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- 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
- ---------
- #. 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
- #. Upon COLD Reset
- - GOTO 1)
- - END
- #. 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
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