MTD的基本介紹可以參考MTD - Memory Technology Devices，這篇文章要透過drivers/mtd/devices/mtdram.c來了解mtd的driver如何運作。
static unsigned long total_size = CONFIG_MTDRAM_TOTAL_SIZE;
#define MTDRAM_TOTAL_SIZE (total_size * 1024)
#ifdef MODULE
module_param(total_size, ulong, 0);
MODULE_PARM_DESC(total_size, "Total device size in KiB");
#endif
static int __init init_mtdram(void)
{
void *addr;
int err;
if (!total_size)
return -EINVAL;
/* Allocate some memory */
mtd_info = kmalloc(sizeof(struct mtd_info), GFP_KERNEL);
if (!mtd_info)
return -ENOMEM;
addr = vmalloc(MTDRAM_TOTAL_SIZE);
if (!addr) {
kfree(mtd_info);
mtd_info = NULL;
return -ENOMEM;
}
err = mtdram_init_device(mtd_info, addr,
MTDRAM_TOTAL_SIZE, "mtdram test device");
if (err) {
vfree(addr);
kfree(mtd_info);
mtd_info = NULL;
return err;
}
memset(mtd_info->priv, 0xff, MTDRAM_TOTAL_SIZE);
return err;
}
首先，看到init_mtdram()，在該function中，我們分配一塊記憶體給mtd_info，以及一塊v-memory給稍候模擬的flash用，接著就呼叫mtdram_init_device()進行mtd的註冊動作，（一個mtd partition需要一個mtd_info來存放所需的資訊），init_mtdram()後面就將mtd_info->priv（即v-memory）的內容全部設成0xff，這是因為一個空的flash裡面預設就是0xff。
int mtdram_init_device(struct mtd_info *mtd, void *mapped_address,
unsigned long size, char *name)
{
memset(mtd, 0, sizeof(*mtd));
/* Setup the MTD structure */
mtd->name = name;
mtd->type = MTD_RAM;
mtd->flags = MTD_CAP_RAM;
mtd->size = size;
mtd->writesize = 1;
mtd->erasesize = MTDRAM_ERASE_SIZE;
mtd->priv = mapped_address;
mtd->owner = THIS_MODULE;
mtd->erase = ram_erase;
mtd->point = ram_point;
mtd->unpoint = ram_unpoint;
mtd->get_unmapped_area = ram_get_unmapped_area;
mtd->read = ram_read;
mtd->write = ram_write;
if (add_mtd_device(mtd)) {
return -EIO;
}
return 0;
}
在mtdram_init_device()主要是填mtd_info相關資訊，然後呼叫add_mtd_device()進行註冊mtd的動作。呼叫del_mtd_device()進行移除mtd的工作。point()/unpoint()可以參考http://www.linux-mtd.infradead.org/faq/general.html#L_point。
#ifdef MODULE
module_param(total_size, ulong, 0);
MODULE_PARM_DESC(total_size, "Total device size in KiB");
module_param(erase_size, ulong, 0);
MODULE_PARM_DESC(erase_size, "Device erase block size in KiB");
#endif
// We could store these in the mtd structure, but we only support 1 device.
static struct mtd_info *mtd_info;
static int ram_erase(struct mtd_info *mtd, struct erase_info *instr)
{
if (instr->addr + instr->len > mtd->size)
return -EINVAL;
memset((char *)mtd->priv + instr->addr, 0xff, instr->len);
instr->state = MTD_ERASE_DONE;
mtd_erase_callback(instr);
return 0;
}
static int ram_point(struct mtd_info *mtd, loff_t from, size_t len,
size_t *retlen, void **virt, resource_size_t *phys)
{
if (from + len > mtd->size)
return -EINVAL;
/* can we return a physical address with this driver? */
if (phys)
return -EINVAL;
*virt = mtd->priv + from;
*retlen = len;
return 0;
}
static void ram_unpoint(struct mtd_info *mtd, loff_t from, size_t len)
{
}
/*
* Allow NOMMU mmap() to directly map the device (if not NULL)
* - return the address to which the offset maps
* - return -ENOSYS to indicate refusal to do the mapping
*/
static unsigned long ram_get_unmapped_area(struct mtd_info *mtd,
unsigned long len,
unsigned long offset,
unsigned long flags)
{
return (unsigned long) mtd->priv + offset;
}
static int ram_read(struct mtd_info *mtd, loff_t from, size_t len,
size_t *retlen, u_char *buf)
{
if (from + len > mtd->size)
return -EINVAL;
memcpy(buf, mtd->priv + from, len);
*retlen = len;
return 0;
}
static int ram_write(struct mtd_info *mtd, loff_t to, size_t len,
size_t *retlen, const u_char *buf)
{
if (to + len > mtd->size)
return -EINVAL;
memcpy((char *)mtd->priv + to, buf, len);
*retlen = len;
return 0;
}
剩下的read()/write()/erase()都是copy from/to memory和清成0xff，所以您可以發現讀寫NOR flash和讀寫memory差不多。
