at91: atmel_nand: Update driver to support Programmable Multibit ECC controller

[oweals/u-boot.git] / doc / README.atmel_pmecc

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+How to enable PMECC(Programmable Multibit ECC) for nand on Atmel SoCs
+-----------------------------------------------------------
+2012-08-22 Josh Wu <josh.wu@atmel.com>
+
+The Programmable Multibit ECC (PMECC) controller is a programmable binary
+BCH(Bose, Chaudhuri and Hocquenghem) encoder and decoder. This controller
+can be used to support both SLC and MLC NAND Flash devices. It supports to
+generate ECC to correct 2, 4, 8, 12 or 24 bits of error per sector (512 or
+1024 bytes) of data.
+
+Following Atmel AT91 products support PMECC.
+- AT91SAM9X25, X35, G25, G15, G35 (tested)
+- AT91SAM9N12 (not tested, Should work)
+
+As soon as your nand flash software ECC works, you can enable PMECC.
+
+To use PMECC in this driver, the user needs to set:
+       1. the PMECC correction error bits capability: CONFIG_PMECC_CAP.
+          It can be 2, 4, 8, 12 or 24.
+       2. The PMECC sector size: CONFIG_PMECC_SECTOR_SIZE.
+          It only can be 512 or 1024.
+       3. The PMECC index lookup table's offsets in ROM code: CONFIG_PMECC_INDEX_TABLE_OFFSET.
+          In the chip datasheet section "Boot Stragegies", you can find
+          two Galois Field Table in the ROM code. One table is for 512-bytes
+          sector. Another is for 1024-byte sector. Each Galois Field includes
+          two sub-table: indext table & alpha table.
+          In the beginning of each Galois Field Table is the index table,
+          Alpha table is in the following.
+          So the index table's offset is same as the Galois Field Table.
+
+          Please set CONFIG_PMECC_INDEX_TABLE_OFFSET correctly according the
+          Galois Field Table's offset base on the sector size you used.
+
+Take AT91SAM9X5EK as an example, the board definition file likes:
+
+/* PMECC & PMERRLOC */
+#define CONFIG_ATMEL_NAND_HWECC                1
+#define CONFIG_ATMEL_NAND_HW_PMECC     1
+#define CONFIG_PMECC_CAP               2
+#define CONFIG_PMECC_SECTOR_SIZE       512
+#define CONFIG_PMECC_INDEX_TABLE_OFFSET        0x8000
+
+NOTE: If you use 1024 as the sector size, then need set 0x10000 as the
+ CONFIG_PMECC_INDEX_TABLE_OFFSET