Booting from NAND flash on i.MX6-based platforms

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Introduction

With respect to the NOR flash memories, NAND devices are known to be quite challenging with regard to the reliability. This is especially true when the NAND flash is used as the boot device. Several techniques such as wear leveling and bad block management have to be implemented to achieve an acceptable reliability.

This document provides information about the NAND device management, in order to handle it properly when it is used as the boot device on NXP i.MX6-based products.

Even though the example shown here refers to an i.MX6SoloX board, the approach is substantially the same across all the i.MX6 family.

Test bed

The test bed used in this example consists of an i.MX6SoloX-board equipped with a 512MB 8-bit asynchronous 1-bit ECC SLC NAND memory which is connected to the EIM bus of the SoC. The boot firmware image is the U-Boot binary file (mx6sx_board_nand_u-boot.imx). Its size is about 400 kB. From the point of view of MTD partitions, the boot partition is mtd0 which is 8MB.

Boot partition organization

The following image shows the organization of the boot partition.


Organization of the boot partition


The bootrom plays a major role in the boot process of any i.MX6 SoC. In case the NAND memory is the boot devices, this implies that the boot partition must be organized in order fulfill the bootrom requirements. Specifically, it contains:

  • A 1MB area (named Flash Control Block, FCB) which, in turn, includes
    • A data structure called NAND Control Block (NCB)
    • Three addresses which indicate where are located
      • The Discovered Bad Block Table (DBBT) which is the data structure used to manage the bad blocks of the NAND flash
      • The first copy of the firmware to load
      • The second (redundant) copy of the firmware to load
  • The DBBT
  • The first copy of the firmware
  • The second copy of the firmware.

For more details about the bootrom e the NAND boot process, please refer to the System Boot chapter of the Reference Manual of the specific i.MX6 Application Processor.

The following section will describe how to burn the boot partition in practice.

How to burn the boot partition

The burning of the boot partition is performed by the kobs-ng tool.

When the NAND flash is not burned yet, the product is usually configured in order to boot from a different device such as an SD card or through serial download mode. Once the Linux kernel is up and running, the kobs-ng can be run as shown in the following example:

