nvme_queue_rq函數用於提交一個req到nvme submission queue中。
1，nvme_setup_cmd用於構造nvme cmd：
nvme_setup_rw函數用於設置nvme讀寫命令的基礎字段：
2，nvme_map_data函數爲req的bio中所有vector建立dma散列表映射
老的磁盤控制器僅僅支持“簡單”的DMA方式：磁盤必須與RAM中連續的內存單元傳送數據。但是新的磁盤控制器支持分散聚集（scatter-gather）DMA方式，磁盤可以與一些非連續的內存區域相互傳送數據。爲了支持分散聚集DMA方式，塊設備驅動程序必須能夠處理稱爲段的數據存儲單元，一次分散聚集DMA可以傳送幾個段（nr_phys_segments），一個段就是一個內存頁或者內存頁的一部分，它們包含磁盤上物理相鄰的數據塊。如果不同的段在RAM中相應的頁框正好是連續的並且在磁盤上相應的數據也是相鄰的，那麼通用塊層就可以進行合併，這種合併方式產生的更大的內存區域就稱爲物理段。當然nvme設置是支持這種DMA傳送方式的。
https://lwn.net/Articles/256368/ 中關於散列表的描述：
Scatter/gather I/O allows the system to perform DMA I/O operations on buffers which are scattered throughout physical memory. Consider, for example, the case of a large (multi-page) buffer created in user space. The application sees a continuous range of virtual addresses, but the physical pages behind those addresses will almost certainly not be adjacent to each other. If that buffer is to be written to a device in a single I/O operation, one of two things must be done: (1) the data must be copied into a physically-contiguous buffer, or (2) the device must be able to work with a list of physical addresses and lengths, grabbing the right amount of data from each segment. Scatter/gather I/O, by eliminating the need to copy data into contiguous buffers, can greatly increase the efficiency of I/O operations while simultaneously getting around the problem that the creation of large, physically-contiguous buffers can be problematic in the first place.
塊設備的request請求中所能攜帶的最大的段數，由下面的函數設置：
blk_rq_map_sq函數是核心的dma映射函數：
散列表sg的結構：
__blk_bvec_map_sq函數用於映射單個bio sector：
__blk_bios_map_sq函數爲req中每個bio所有的vector創建dma映射：
3，nvme_pci_setup_prps函數設置nvme cmd中prp字段
在設置prp之前，會判斷是用sgl還是prp：
設置prp字段：
4，提交nvme cmd
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