410_dma_devconfig(int channel,
			  enum s3c2410_dmasrc source,
			  int hwcfg,
			  unsigned long devaddr)
{
	/*根据虚拟通道号找到真实通道*/
	struct s3c2410_dma_chan *chan = lookup_dma_channel(channel);

	if (chan == NULL)
		return -EINVAL;

	pr_debug("%s: source=%d, hwcfg=%08x, devaddr=%08lx\n",
		 __func__, (int)source, hwcfg, devaddr);
	
	chan->source = source;     //保存dma源
	chan->dev_addr = devaddr;  //保存dma源地址
	chan->hw_cfg = hwcfg;      //保存dma源的控制信息

	switch (source) {
	case S3C2410_DMASRC_HW:                       //源是外设，从外设读数据到内存，源的地址是固定的
		/* source is hardware */
		pr_debug("%s: hw source, devaddr=%08lx, hwcfg=%d\n",
			 __func__, devaddr, hwcfg);
		dma_wrreg(chan, S3C2410_DMA_DISRCC, hwcfg & 3);         //初始化源控制寄存器
		dma_wrreg(chan, S3C2410_DMA_DISRC,  devaddr);           //将源地址写入初始源寄存器
		dma_wrreg(chan, S3C2410_DMA_DIDSTC, (0<<1) | (0<<0));   //目的地在AHB总线上

		chan->addr_reg = dma_regaddr(chan, S3C2410_DMA_DIDST);  
		break;

	case S3C2410_DMASRC_MEM:                    //源是内存，从内存读数据到外设上，目的地址是固定的
		/* source is memory */
		pr_debug("%s: mem source, devaddr=%08lx, hwcfg=%d\n",
			 __func__, devaddr, hwcfg);
		dma_wrreg(chan, S3C2410_DMA_DISRCC, (0<<1) | (0<<0));   //目的地址在AHB总线上
		dma_wrreg(chan, S3C2410_DMA_DIDST,  devaddr);           //把目的地址写到初始目的寄存器
		dma_wrreg(chan, S3C2410_DMA_DIDSTC, hwcfg & 3);         //初始化目的控制寄存器

		chan->addr_reg = dma_regaddr(chan, S3C2410_DMA_DISRC);  
		break;
		/*无论内存是源还是目的，这个地址始终是保存在chan->addr_reg*/
	default:
		printk(KERN_ERR "dma%d: invalid source type (%d)\n",
		       channel, source);

		return -EINVAL;
	}

	if (dma_sel.direction != NULL)
		(dma_sel.direction)(chan, chan->map, source);

	return 0;
}

int s3c2410_dma_config(unsigned int channel,
		       int xferunit,
		       int dcon)
{
	/*找到虚拟通道对应的实际通道*/
	struct s3c2410_dma_chan *chan = lookup_dma_channel(channel);

	pr_debug("%s: chan=%d, xfer_unit=%d, dcon=%08x\n",
		 __func__, channel, xferunit, dcon);

	if (chan == NULL)
		return -EINVAL;
	
	pr_debug("%s: Initial dcon is %08x\n", __func__, dcon);
	/*清除DMA源的选择位*/
	dcon |= chan->dcon & dma_sel.dcon_mask;          

	pr_debug("%s: New dcon is %08x\n", __func__, dcon);
	/*传输数据的大小*/
	switch (xferunit) {
	case 1:
		dcon |= S3C2410_DCON_BYTE;
		break;

	case 2:
		dcon |= S3C2410_DCON_HALFWORD;
		break;

	case 4:
		dcon |= S3C2410_DCON_WORD;
		break;

	default:
		pr_debug("%s: bad transfer size %d\n", __func__, xferunit);
		return -EINVAL;
	}

	dcon |= S3C2410_DCON_HWTRIG;     //DMA源是硬件
	dcon |= S3C2410_DCON_INTREQ;     //中断使能

	pr_debug("%s: dcon now %08x\n", __func__, dcon);
	/*将通道控制寄存器和传输大小存于chan中*/
	chan->dcon = dcon;             
	chan->xfer_unit = xferunit;

	return 0;
}

设置回调函数：

int s3c2410_dma_set_buffdone_fn(unsigned int channel, s3c2410_dma_cbfn_t rtn)
{
	。。。。。。。
	chan->callback_fn = rtn;

	return 0;
}

设置标志：

int s3c2410_dma_setflags(unsigned int channel, unsigned int flags)
{
	。。。。。。。。。。。。。。
	chan->flags = flags;

	return 0;
}

将数据放入队列，先看一下一个结构：

struct s3c2410_dma_buf {
       struct s3c2410_dma_buf    *next;
       int                  magic;         /* magic */
       int                  size;             /* buffer size in bytes */
       dma_addr_t          data;            /* start of DMA data */
       dma_addr_t          ptr;              /* where the DMA got to [1] */
       void               *id;        /* client's id */
};

