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Linux块设备驱动程序原理
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顧名思義，塊設備驅動程序就是支持以塊的方式進行讀寫的設備。塊設備和字符設備最大的區別在于讀寫數據的基本單元不同。塊設備讀寫數據的基本單元為塊，例如磁盤通常為一個sector，而字符設備的基本單元為字節。從實現角度來看，字符設備的實現比較簡單，內核例程和用戶態API一一對應，這種映射關系由字符設備的file_operations維護。塊設備接口則相對復雜，讀寫API沒有直接到塊設備層，而是直接到文件系統層，然后再由文件系統層發起讀寫請求。
block_device結構代表了內核中的一個塊設備。它可以表示整個磁盤或一個特定的分區。當這個結構代表一個分區時，它的bd_contains成員指向包含這個分區的設備，bd_part成員指向設備的分區結構。當這個結構代表一個塊設備時，bd_disk成員指向設備的gendisk結構。
struct?block_device?{ ?????dev_t???????????bd_dev; ?????struct?inode?*??bd_inode;???/*分區結點*/ ?????int?????????bd_openers; ?????struct?semaphore????bd_sem;?/*打開/關閉鎖*/ ?????struct?semaphore????bd_mount_sem;???/*?加載互斥鎖*/ ?????struct?list_head????bd_inodes; ?????void?*??????bd_holder; ?????int?????????bd_holders; ?????struct?block_device?*???bd_contains; ?????unsigned????????bd_block_size;//分區塊大小 ?????struct?hd_struct?*??bd_part; ?????unsigned????????bd_part_count;//打開次數 ?????int?????????bd_invalidated; ?????struct?gendisk?*????bd_disk; ?????struct?list_head????bd_list; ?????struct?backing_dev_info?*bd_inode_backing_dev_info; ?????unsigned?long???bd_private; ?}; ?
gendisk是一個單獨的磁盤驅動器的內核表示。內核還使用gendisk來表示分區。
struct?gendisk?{ ?????int?major;??????????//主設備號 ?????int?first_minor;??? ?????int?minors;?????????//最大的次設備號數量，如果設備不能分區，該值為1????????????????????????????????? ?????char?disk_name[32];?//主設備名 ?????struct?hd_struct?**part;????//分區信息，有minors個 ?????struct?block_device_operations?*fops;//設備操作 ?????struct?request_queue?*queue;????//設備管理I/O請求 ?????void?*private_data; ?????sector_t?capacity; ?????int?flags; ?????char?devfs_name[64]; ?????int?number; ?????struct?device?*driverfs_dev; ?????struct?kobject?kobj; ?????struct?timer_rand_state?*random; ?????int?policy; ?????atomic_t?sync_io;??? ?????unsigned?long?stamp,?stamp_idle; ?????int?in_flight; ?#ifdef??CONFIG_SMP ?????struct?disk_stats?*dkstats; ?#else ?????struct?disk_stats?dkstats; ?#endif ?}; ?
