SD卡與fatfs檔案系統(4)

FatFs的初始化和加載的操作是在函數auto_mount中進行的。

static
FRESULT auto_mount (	/* FR_OK(0): successful, !=0: any error occured */
	const char **path,	/* Pointer to pointer to the path name (drive number) */
	FATFS **rfs,		/* Pointer to pointer to the found file system object */
	BYTE chk_wp			/* !=0: Check media write protection for write access */
)
{
	BYTE drv, fmt, *tbl;
	DSTATUS stat;
	DWORD bootsect, fatsize, totalsect, maxclust;
	const char *p = *path;
	FATFS *fs;

	//分析路徑字元串
	/* Get drive number from the path name */
	while (*p == ' ') p++;		/* Strip leading spaces */
	drv = p[0] - '0';			/* Is there a drive number? */
	if (drv <= 9 && p[1] == ':')
		p += 2;				/* Found a drive number, get and strip it */
	else
		drv = 0;			/* No drive number is given, use drive number 0 as default */
	if (*p == '/') p++;		/* Strip heading slash */
	*path = p;				/* Return pointer to the path name */

	/* Check if the drive number is valid or not */
	if (drv >= _DRIVES) return FR_INVALID_DRIVE;	/* Is the drive number valid? */
	*rfs = fs = FatFs[drv];					/* Returen pointer to the corresponding file system object */
	if (!fs) return FR_NOT_ENABLED;			/* Is the file system object registered? */

	if (fs->fs_type) {						/* If the logical drive has been mounted */
		stat = disk_status(fs->drive);
		if (!(stat & STA_NOINIT)) {			/* and physical drive is kept initialized (has not been changed), */
#if !_FS_READONLY
			if (chk_wp && (stat & STA_PROTECT))	/* Check write protection if needed */
				return FR_WRITE_PROTECTED;
#endif
			return FR_OK;					/* The file system object is valid */
		}
	}

	/* The logical drive must be re-mounted. Following code attempts to mount the logical drive */

	memset(fs, 0, sizeof(FATFS));		/* Clean-up the file system object */
	fs->drive = LD2PD(drv);				/* Bind the logical drive and a physical drive */
	stat = disk_initialize(fs->drive);	/* Initialize low level disk I/O layer */
	if (stat & STA_NOINIT)				/* Check if the drive is ready */
		return FR_NOT_READY;
#if S_MAX_SIZ > 512						/* Get disk sector size if needed */
	if (disk_ioctl(drv, GET_SECTOR_SIZE, &SS(fs)) != RES_OK || SS(fs) > S_MAX_SIZ)
		return FR_NO_FILESYSTEM;
#endif
#if !_FS_READONLY
	if (chk_wp && (stat & STA_PROTECT))	/* Check write protection if needed */
		return FR_WRITE_PROTECTED;
#endif
	/* Search FAT partition on the drive */
	fmt = check_fs(fs, bootsect = 0);	/* Check sector 0 as an SFD format */  讀取MBR和分區表
	if (fmt == 1) {						/* Not an FAT boot record, it may be patitioned */
		/* Check a partition listed in top of the partition table */
		tbl = &fs->win[MBR_Table + LD2PT(drv) * 16];	/* Partition table */
		if (tbl[4]) {									/* Is the partition existing? */
			bootsect = LD_DWORD(&tbl[8]);				/* Partition offset in LBA */
			fmt = check_fs(fs, bootsect);				/* Check the partition */  讀取分區DBR資訊
		}
	}
	if (fmt || LD_WORD(&fs->win[BPB_BytsPerSec]) != SS(fs))	/* No valid FAT patition is found */
		return FR_NO_FILESYSTEM;

	/* Initialize the file system object */  從讀取的資料中初始化fs結構體
	fatsize = LD_WORD(&fs->win[BPB_FATSz16]);			/* Number of sectors per FAT */
	if (!fatsize) fatsize = LD_DWORD(&fs->win[BPB_FATSz32]);
	fs->sects_fat = fatsize;
	fs->n_fats = fs->win[BPB_NumFATs];					/* Number of FAT copies */
	fatsize *= fs->n_fats;								/* (Number of sectors in FAT area) */
	fs->fatbase = bootsect + LD_WORD(&fs->win[BPB_RsvdSecCnt]); /* FAT start sector (lba) */
	fs->csize = fs->win[BPB_SecPerClus];				/* Number of sectors per cluster */
	fs->n_rootdir = LD_WORD(&fs->win[BPB_RootEntCnt]);	/* Nmuber of root directory entries */
	totalsect = LD_WORD(&fs->win[BPB_TotSec16]);		/* Number of sectors on the file system */
	if (!totalsect) totalsect = LD_DWORD(&fs->win[BPB_TotSec32]);
	fs->max_clust = maxclust = (totalsect				/* max_clust = Last cluster# + 1 */
		- LD_WORD(&fs->win[BPB_RsvdSecCnt]) - fatsize - fs->n_rootdir / (SS(fs)/32)
		) / fs->csize + 2;

	fmt = FS_FAT12;										/* Determine the FAT sub type */
	if (maxclust >= 0xFF7) fmt = FS_FAT16;
	if (maxclust >= 0xFFF7) fmt = FS_FAT32;

	if (fmt == FS_FAT32)
		fs->dirbase = LD_DWORD(&fs->win[BPB_RootClus]);	/* Root directory start cluster */
	else
		fs->dirbase = fs->fatbase + fatsize;			/* Root directory start sector (lba) */
	fs->database = fs->fatbase + fatsize + fs->n_rootdir / (SS(fs)/32);	/* Data start sector (lba) */

#if !_FS_READONLY
	/* Initialize allocation information */
	fs->free_clust = 0xFFFFFFFF;
#if _USE_FSINFO
	/* Get fsinfo if needed */
	if (fmt == FS_FAT32) {   //讀取額外的FAT32資訊
		fs->fsi_sector = bootsect + LD_WORD(&fs->win[BPB_FSInfo]);
		if (disk_read(fs->drive, fs->win, fs->fsi_sector, 1) == RES_OK &&
			LD_WORD(&fs->win[BS_55AA]) == 0xAA55 &&
			LD_DWORD(&fs->win[FSI_LeadSig]) == 0x41615252 &&
			LD_DWORD(&fs->win[FSI_StrucSig]) == 0x61417272) {
			fs->last_clust = LD_DWORD(&fs->win[FSI_Nxt_Free]);
			fs->free_clust = LD_DWORD(&fs->win[FSI_Free_Count]);
		}
	}
#endif
#endif

	fs->fs_type = fmt;			/* FAT syb-type */
	fs->id = ++fsid;			/* File system mount ID */
	return FR_OK;
}
           

(1)disk_status(fs->drive);擷取裝置的狀态，判斷裝置是否存在；

(2)disk_initialize(fs->drive);它對接口做初始化；

(3)check_fs(fs, bootsect = 0);讀取MBR和分區表，判斷分區是否是FAT或FAT32；