每个struct s3c2410_dma_chan维护了一个缓冲区队列，每个缓冲区用上边的结构表示。在struct s3c2410_dma_chan中的结构是：

      /* buffer list and information */
       struct s3c2410_dma_buf    *curr;            /* current dma buffer */
       struct s3c2410_dma_buf    *next;            /* next buffer to load */
       struct s3c2410_dma_buf    *end;             /* end of queue */

下边这个函数就是完成将s3c2410_dma_buf放入这个队列中排队：

int s3c2410_dma_enqueue(unsigned int channel, void *id,
			dma_addr_t data, int size)
{
	/*找到虚拟通道对应的实际通道*/
	struct s3c2410_dma_chan *chan = lookup_dma_channel(channel);
	struct s3c2410_dma_buf *buf;
	unsigned long flags;

	if (chan == NULL)
		return -EINVAL;

	pr_debug("%s: id=%p, data=%08x, size=%d\n",
		 __func__, id, (unsigned int)data, size);
	/*分配s3c2410_dma_chan结构的buffer*/
	buf = kmem_cache_alloc(dma_kmem, GFP_ATOMIC);
	if (buf == NULL) {
		pr_debug("%s: out of memory (%ld alloc)\n",
			 __func__, (long)sizeof(*buf));
		return -ENOMEM;
	}

	//pr_debug("%s: new buffer %p\n", __func__, buf);
	//dbg_showchan(chan);
	/*设置这个buffer*/
	buf->next  = NULL;
	buf->data  = buf->ptr = data;  //指向要传输数据的地址
	buf->size  = size;             //该段buffer的大小
	buf->id    = id;
	buf->magic = BUF_MAGIC;

	local_irq_save(flags);
	/*加载的是该通道的第一段buf*/
	if (chan->curr == NULL) {
		/* we've got nothing loaded... */
		pr_debug("%s: buffer %p queued onto empty channel\n",
			 __func__, buf);
			
		chan->curr = buf;   //curr指向现在生成的buf
		chan->end  = buf;
		chan->next = NULL;
	} else {
		pr_debug("dma%d: %s: buffer %p queued onto non-empty channel\n",
			 chan->number, __func__, buf);

		if (chan->end == NULL)
			pr_debug("dma%d: %s: %p not empty, and chan->end==NULL?\n",
				 chan->number, __func__, chan);
		/*从链表尾加入链表*/
		chan->end->next = buf;  
		chan->end = buf;
	}

	/* if necessary, update the next buffer field */
	if (chan->next == NULL)
		chan->next = buf;
	
	if (chan->state == S3C2410_DMA_RUNNING) {                               //该channel正在运行
		if (chan->load_state == S3C2410_DMALOAD_1LOADED && 1) {         //已有buf load了
			if (s3c2410_dma_waitforload(chan, __LINE__) == 0) {     //等待load
				printk(KERN_ERR "dma%d: loadbuffer:"
				       "timeout loading buffer\n",
				       chan->number);
				dbg_showchan(chan);
				local_irq_restore(flags);
				return -EINVAL;
			}
		}

		while (s3c2410_dma_canload(chan) && chan->next != NULL) {        //检查能否load
			s3c2410_dma_loadbuffer(chan, chan->next);                //load buffer
		}
	} else if (chan->state == S3C2410_DMA_IDLE) {                            //该channel空闲着
		if (chan->flags & S3C2410_DMAF_AUTOSTART) {                      //如果设了自动启动标记，则直接启动该次传输
			s3c2410_dma_ctrl(chan->number | DMACH_LOW_LEVEL,         //启动传输
					 S3C2410_DMAOP_START);
		}
	}

	local_irq_restore(flags);
	return 0;
}

channel在运行的时候会有很多状态，在arch/arm/mach-s3c2410/include/mach/dma.h，注意已经很清楚了，我就不多解释了。

/* enum s3c2410_dma_loadst
 *
 * This represents the state of the DMA engine, wrt to the loaded / running
 * transfers. Since we don't have any way of knowing exactly the state of
 * the DMA transfers, we need to know the state to make decisions on wether
 * we can
 *
 * S3C2410_DMA_NONE
 *
 * There are no buffers loaded (the channel should be inactive)
 *
 * S3C2410_DMA_1LOADED
 *
 * There is one buffer loaded, however it has not been confirmed to be
 * loaded by the DMA engine. This may be because the channel is not
 * yet running, or the DMA driver decided that it was too costly to
 * sit and wait for it to happen.
 *
 * S3C2410_DMA_1RUNNING
 *
 * The buffer has been confirmed running, and not finisged
 *
 * S3C2410_DMA_1LOADED_1RUNNING
 *
 * There is a buffer waiting to be loaded by the DMA engine, and one
 * currently running.
*/

enum s3c2410_dma_loadst {
	S3C2410_DMALOAD_NONE,
	S3C2410_DMALOAD_1LOADED,
	S3C2410_DMALOAD_1RUNNING,
	S3C2410_DMALOAD_1LOADED_1RUNNING,
};

中断处理函数：

static irqreturn_t
s3c2410_dma_irq(int irq, void *devpw)
{
	struct s3c2410_dma_chan *chan = (struct s3c2410_dma_chan *)devpw;
	struct s3c2410_dma_buf  *buf;

	buf = chan->curr;                                   //当前传输完毕的buf

	dbg_showchan(chan);

