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