linux驱动子系统--MTD

MTD是各种类型存储设备的抽象，是介于高层抽象和底层物理硬件之间的桥梁，它屏蔽了底层硬件的技术细节，对上层模块提供无差别的访问控制。分析MTD部分的代码就不难发现，MTD采用3层设计思路，最上面一层用于和块设备层对接、中间层是MTD实现，最底层是硬件驱动层。

MTD下层注册接口为mtd_device_register，现有支持MTD架构的设备类型包括：CFI、JEDEC、absent（假设备）、RAM、ROM、LPDDR接口设备、NAND设备。

MTD层向块设备层注册的接口为register_mtd_blktrans，现在支持的包括Flash Translation Layer, used on PCMCIA devices、Inverse Flash Translation Layer等；

 

既然MTD是整个架构的核心，那么首先来看struct mtd_info这个结构体；

struct mtd_info {
	u_char type;
	uint32_t flags;
	uint64_t size;	 // Total size of the MTD

	/* "Major" erase size for the device. Naïve users may take this
	 * to be the only erase size available, or may use the more detailed
	 * information below if they desire
	 */
	uint32_t erasesize;
	/* Minimal writable flash unit size. In case of NOR flash it is 1 (even
	 * though individual bits can be cleared), in case of NAND flash it is
	 * one NAND page (or half, or one-fourths of it), in case of ECC-ed NOR
	 * it is of ECC block size, etc. It is illegal to have writesize = 0.
	 * Any driver registering a struct mtd_info must ensure a writesize of
	 * 1 or larger.
	 */
	uint32_t writesize;

	/*
	 * Size of the write buffer used by the MTD. MTD devices having a write
	 * buffer can write multiple writesize chunks at a time. E.g. while
	 * writing 4 * writesize bytes to a device with 2 * writesize bytes
	 * buffer the MTD driver can (but doesn't have to) do 2 writesize
	 * operations, but not 4. Currently, all NANDs have writebufsize
	 * equivalent to writesize (NAND page size). Some NOR flashes do have
	 * writebufsize greater than writesize.
	 */
	uint32_t writebufsize;

	uint32_t oobsize;   // Amount of OOB data per block (e.g. 16)
	uint32_t oobavail;  // Available OOB bytes per block

	/*
	 * If erasesize is a power of 2 then the shift is stored in
	 * erasesize_shift otherwise erasesize_shift is zero. Ditto writesize.
	 */
	unsigned int erasesize_shift;
	unsigned int writesize_shift;
	/* Masks based on erasesize_shift and writesize_shift */
	unsigned int erasesize_mask;
	unsigned int writesize_mask;

	// Kernel-only stuff starts here.
	const char *name;
	int index;

	/* ECC layout structure pointer - read only! */
	struct nand_ecclayout *ecclayout;

	/* max number of correctible bit errors per writesize */
	unsigned int ecc_strength;

	/* Data for variable erase regions. If numeraseregions is zero,
	 * it means that the whole device has erasesize as given above.
	 */
	int numeraseregions;
	struct mtd_erase_region_info *eraseregions;

	/*
	 * Do not call via these pointers, use corresponding mtd_*()
	 * wrappers instead.
	 */
	int (*_erase) (struct mtd_info *mtd, struct erase_info *instr);
	int (*_point) (struct mtd_info *mtd, loff_t from, size_t len,
		       size_t *retlen, void **virt, resource_size_t *phys);
	int (*_unpoint) (struct mtd_info *mtd, loff_t from, size_t len);
	unsigned long (*_get_unmapped_area) (struct mtd_info *mtd,
					     unsigned long len,
					     unsigned long offset,
					     unsigned long flags);
	int (*_read) (struct mtd_info *mtd, loff_t from, size_t len,
		      size_t *retlen, u_char *buf);
	int (*_write) (struct mtd_info *mtd, loff_t to, size_t len,
		       size_t *retlen, const u_char *buf);
	int (*_panic_write) (struct mtd_info *mtd, loff_t to, size_t len,
			     size_t *retlen, const u_char *buf);
	int (*_read_oob) (struct mtd_info *mtd, loff_t from,
			  struct mtd_oob_ops *ops);
	int (*_write_oob) (struct mtd_info *mtd, loff_t to,
			   struct mtd_oob_ops *ops);
	int (*_get_fact_prot_info) (struct mtd_info *mtd, struct otp_info *buf,
				    size_t len);
	int (*_read_fact_prot_reg) (struct mtd_info *mtd, loff_t from,
				    size_t len, size_t *retlen, u_char *buf);
	int (*_get_user_prot_info) (struct mtd_info *mtd, struct otp_info *buf,
				    size_t len);
	int (*_read_user_prot_reg) (struct mtd_info *mtd, loff_t from,
				    size_t len, size_t *retlen, u_char *buf);
	int (*_write_user_prot_reg) (struct mtd_info *mtd, loff_t to,
				     size_t len, size_t *retlen, u_char *buf);
	int (*_lock_user_prot_reg) (struct mtd_info *mtd, loff_t from,
				    size_t len);
	int (*_writev) (struct mtd_info *mtd, const struct kvec *vecs,
			unsigned long count, loff_t to, size_t *retlen);
	void (*_sync) (struct mtd_info *mtd);
	int (*_lock) (struct mtd_info *mtd, loff_t ofs, uint64_t len);
	int (*_unlock) (struct mtd_info *mtd, loff_t ofs, uint64_t len);
	int (*_is_locked) (struct mtd_info *mtd, loff_t ofs, uint64_t len);
	int (*_block_isbad) (struct mtd_info *mtd, loff_t ofs);
	int (*_block_markbad) (struct mtd_info *mtd, loff_t ofs);
	int (*_suspend) (struct mtd_info *mtd);
	void (*_resume) (struct mtd_info *mtd);
	/*
	 * If the driver is something smart, like UBI, it may need to maintain
	 * its own reference counting. The below functions are only for driver.
	 */
	int (*_get_device) (struct mtd_info *mtd);
	void (*_put_device) (struct mtd_info *mtd);

	/* Backing device capabilities for this device
	 * - provides mmap capabilities
	 */
	struct backing_dev_info *backing_dev_info;

	struct notifier_block reboot_notifier;  /* default mode before reboot */

	/* ECC status information */
	struct mtd_ecc_stats ecc_stats;
	/* Subpage shift (NAND) */
	int subpage_sft;

	void *priv;

	struct module *owner;
	struct device dev;
	int usecount;
};