	/* modify the channel state */

	switch (chan->load_state) {                         //改变状态，如果对上边那4个状态理解了很容易看懂的
	case S3C2410_DMALOAD_1RUNNING:
		/* TODO - if we are running only one buffer, we probably
		 * want to reload here, and then worry about the buffer
		 * callback */

		chan->load_state = S3C2410_DMALOAD_NONE;
		break;

	case S3C2410_DMALOAD_1LOADED:
		/* iirc, we should go back to NONE loaded here, we
		 * had a buffer, and it was never verified as being
		 * loaded.
		 */

		chan->load_state = S3C2410_DMALOAD_NONE;
		break;

	case S3C2410_DMALOAD_1LOADED_1RUNNING:
		/* we'll worry about checking to see if another buffer is
		 * ready after we've called back the owner. This should
		 * ensure we do not wait around too long for the DMA
		 * engine to start the next transfer
		 */

		chan->load_state = S3C2410_DMALOAD_1LOADED;
		break;

	case S3C2410_DMALOAD_NONE:
		printk(KERN_ERR "dma%d: IRQ with no loaded buffer?\n",
		       chan->number);
		break;

	default:
		printk(KERN_ERR "dma%d: IRQ in invalid load_state %d\n",
		       chan->number, chan->load_state);
		break;
	}

	if (buf != NULL) {                                                       //如果不为空
		/* update the chain to make sure that if we load any more
		 * buffers when we call the callback function, things should
		 * work properly */

		chan->curr = buf->next;                                         //指向下一个buf
		buf->next  = NULL;

		if (buf->magic != BUF_MAGIC) {
			printk(KERN_ERR "dma%d: %s: buf %p incorrect magic\n",
			       chan->number, __func__, buf);
			return IRQ_HANDLED;
		}

		s3c2410_dma_buffdone(chan, buf, S3C2410_RES_OK);                 //buf传输完成后的操作

		/* free resouces */
		s3c2410_dma_freebuf(buf);                                        //释放buf
	} else {
	}

	/* only reload if the channel is still running... our buffer done
	 * routine may have altered the state by requesting the dma channel
	 * to stop or shutdown... */

	/* todo: check that when the channel is shut-down from inside this
	 * function, we cope with unsetting reload, etc */

	if (chan->next != NULL && chan->state != S3C2410_DMA_IDLE) {            //还有要传输的buf,则继续传输
		unsigned long flags;

		switch (chan->load_state) {
		case S3C2410_DMALOAD_1RUNNING:
			/* don't need to do anything for this state */
			break;

		case S3C2410_DMALOAD_NONE:
			/* can load buffer immediately */
			break;

		case S3C2410_DMALOAD_1LOADED:
			if (s3c2410_dma_waitforload(chan, __LINE__) == 0) {     //如果已经有载入的，则等待被载入
				/* flag error? */
				printk(KERN_ERR "dma%d: timeout waiting for load (%s)\n",
				       chan->number, __func__);
				return IRQ_HANDLED;
			}

			break;

		case S3C2410_DMALOAD_1LOADED_1RUNNING:
			goto no_load;

		default:
			printk(KERN_ERR "dma%d: unknown load_state in irq, %d\n",
			       chan->number, chan->load_state);
			return IRQ_HANDLED;
		}

		local_irq_save(flags);
		s3c2410_dma_loadbuffer(chan, chan->next);                       //载入buf
		local_irq_restore(flags);
	} else {                                                                    //所有传输完成
		s3c2410_dma_lastxfer(chan);                                         //完成处理工作

		/* see if we can stop this channel.. */
		if (chan->load_state == S3C2410_DMALOAD_NONE) {
			pr_debug("dma%d: end of transfer, stopping channel (%ld)\n",
				 chan->number, jiffies);
			s3c2410_dma_ctrl(chan->number | DMACH_LOW_LEVEL,            //停止DMA传输
					 S3C2410_DMAOP_STOP);
		}
	}

 no_load:
	return IRQ_HANDLED;
}

可以选择不同的dma操作：

int
s3c2410_dma_ctrl(unsigned int channel, enum s3c2410_chan_op op)
{
	struct s3c2410_dma_chan *chan = lookup_dma_channel(channel);

	if (chan == NULL)
		return -EINVAL;

	switch (op) {
	case S3C2410_DMAOP_START:
		return s3c2410_dma_start(chan);

	case S3C2410_DMAOP_STOP:
		return s3c2410_dma_dostop(chan);

	case S3C2410_DMAOP_PAUSE:
	case S3C2410_DMAOP_RESUME:
		return -ENOENT;

	case S3C2410_DMAOP_FLUSH:
		return s3c2410_dma_flush(chan);

	case S3C2410_DMAOP_STARTED:
		return s3c2410_dma_started(chan);

	case S3C2410_DMAOP_TIMEOUT:
		return 0;
	}
	return -ENOENT;      /* unknown, don't bother */
}