./kobs-ng-5.4 init -x -v -w --chip_0_device_path=/dev/mtd0 mx6sx_board_nand_u-boot.imx
MTD CONFIG:
  chip_0_device_path = "/dev/mtd0"
  chip_1_device_path = "(null)"
  search_exponent = 2
  data_setup_time = 80
  data_hold_time = 60
  address_setup_time = 25
  data_sample_time = 6
  row_address_size = 3
  column_address_size = 2
  read_command_code1 = 0
  read_command_code2 = 48
  boot_stream_major_version = 1
  boot_stream_minor_version = 0
  boot_stream_sub_version = 0
  ncb_version = 3
  boot_stream_1_address = 0
  boot_stream_2_address = 0
         -- We add the 1k-padding to the uboot.
.tmp_kobs_ng: verifying using key '00000000000000000000000000000000'
.tmp_kobs_ng: is a valid bootstream for key '00000000000000000000000000000000'
mtd: use legacy raw access mode
mtd: opening: "/dev/mtd0"
NFC geometry :
        ECC Strength       : 2
        Page Size in Bytes : 2071
        Metadata size      : 10
        ECC Chunk Size in byte : 512
        ECC Chunk count        : 4
        Block Mark Byte Offset : 2028
        Block Mark Bit Offset  : 2
====================================================
mtd: opened '/dev/mtd0' - '(null)'
mtd: max_boot_stream_size_in_bytes = 3670016
mtd: boot_stream_size_in_bytes = 398380
mtd: boot_stream_size_in_pages = 195
mtd: #1 0x00100000 - 0x00480000 (0x0016142c)
mtd: #2 0x00480000 - 0x00800000 (0x004e142c)
FCB
  m_u32Checksum = 0x00000000
  m_u32FingerPrint = 0x20424346
  m_u32Version = 0x01000000
  m_NANDTiming.m_u8DataSetup = 80
  m_NANDTiming.m_u8DataHold = 60
  m_NANDTiming.m_u8AddressSetup = 25
  m_NANDTiming.m_u8DSAMPLE_TIME = 6
  m_u32PageDataSize = 2048
  m_u32TotalPageSize = 2112
  m_u32SectorsPerBlock = 64
  m_u32NumberOfNANDs = 0
  m_u32TotalInternalDie = 0
  m_u32CellType = 0
  m_u32EccBlockNEccType = 1
  m_u32EccBlock0Size = 512
  m_u32EccBlockNSize = 512
  m_u32EccBlock0EccType = 1
  m_u32MetadataBytes = 10
  m_u32NumEccBlocksPerPage = 3
  m_u32EccBlockNEccLevelSDK = 0
  m_u32EccBlock0SizeSDK = 0
  m_u32EccBlockNSizeSDK = 0
  m_u32EccBlock0EccLevelSDK = 0
  m_u32NumEccBlocksPerPageSDK = 0
  m_u32MetadataBytesSDK = 0
  m_u32EraseThreshold = 0
  m_u32Firmware1_startingPage = 512
  m_u32Firmware2_startingPage = 2304
  m_u32PagesInFirmware1 = 195
  m_u32PagesInFirmware2 = 195
  m_u32DBBTSearchAreaStartAddress = 256
  m_u32BadBlockMarkerByte = 2028
  m_u32BadBlockMarkerStartBit = 2
  m_u32BBMarkerPhysicalOffset = 2048
  m_u32BCHType = 0
  m_NANDTMTiming.m_u32TMTiming2_ReadLatency = 0
  m_NANDTMTiming.m_u32TMTiming2_PreambleDelay = 0
  m_NANDTMTiming.m_u32TMTiming2_CEDelay = 0
  m_NANDTMTiming.m_u32TMTiming2_PostambleDelay = 0
  m_NANDTMTiming.m_u32TMTiming2_CmdAddPause = 0
  m_NANDTMTiming.m_u32TMTiming2_DataPause = 0
  m_NANDTMTiming.m_u32TMSpeed = 0
  m_NANDTMTiming.m_u32TMTiming1_BusyTimeout = 0
  m_u32DISBBM = 0
  m_u32BBMarkerPhysicalOffsetInSpareData = 0
  m_u32OnfiSyncEnable = 0
  m_NANDONFITiming.m_u32ONFISpeed = 0
  m_NANDONFITiming.m_u32ONFITiming_ReadLatency = 0
  m_NANDONFITiming.m_u32ONFITiming_CEDelay = 0
  m_NANDONFITiming.m_u32ONFITiming_PreambleDelay = 0
  m_NANDONFITiming.m_u32ONFITiming_PostambleDelay = 0
  m_NANDONFITiming.m_u32ONFITiming_CmdAddPause = 0
  m_NANDONFITiming.m_u32ONFITiming_DataPause = 0
  m_NANDONFITiming.m_u32ONFITiming_BusyTimeout = 0
  m_u32DISBBSearch = 0
  m_u32RandomizerEnable = 0
  m_u32ReadRetryEnable = 0
  m_u32ReadRetrySeqLength = 0
DBBT
  m_u32Checksum = 0x00000000
  m_u32FingerPrint = 0x54424244
  m_u32Version = 0x01000000
  m_u32DBBTNumOfPages = 0
Firmware: image #0 @ 0x100000 size 0x61800 - available 0x380000
Firmware: image #1 @ 0x480000 size 0x61800 - available 0x380000
-------------- Start to write the [ FCB ] -----
mtd: erasing @0:0x0-0x20000
mtd: Writing FCB0 [ @0:0x0 ] (840) *
mtd: erasing @0:0x20000-0x40000
mtd: Writing FCB1 [ @0:0x20000 ] (840) *
mtd: erasing @0:0x40000-0x60000
mtd: Writing FCB2 [ @0:0x40000 ] (840) *
mtd: erasing @0:0x60000-0x80000
mtd: Writing FCB3 [ @0:0x60000 ] (840) *
mtd_commit_bcb(FCB): status 0

-------------- Start to write the [ DBBT ] -----
mtd: erasing @0:0x80000-0xa0000
mtd: Writing DBBT0 [ @0:0x80000 ] (800) *
mtd: erasing @0:0xa0000-0xc0000
mtd: Writing DBBT1 [ @0:0xa0000 ] (800) *
mtd: erasing @0:0xc0000-0xe0000
mtd: Writing DBBT2 [ @0:0xc0000 ] (800) *
mtd: erasing @0:0xe0000-0x100000
mtd: Writing DBBT3 [ @0:0xe0000 ] (800) *
mtd_commit_bcb(DBBT): status 0

---------- Start to write the [ .tmp_kobs_ng ]----
mtd: Writting .tmp_kobs_ng: #0 @0: 0x00100000 - 0x00161800
mtd: erasing @0:0x100000-0x120000
mtd: erasing @0:0x120000-0x140000
mtd: erasing @0:0x140000-0x160000
mtd: erasing @0:0x160000-0x180000
mtd: The last page is not full : 1068
mtd: We write one page for save guard. *
mtd: Writting .tmp_kobs_ng: #1 @0: 0x00480000 - 0x004e1800
mtd: erasing @0:0x480000-0x4a0000
mtd: erasing @0:0x4a0000-0x4c0000
mtd: erasing @0:0x4c0000-0x4e0000
mtd: erasing @0:0x4e0000-0x500000
mtd: The last page is not full : 1068
mtd: We write one page for save guard. *

In this case, the boot partition–/dev/mtd0, indicated as a parameter of the command line–is 8MB. For this reason, the secondary firmware image is stored at the address 0x480000, that is at the half of the available space after the first MB ( (0x800000-0x100000)/2 ).