how to write nand driver

In Linux , the upper layer will not access nand driver directly . The upper File System and MTD utility will access MTD layer , and MTD layer will access your nand driver .

           FS   or  MTD util

 ------------------------------------

            MTD Layer

  -------------------------------

          Nand driver

MTD can support Nand and NOR Flash , For Nand , the source code are placed in drivers\mtd\nand , and the important files are nand_base.c,nand_ecc.c,nand_bbt.c . They are the skeleton of nand driver.

 nand_bbt.c is a file for bad block management and nand_ecc.c is used to calculate the ECC and correct the data .

nand_base.c is the most important file , it implements the struct mtd_info . All the upper layers will use the mtd_info variables to do something .

  Let's have an insight of the struct mtd_info which is defined in mtd.h.

oobsize: Amount of OOB data per block

writesize: chunk (page) size

erasesize: block size

size:   the nand chip size , for only one chip , size is equal to nand->chipsize

The above variables are related to the nand flash type . they are attributes of nand flash.  In the function nand_get_flash_type , it emits NAND_CMD_READID command and then read_byte to get the attributes .

oobavail: In the nand spare area , some bytes are used for bad block flags , some bytes are used for SW or HW ECC , and other bytes are reserved for upper layer, for example , yaffs will use these bytes to store FS information . the reserved bytes size is oobavail .  It is related to the chip ecc.layout . It will calculate in the function nand_scan_tail .

erase :  the nand flash erase function . it will call nand_erase_nand

read : the read function ,nand_read

write: the write function , nand_write

read_oob:nand_read_oob, it will read out-of-band data with or without data

write_oob:nand_write_oob,it will write  out-of-band data with or without data

sync:nand_sync,wait for chip ready

block_isbad: check the block is bad or not , if the chip->bbt (bad block table) is not created , it will call chip->block_bad function , otherwise , it will use bbt to confirm the block is bad or not . the default function is nand_block_isbad .

block_markbad:nand_block_markbad, mark  the block is bad . If the block is already marked bad , it will do nothing ,otherwise , it will call :chip->block_markbad.

Let's parse the flow :

1) first, nand_scan will be called .  nand_scan will call nand_scan_ident , which will set the default chip function and get the flash type , according to the flash type , the other chip attributes can be defined such as writesize , oobsize

2) nand_scan_tail will set the default chip ecc function according to the ECC mode , calculate the oobavail , ecc.steps and then set the mtd function

3) the mtd functions are nand_erase , nand_read , nand_write , nand_read_oob , nand_write_oob , nand_sync , nand_suspend , nand_resume,

nand_block_isbad , nand_block_markbad

4) call the  chip scan_bbt function to build up the bad block table

which will call the chip read_oob .

there are several functions need to describle .

nand_get_device:

nand_release_device:

Multi process will access the nand driver , but only one process can access it at the same time . nand_get_device will check the chip state , if it is not ready , the process will add itself into wait queue and shedule .  if the chip state is ready now , the process will change the chip state to new_state (so other process can't access the nand ) and return .

nand_release_device change the chip state to READY and then wake up the wait queue .

nand_check_wp:  check if the device is write protected .

Now We will parse the mtd nand functions one by one .

nand_erase -> nand_erase_nand

  1 nand_get_device to get the device

  2 select_chip to enable the CE pin of NAND

  3 nand_check_wp to check it is write proected or not

  4 enter into the loop

      check whether the block is bad , if it is bad , failed and exit

      chip->erase_cmd to erase the block

      chip_waitfunc to wait the operation finished

      according to the status from waitfunc function , if failed ,exit

      add the page to erase next block

   5 release the device

   6 return