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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
=> 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
|