gendisk結構的操作函數包括以下幾個：
struct?gendisk?*alloc_disk(int?minors);?????//分配磁盤 ?void?add_disk(struct?gendisk?*disk);????????//增加磁盤信息 ?void?unlink_gendisk(struct?gendisk?*disk)???//刪除磁盤信息 ?void?delete_partition(struct?gendisk?*disk,?int?part)；??//刪除分區 ?void?add_partition(struct?gendisk?*disk,?int?part,?sector_t?start,?sector_t?len,?int?flags)；//添加分區 ?block_device_operations結構是塊設備對應的操作接口，是連接抽象的塊設備操作與具體塊設備操作之間的樞紐。
struct?block_device_operations?{ ?????int?(*open)?(struct?inode?*,?struct?file?*); ?????int?(*release)?(struct?inode?*,?struct?file?*); ?????int?(*ioctl)?(struct?inode?*,?struct?file?*,?unsigned,?unsigned?long); ?????long?(*unlocked_ioctl)?(struct?file?*,?unsigned,?unsigned?long); ?????long?(*compat_ioctl)?(struct?file?*,?unsigned,?unsigned?long); ?????int?(*direct_access)?(struct?block_device?*,?sector_t,?unsigned?long?*); ?????int?(*media_changed)?(struct?gendisk?*); ?????int?(*revalidate_disk)?(struct?gendisk?*); ?????int?(*getgeo)(struct?block_device?*,?struct?hd_geometry?*); ?????struct?module?*owner; ?}; ?block_device_operations并不能完全提供文件操作全部的API，實際上只提供了open、release等函數，其他的文件操作依賴于def_blk_fops：const?struct?file_operations?def_blk_fops?=?{ ?????.open???=?blkdev_open, ?????.release????=?blkdev_close, ?????.llseek?=?block_llseek, ?????.read???????=?do_sync_read, ?????.write??=?do_sync_write, ?????.aio_read???=?generic_file_aio_read, ?????.aio_write=?generic_file_aio_write_nolock, ?????.mmap???=?generic_file_mmap, ?????.fsync??=?block_fsync, ?????.unlocked_ioctl?=?block_ioctl, ?#ifdef?CONFIG_COMPAT ?????.compat_ioctl???=?compat_blkdev_ioctl, ?#endif ?????.splice_read????????=?generic_file_splice_read, ?????.splice_write???=?generic_file_splice_write, ?}; ?系統對塊設備進行讀寫操作時，通過塊設備通用的讀寫操作函數將一個請求保存在該設備的操作請求隊列（request queue）中，然后調用這個塊設備的底層處理函數，對請求隊列中的操作請求進行逐一執行。request_queue結構描述了塊設備的請求隊列，該結構定義如下：struct?request_queue ?{ ?????struct?list_head????queue_head; ?????struct?request??????*last_merge; ?????elevator_t??????elevator; ?????/*請求隊列列表*/ ?????struct?request_list?????rq; ?????request_fn_proc?????*request_fn; ?????merge_request_fn????*back_merge_fn; ?????merge_request_fn????*front_merge_fn; ?????merge_requests_fn???*merge_requests_fn; ?????make_request_fn?????*make_request_fn; ?????prep_rq_fn??????????*prep_rq_fn; ?????unplug_fn???????????*unplug_fn; ?????merge_bvec_fn???????*merge_bvec_fn; ?????activity_fn?????????*activity_fn; ?????/*自動卸載狀態*/ ?????struct?timer_list???unplug_timer; ?????int?????????unplug_thresh;?? ?????unsigned?long???????unplug_delay;???/*自動卸載延時*/ ?????struct?work_struct??unplug_work; ?????struct?backing_dev_info?backing_dev_info; ?????void????????????????*queuedata; ?????void????????????????*activity_data; ?????unsigned?long???????bounce_pfn; ?????int?????????????bounce_gfp; ?????unsigned?long???????queue_flags;//各種隊列標志 ?????/*保護隊列結構，避免重入*/ ?????spinlock_t??????????*queue_lock; ?????/*?請求的核心結構*/ ?????struct?kobject?kobj; ?????/*請求的配置*/ ?????unsigned?long???????nr_requests;????/*?