i.MX7D/i.MX8MM SRC_GPR10 PERSIST_SECONDARY_BOOT for bootloader A/B switching

Introduction

Since at least iMX53 until iMX8MM, it is possible to have two copies of bootloader in SD/eMMC and switch between them. The switch is triggered either by the BootROM in case the bootloader image is faulty OR can be enforced by the user.

Operation

  1. Upon Power-On Reset (POR)

    • SRC_GPR10 bit PERSIST_SECONDARY_BOOT is set to 0

    • BootROM attempts to start bootloader A-copy

      • if A-copy valid

        • BootROM starts A-copy
        • END

      • if A-copy NOT valid

        • BootROM sets SRC_GPR10 bit PERSIST_SECONDARY_BOOT to 1
        • BootROM triggers WARM reset, GOTO 1)
        • END

  2. Upon COLD Reset

    • GOTO 1)
    • END

  3. Upon WARM Reset

    • SRC_GPR10 bit PERSIST_SECONDARY_BOOT is retained
      • if SRC_GPR10 bit PERSIST_SECONDARY_BOOT is 0
        • BootROM attempts to start bootloader A-copy
          • if A-copy valid
            • BootROM starts A-copy
            • END
          • if A-copy NOT valid
            • BootROM sets SRC_GPR10 bit PERSIST_SECONDARY_BOOT to 1
            • BootROM triggers WARM reset. GOTO 1.3)
            • END
      • if SRC_GPR10 bit PERSIST_SECONDARY_BOOT is 1
        • BootROM attempts to start bootloader B-copy
          • if B-copy valid
            • BootROM starts B-copy
            • END
          • if B-copy NOT valid - System hangs - END

Setup

The bootloader A-copy must be placed at predetermined offset in SD/eMMC. The bootloader B-copy area offset is determined by an offset stored in Secondary Image Table (SIT). The SIT must be placed at predetermined offset in SD/eMMC.

The following table contains offset of SIT, bootloader A-copy and recommended bootloader B-copy offset. The offsets are in 512 Byte sector units (that is offset 0x1 means 512 Bytes from the start of SD/eMMC card data partition). For details on the addition of two numbers in recommended B-copy offset, see SIT format below.

SoC SIT offset (fixed) A-copy offset (fixed) B-copy offset (recommended)
iMX7D 0x1 0x2 0x800+0x2
iMX8MM 0x41 0x42 0x1000+0x42

SIT format

SIT is a 20 byte long structure containing of 5 32-bit words. Those encode bootloader B-copy area offset (called “firstSectorNumber”), magic value (called “tag”) that is always 0x00112233, and three unused words set to 0. SIT is documented in [1] and [2]. Example SIT are below:

$ hexdump -vC sit-mx7d.bin
  00000000  00 00 00 00
  00000004  00 00 00 00
  00000008  33 22 11 00 <--- This is the "tag"
  0000000c  00 08 00 00 <--- This is the "firstSectorNumber"
  00000010  00 00 00 00

$ hexdump -vC sit-mx8mm.bin
  00000000  00 00 00 00
  00000004  00 00 00 00
  00000008  33 22 11 00 <--- This is the "tag"
  0000000c  00 10 00 00 <--- This is the "firstSectorNumber"
  00000010  00 00 00 00

B-copy area offset (“firstSectorNumber”) is offset, in units of 512 Byte sectors, that is added to the start of boot media when switching between A-copy and B-copy. For A-copy, this offset is 0x0. For B-copy, this offset is determined by SIT (e.g. if firstSectorNumber is 0x1000 as it is above in sit-mx8mm.bin, then the B-copy offset is 0x1000 sectors = 2 MiB).

Bootloader A-copy (e.g. u-boot.imx or flash.bin) is placed at fixed offset from A-copy area offset (e.g. 0x2 sectors from sector 0x0 for iMX7D, which means u-boot.imx A-copy must be written to sector 0x2).

The same applies to bootloader B-copy, which is placed at fixed offset from B-copy area offset determined by SIT (e.g. 0x2 sectors from sector 0x800 [see sit-mx7d.bin example above, this can be changed in SIT firstSectorNumber] for iMX7D, which means u-boot.imx B-copy must be written to sector 0x802)

WARNING: B-copy area offset (“firstSectorNumber”) is NOT equal to bootloader (image, which is u-boot.imx or flash.bin) B-copy offset.

To generate SIT, use for example the following bourne shell printf command:

$ printf '\x0\x0\x0\x0\x0\x0\x0\x0\x33\x22\x11\x00\x00\x08\x00\x00\x0\x0\x0\x0' > sit-mx7d.bin
$ printf '\x0\x0\x0\x0\x0\x0\x0\x0\x33\x22\x11\x00\x00\x10\x00\x00\x0\x0\x0\x0' > sit-mx8mm.bin

Write bootloader A/B copy and SIT to SD/eMMC

Examples of writing SIT and two copies of bootloader to SD or eMMC:

  • iMX8MM, SD card at /dev/sdX, Linux command line

    $ dd if=sit-mx8mm.bin of=/dev/sdX bs=512 seek=65
$ dd if=flash.bin     of=/dev/sdX bs=512 seek=66
$ dd if=flash.bin     of=/dev/sdX bs=512 seek=4162

  • iMX8MM, eMMC 1 data partition, U-Boot command line

    => mmc partconf 1 0 0 0

=> dhcp ${loadaddr} sit-mx8mm.bin
=> mmc dev 1
=> mmc write ${loadaddr} 0x41 0x1

=> dhcp ${loadaddr} flash.bin
=> setexpr blkcnt ${filesize} + 0x1ff && setexpr blkcnt ${blkcnt} / 0x200
=> mmc dev 1
=> mmc write ${loadaddr} 0x42   ${blkcnt}
=> mmc write ${loadaddr} 0x1042 ${blkcnt}

WARM reset into B-copy using WDT

To perform a reboot into B-copy, the PERSIST_SECONDARY_BOOT must be set in SRC_GPR0 register. Example on iMX8MM:

=> mw 0x30390098 0x40000000

A WARM reset can be triggered using WDT as follows:

=> mw.w 0x30280000 0x25

References

[1]i.MX 7Dual Applications Processor Reference Manual, Rev. 1, 01/2018 ; section 6.6.5.3.5 Redundant boot support for expansion device
[2]i.MX 8M Mini Applications Processor Reference Manual, Rev. 3, 11/2020 ; section 6.1.5.4.5 Redundant boot support for expansion device