linux内核dma机制-凯发app官方网站

struct dma_pool {	/* the pool */
	struct list_head	page_list;//页链表
	spinlock_t		lock;
	size_t			blocks_per_page;　//每页的块数
	size_t			size;     //dma池里的一致内存块的大小
	struct device		*dev; //将做dma的设备
	size_t			allocation; //分配的没有跨越边界的块数，是size的整数倍
	char			name [32];　//池的名字
	wait_queue_head_t	waitq;  //等待队列
	struct list_head	pools;
};

struct dma_pool *dma_pool_create (const char *name, struct device *dev,
	　　　　　　　　　　size_t size, size_t align, size_t allocation)
{
	struct dma_pool		*retval;
 
	if (align == 0)
		align = 1;
	if (size == 0)
		return null;
	else if (size < align)
		size = align;
	else if ((size % align) != 0) {//对齐处理
		size  = align  1;
		size &= ~(align - 1);
	}
　　//如果一致内存块比页大，是分配为一致内存块大小，否则，分配为页大小
	if (allocation == 0) {
		if (page_size < size)//页比一致内存块小
			allocation = size;
		else
			allocation = page_size;//页大小
		// fixme: round up for less fragmentation
	} else if (allocation < size)
		return null;
　　//分配dma_pool结构对象空间
	if (!(retval = kmalloc (sizeof *retval, slab_kernel)))
		return retval;
 
	strlcpy (retval->name, name, sizeof retval->name);
 
	retval->dev = dev;
　　//初始化dma_pool结构对象retval
	init_list_head (&retval->page_list);//初始化页链表
	spin_lock_init (&retval->lock);
	retval->size = size;
	retval->allocation = allocation;
	retval->blocks_per_page = allocation / size;
	init_waitqueue_head (&retval->waitq);//初始化等待队列
 
	if (dev) {//设备存在时
		down (&pools_lock);
		if (list_empty (&dev->dma_pools))
　　　　　　//给设备创建sysfs文件系统属性文件
			device_create_file (dev, &dev_attr_pools);
		/* note:  not currently insisting "name" be unique */
		list_add (&retval->pools, &dev->dma_pools);　//将dma池加到dev中
		up (&pools_lock);
	} else
		init_list_head (&retval->pools);
 
	return retval;
}

void *dma_pool_alloc (struct dma_pool *pool, int mem_flags, dma_addr_t *handle)
{
	unsigned long		flags;
	struct dma_page		*page;
	int			map, block;
	size_t			offset;
	void			*retval;
 
restart:
	spin_lock_irqsave (&pool->lock, flags);
	list_for_each_entry(page, &pool->page_list, page_list) {
		int		i;
		/* only cachable accesses here ... */
        //遍历一页的每块，而每块又以32字节递增
		for (map = 0, i = 0;
				i < pool->blocks_per_page;　//每页的块数
				i  = bits_per_long, map) {　// bits_per_long定义为32
			if (page->bitmap [map] == 0)
				continue;
			block = ffz (~ page->bitmap [map]);//找出第一个0
			if ((i  block) < pool->blocks_per_page) {
				clear_bit (block, &page->bitmap [map]);
　　　　　　　//得到相对于页边界的偏移
				offset = (bits_per_long * map)  block;
				offset *= pool->size;
				goto ready;
			}
		}
	}
//给dma池分配dma_page结构空间，加入到pool->page_list链表，
//并作dma一致映射，它包括分配给dma池一页。
// slab_atomic表示调用 kmalloc（gfp_atomic） 直到失败为止，
//然后它等待内核释放若干页面，接下来再一次进行分配。
	if (!(page = pool_alloc_page (pool, slab_atomic))) {
		if (mem_flags & __gfp_wait) {
			declare_waitqueue (wait, current);
 
			current->state = task_interruptible;
			add_wait_queue (&pool->waitq, &wait);
			spin_unlock_irqrestore (&pool->lock, flags);
 
			schedule_timeout (pool_timeout_jiffies);
 
			remove_wait_queue (&pool->waitq, &wait);
			goto restart;
		}
		retval = null;
		goto done;
	}
 
	clear_bit (0, &page->bitmap [0]);
	offset = 0;
ready:
	page->in_use  ;
	retval = offset  page->vaddr;　//返回虚拟地址
	*handle = offset  page->dma;　//相对dma地址
#ifdef	config_debug_slab
	memset (retval, pool_poison_allocated, pool->size);
#endif
done:
	spin_unlock_irqrestore (&pool->lock, flags);
	return retval;
}