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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