How to enable PMECC(Programmable Multibit ECC) for nand on Atmel SoCs
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  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.

  Take 'configs/at91sam9x5ek_nandflash_defconfig' as an example, the board
  configuration file has the following entries:

  	CONFIG_PMECC_CAP=2
  	CONFIG_PMECC_SECTOR_SIZE=512
  	CONFIG_SPL_GENERATE_ATMEL_PMECC_HEADER=y

  
  How to enable PMECC header for direct programmable boot.bin
  -----------------------------------------------------------

  2014-05-19 Andreas Bießmann <andreas@biessmann.org>

  
  The usual way to program SPL into NAND flash is to use the SAM-BA Atmel tool.
  This however is often not usable when doing field updates. To be able to
  program a SPL binary into NAND flash we need to add the PMECC header to the
  binary before. Chapter '12.4.4.1 NAND Flash Boot: NAND Flash Detection' in
  sama5d3 SoC spec (as of 03. April 2014) defines how this PMECC header has to
  look like. In order to do so we have a new image type added to mkimage to
  generate this PMECC header and integrated this into the build process of SPL.

  To enable the generation of atmel PMECC header for SPL one needs to define

  CONFIG_SPL_GENERATE_ATMEL_PMECC_HEADER. The required parameters are taken from
  board configuration and compiled into the host tools atmel_pmecc_params. This
  tool will be called in build process to parametrize mkimage for atmelimage
  type. The mkimage tool has intentionally _not_ compiled in those parameters.
  
  The mkimage image type atmelimage also set the 6'th interrupt vector to the
  correct value. This feature can also be used to setup a boot.bin for MMC boot.