請求的最大數*/ ?????unsigned?int????????nr_congestion_on; ?????unsigned?int????????nr_congestion_off; ?????unsigned?short??????max_sectors; ?????unsigned?short??????max_phys_segments; ?????unsigned?short??????max_hw_segments; ?????unsigned?short??????hardsect_size; ?????unsigned?int????????max_segment_size; ?????unsigned?long???????seg_boundary_mask; ?????unsigned?int????????dma_alignment; ?????struct?blk_queue_tag????*queue_tags; ?????atomic_t????????refcnt; ?????unsigned?int????????in_flight; ?????/*sg?參數配置*/ ?????unsigned?int????????sg_timeout; ?????unsigned?int????????sg_reserved_size; ?}; ?請求隊列相關的處理函數包括：//創建隊列時提供了一個自旋鎖。 ?request_queue_t?*blk_init_queue(request_fn_proc?*rfn,?spinlock_t?*lock)； ?//獲得隊列中第一個未完成的請求。 ?struct?request?*elv_next_request(request_queue_t?*q)； ?void?end_request(struct?request?*req,?int?uptodate)；//請求完成 ?void?blk_stop_queue(request_queue_t?*queue);?//停止請求 ?void?blk_start_queue(request_queue_t?*queue);?//開始請求 ?void?blk_cleanup_queue(request_queue_t?*);//清除請求隊列 ?簡單的塊設備驅動程序實例
向內核注冊和注銷一個塊設備可使用如下函數：
int?register_blkdev(unsigned?int?major,?const?char?*name)； ?int?unregister_blkdev(unsigned?int?major,?const?char?*name)；?例1.10? 簡單的塊設備驅動程序實例
代碼見光盤\src\1drivermodel\1-10block。核心代碼如下所示：
static?struct?request_queue?*Queue; ?//自定義塊設備結構 ?static?struct?simpleblockdevice? ?{ ?????unsigned?long?size; ?????spinlock_t?lock; ?????u8?*data; ?????struct?gendisk?*gd; ?}?Device; ?//處理I/O請求 ?static?void?simpleblocktransfer(struct?simpleblockdevice?*dev,?unsigned?long?sector, ?????????????????????????????????unsigned?long?nsect,?char?*buffer,?int?write) ?{ ?????unsigned?long?offset?=?sector*hardsect_size; ?????unsigned?long?nbytes?=?nsect*hardsect_size; ?????//判斷I/O請求是否超出范圍 ?????if?((offset?+?nbytes)?>?dev->size) ?????{ ?????????printk?(KERN_NOTICE?"sbd:?Beyond-end?write?(%ld?%ld)\n",?offset,?nbytes); ?????????return; ?????} ?????if?(write) ?????????memcpy(dev->data?+?offset,?buffer,?nbytes); ?????else ?????????memcpy(buffer,?dev->data?+?offset,?nbytes); ?} ?//簡單請求處理 ?static?void?simpleblockrequest(struct?request_queue?*q) ?{ ?????struct?request?*req; ?????//獲取下一個請求 ?????while?((req?=?elv_next_request(q))?!=?NULL)? ?????{ ?????????if?(!?blk_fs_request(req))? ?????????{ ?????????????printk?(KERN_NOTICE?"Skip?non-CMD?request\n"); ?????????????end_request(req,?0); ?????????????continue; ?????????} ?????????simpleblocktransfer(&Device,?req->sector,?req->current_nr_sectors, ?????????????req->buffer,?rq_data_dir(req)); ?????????end_request(req,?1); ?????} ?} ?//簡單的塊設備ioctl函數 ?int?simpleblockioctl?(struct?inode?*inode,?struct?file?*filp,unsigned?int?cmd,?unsigned?long?arg) ?{ ?????long?size; ?????struct?hd_geometry?geo; ?????switch(cmd)? ?????{ ?????//獲取磁盤信息 ?????case?HDIO_GETGEO: ?????????size?=?Device.size*(hardsect_size/KERNEL_SECTOR_SIZE); ?????????geo.cylinders?=?(size?&?~0x3f)?>>?6; ?????????geo.heads?=?4; ?????????geo.sectors?=?16; ?????????geo.start?=?4; ?????????if?(copy_to_user((void?*)?arg,?&geo,?sizeof(geo))) ?????????????return?-EFAULT; ?????????return?0; ?????} ?????return?-ENOTTY;?/*?未知命令?*/ ?} ?//設備操作結構 ?static?struct?block_device_operations?simpleblockops?=?{ ?????.owner???????????=?THIS_MODULE, ?????.ioctl???????=?simpleblockioctl?}; ?static?int?__init?simpleblockinit(void) ?