Most NAND chips mark the bad blocks at a defined position in the spare area. Those blocks must not be erased under any circumstances as the bad block information would be lost. It is possible to check the bad block mark each time when the blocks are accessed by reading the spare area of the first page in the block. This is time consuming so a bad block table is used.
The nand driver supports various types of bad block tables.
The bad block table contains all bad block information of the device which can consist of multiple chips.
A bad block table is used per chip and contains the bad block information for this particular chip.
The bad block table is located at a fixed offset in the chip (device). This applies to various DiskOnChip devices.
The bad block table is automatically placed and detected either at the end or at the beginning of a chip (device)
The bad block table is mirrored on the chip (device) to allow updates of the bad block table without data loss.
nand_scan() calls the function nand_default_bbt(). nand_default_bbt() selects appropriate default bad block table descriptors depending on the chip information which was retrieved by nand_scan().
The standard policy is scanning the device for bad blocks and build a ram based bad block table which allows faster access than always checking the bad block information on the flash chip itself.
It may be desired or necessary to keep a bad block table in FLASH. For AG-AND chips this is mandatory, as they have no factory marked bad blocks. They have factory marked good blocks. The marker pattern is erased when the block is erased to be reused. So in case of powerloss before writing the pattern back to the chip this block would be lost and added to the bad blocks. Therefore we scan the chip(s) when we detect them the first time for good blocks and store this information in a bad block table before erasing any of the blocks.
The blocks in which the tables are stored are protected against accidental access by marking them bad in the memory bad block table. The bad block table management functions are allowed to circumvent this protection.
The simplest way to activate the FLASH based bad block table support is to set the option NAND_BBT_USE_FLASH in the bbt_option field of the nand chip structure before calling nand_scan(). For AG-AND chips is this done by default. This activates the default FLASH based bad block table functionality of the NAND driver. The default bad block table options are
Store bad block table per chip
Use 2 bits per block
Automatic placement at the end of the chip
Use mirrored tables with version numbers
Reserve 4 blocks at the end of the chip
User defined tables are created by filling out a nand_bbt_descr structure and storing the pointer in the nand_chip structure member bbt_td before calling nand_scan(). If a mirror table is necessary a second structure must be created and a pointer to this structure must be stored in bbt_md inside the nand_chip structure. If the bbt_md member is set to NULL then only the main table is used and no scan for the mirrored table is performed.
The most important field in the nand_bbt_descr structure is the options field. The options define most of the table properties. Use the predefined constants from nand.h to define the options.
Number of bits per block
The supported number of bits is 1, 2, 4, 8.
Table per chip
Setting the constant NAND_BBT_PERCHIP selects that a bad block table is managed for each chip in a chip array. If this option is not set then a per device bad block table is used.
Table location is absolute
Use the option constant NAND_BBT_ABSPAGE and define the absolute page number where the bad block table starts in the field pages. If you have selected bad block tables per chip and you have a multi chip array then the start page must be given for each chip in the chip array. Note: there is no scan for a table ident pattern performed, so the fields pattern, veroffs, offs, len can be left uninitialized
Table location is automatically detected
The table can either be located in the first or the last good blocks of the chip (device). Set NAND_BBT_LASTBLOCK to place the bad block table at the end of the chip (device). The bad block tables are marked and identified by a pattern which is stored in the spare area of the first page in the block which holds the bad block table. Store a pointer to the pattern in the pattern field. Further the length of the pattern has to be stored in len and the offset in the spare area must be given in the offs member of the nand_bbt_descr structure. For mirrored bad block tables different patterns are mandatory.
Set the option NAND_BBT_CREATE to enable the table creation if no table can be found during the scan. Usually this is done only once if a new chip is found.
Table write support
Set the option NAND_BBT_WRITE to enable the table write support. This allows the update of the bad block table(s) in case a block has to be marked bad due to wear. The MTD interface function block_markbad is calling the update function of the bad block table. If the write support is enabled then the table is updated on FLASH.
Note: Write support should only be enabled for mirrored tables with version control.
Table version control
Set the option NAND_BBT_VERSION to enable the table version control. It's highly recommended to enable this for mirrored tables with write support. It makes sure that the risk of losing the bad block table information is reduced to the loss of the information about the one worn out block which should be marked bad. The version is stored in 4 consecutive bytes in the spare area of the device. The position of the version number is defined by the member veroffs in the bad block table descriptor.
Save block contents on write
In case that the block which holds the bad block table does contain other useful information, set the option NAND_BBT_SAVECONTENT. When the bad block table is written then the whole block is read the bad block table is updated and the block is erased and everything is written back. If this option is not set only the bad block table is written and everything else in the block is ignored and erased.
Number of reserved blocks
For automatic placement some blocks must be reserved for bad block table storage. The number of reserved blocks is defined in the maxblocks member of the bad block table description structure. Reserving 4 blocks for mirrored tables should be a reasonable number. This also limits the number of blocks which are scanned for the bad block table ident pattern.