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struct dma_pool {	/* the pool */            struct list_head	page_list;//頁(yè)鏈表            spinlock_t		lock;            size_t			blocks_per_page;　//每頁(yè)的塊數(shù)            size_t			size;     //DMA池里的一致內(nèi)存塊的大小            struct device		*dev; //將做DMA的設(shè)備            size_t			allocation; //分配的沒(méi)有跨越邊界的塊數(shù)，是size的整數(shù)倍            char			name [32];　//池的名字            wait_queue_head_t	waitq;  //等待隊(duì)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) {//對(duì)齊處理            size += align + 1;            size &= ~(align - 1);            }            //如果一致內(nèi)存塊比頁(yè)大，是分配為一致內(nèi)存塊大小，否則，分配為頁(yè)大小            if (allocation == 0) {            if (PAGE_SIZE < size)//頁(yè)比一致內(nèi)存塊小            allocation = size;            else            allocation = PAGE_SIZE;//頁(yè)大小            // FIXME: round up for less fragmentation            } else if (allocation < size)            return NULL;            //分配dma_pool結(jié)構(gòu)對(duì)象空間            if (!(retval = kmalloc (sizeof *retval, SLAB_KERNEL)))            return retval;                         strlcpy (retval->name, name, sizeof retval->name);                         retval->dev = dev;            //初始化dma_pool結(jié)構(gòu)對(duì)象retval            INIT_LIST_HEAD (&retval->page_list);//初始化頁(yè)鏈表            spin_lock_init (&retval->lock);            retval->size = size;            retval->allocation = allocation;            retval->blocks_per_page = allocation / size;            init_waitqueue_head (&retval->waitq);//初始化等待隊(duì)列                         if (dev) {//設(shè)備存在時(shí)            down (&pools_lock);            if (list_empty (&dev->dma_pools))            //給設(shè)備創(chuàng)建sysfs文件系統(tǒng)屬性文件            device_create_file (dev, &dev_attr_pools);            /* note:  not currently insisting "name" be unique */            list_add (&retval->pools, &dev->dma_pools);　//將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 ... */            //遍歷一頁(yè)的每塊，而每塊又以32字節(jié)遞增            for (map = 0, i = 0;            i < pool->blocks_per_page;　//每頁(yè)的塊數(shù)            i += BITS_PER_LONG, map++) {　// BITS_PER_LONG定義為32            if (page->bitmap [map] == 0)            continue;            block = ffz (~ page->bitmap [map]);//找出第一個(gè)0            if ((i + block) < pool->blocks_per_page) {            clear_bit (block, &page->bitmap [map]);            //得到相對(duì)于頁(yè)邊界的偏移            offset = (BITS_PER_LONG * map) + block;            offset *= pool->size;            goto ready;            }            }            }            //給DMA池分配dma_page結(jié)構(gòu)空間，加入到pool->page_list鏈表，            //并作DMA一致映射，它包括分配給DMA池一頁(yè)。            // SLAB_ATOMIC表示調(diào)用 kmalloc（GFP_ATOMIC） 直到失敗為止，            //然后它等待內(nèi)核釋放若干頁(yè)面，接下來(lái)再一次進(jìn)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;　//返回虛擬地址            *handle = offset + page->dma;　//相對(duì)DMA地址            #ifdef	CONFIG_DEBUG_SLAB            memset (retval, POOL_POISON_ALLOCATED, pool->size);            #endif            done:            spin_unlock_irqrestore (&pool->lock, flags);            return retval;            }