{ ?????Device.size?=?nsectors*hardsect_size; ?????spin_lock_init(&Device.lock); ?????Device.data?=?vmalloc(Device.size); ?????if?(Device.data?==?NULL) ?????????return?-ENOMEM; ?????//初始化請求隊列，配置處理函數為sbd_request ?????Queue?=?blk_init_queue(simpleblockrequest,?&Device.lock); ?????if?(Queue?==?NULL) ?????????goto?out; ?????blk_queue_hardsect_size(Queue,?hardsect_size); ?????//注冊塊設備 ?????major_num?=?register_blkdev(major_num,?"sbd"); ?????if?(major_num?<=?0)?{ ?????????printk(KERN_WARNING?"sbd:?unable?to?get?major?number\n"); ?????????goto?out; ?????} ?????Device.gd?=?alloc_disk(16); ?????if?(!?Device.gd) ?????????goto?out_unregister; ?????Device.gd->major?=?major_num; ?????Device.gd->first_minor?=?0; ?????Device.gd->fops?=?&simpleblockops; ?????Device.gd->private_data?=?&Device; ?????strcpy?(Device.gd->disk_name,?"sbd0"); ?????//配置容量 ?????set_capacity(Device.gd,?nsectors*(hardsect_size/KERNEL_SECTOR_SIZE)); ?????Device.gd->queue?=?Queue; ?????add_disk(Device.gd); ?????return?0; ?out_unregister: ?????unregister_blkdev(major_num,?"sbd"); ?out: ?????vfree(Device.data); ?????return?-ENOMEM; ?} ?static?void?__exit?simpleblockexit(void) ?{ ?????del_gendisk(Device.gd); ?????put_disk(Device.gd); ?????unregister_blkdev(major_num,?"sbd"); ?????blk_cleanup_queue(Queue); ?????vfree(Device.data); ?} ?module_init(simpleblockinit); ?module_exit(simpleblockexit); ?運行結果如下：[root@/home]#cat?/proc/filesystems ?nodev???sysfs ?nodev???rootfs ?nodev???bdev ?nodev???proc ?nodev???binfmt_misc ?nodev???debugfs ?nodev???securityfs ?nodev???sockfs ?nodev???usbfs ?nodev???pipefs ?nodev???anon_inodefs ?nodev???futexfs ?nodev???tmpfs ?nodev???inotifyfs ?????????ext3 ?????????cramfs ?nodev???ramfs ?????????msdos ?????????vfat ?????????iso9660 ?nodev???nfs ?nodev???nfs4 ?nodev???mqueue ?nodev???rpc_pipefs ?[root@/home]#insmod?demo.ko ??sbd0:?unknown?partition?table ?[root@/home]#mknod?/dev/sbd?b?253?0 ?[root@/home]#./mkfs.ext3?/dev/sbd ?mke2fs?1.40.9?(27-Apr-2008) ?Filesystem?label= ?OS?type:?Linux ?Block?size=1024?(log=0) ?Fragment?size=1024?(log=0) ?1280?inodes,?5120?blocks ?256?blocks?(5.00%)?reserved?for?the?super?user ?First?data?block=1?Maximum?filesystem?blocks=5242880?1?block?group ?8192?blocks?per?group,?8192?fragments?per?group ?1280?inodes?per?group ??Writing?inode?tables:?done ?Creating?journal?(1024?blocks):?done ?Writing?superblocks?and?filesystem?accounting?information:?done ??This?filesystem?will?be?automatically?checked?every?39?mounts?or ?180?days,?whichever?comes?first.??Use?tune2fs?-c?or?-i?to?override. ?[root@/home]#mount?-t?ext3?/dev/sbd?/mnt/u ?kjournald?starting.??Commit?interval?5?seconds ?EXT3?FS?on?sbd0,?internal?journal ?EXT3-fs:?mounted?filesystem?with?ordered?data?mode. ?[root@/home]#df ?Filesystem???????????1k-blocks??????Used?Available?Use%?Mounted?on ?rootfs?????????????????2063504???1191136????767548??61%?/ ?/dev/root??????????????2063504???1191136????767548??61%?/ ?/dev/sbd??????????????????4955??????1063??????3636??23%?/mnt/u ?[root@/home]#cd?/mnt/u ?[root@/mnt/u]#ls ?lost+found ?
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