PARTITION parameters: Preliminary documentation for remapping partition numbers

The latest firmware provides a basic method for remapping a high
partition number to a regular partition. This makes it more flexible
as generic reboot parameter.
E.g. It would be possible reboot into a different boot-order or
even network install.
This will probably evolve as we use it with rpi-image-gen

Author: timg236

documentation/asciidoc/computers/config_txt/conditional.adoc

@@ -96,6 +96,34 @@ Some models of Raspberry Pi, including Zero, Compute Module, and Keyboard models
 
 The `[none]` filter prevents any settings that follow from being applied to any hardware. Although there is nothing that you can't do without `[none]`, it can be a useful way to keep groups of unused settings in config.txt without having to comment out every line.
 
+=== The `[partition=N]` filter
+This `partition` filter can be be used to select alternate boot flows according to the requested partition number (`sudo reboot N`) or via direct usage of the `PM_RSTS` watchdog register.
+
+[source,ini]
+----
+# Bootloader EEPROM config.
+# If PM_RSTS is partition 62 then set bootloader properties to disable
+# SD high speed and show HDMI diagnostics
+# Boot from partition 2 with debug option.
+[partition=62]
+# Only high (>31) partition can be remapped.
+PARTITION=2
+SD_QUIRKS=0x1
+HDMI_DELAY=0
+----
+
+Example `config.txt` - (Currently Raspberry Pi 5 onwards)
+[source,ini]
+----
+# config.txt - If the original requested partition number in PM_RSTS was a
+# special number then use an alternate cmdline.txt
+[partition=62]
+cmdline=cmdline-recovery.txt
+----
+
+The raw value of the `PM_RSTS` register at bootup is available via `/proc/device-tree/chosen/bootloader/rsts` and the final partition number used for booting is available via `/proc/device-tree/chosen/bootloader/partition`. These are big-endian binary values.
+
+
 === The `[tryboot]` filter
 
 This filter succeeds if the `tryboot` reboot flag was set.

documentation/asciidoc/computers/raspberry-pi/eeprom-bootloader.adoc

@@ -499,6 +499,19 @@ Max length: 32 characters
 The `PARTITION` option may be used to specify the boot partition number, if it has not explicitly been set by the `reboot` command (e.g. `sudo reboot N`) or by `boot_partition=N` in `autoboot.txt`.
 This could be used to boot from a rescue partition if the user presses a button.
 
+The latest firmware also allows high partition numbers (> 31) to be overriden. This allows a custom setup of the system hardware watchdog to trigger a reboot with a special high partition number (e.g. 62) which can be detected by the bootloader (using a conditional filter) and remapped to a recovery partition.
+
+Example:
+[source,ini]
+----
+# System watchdog fired - boot the rescue partition with additional options
+# Disable SD high speed mode and enable HDMI diagnostics immediately.
+[partition=62]
+PARTITION=2
+HDMI_DELAY=0
+SD_QUIRKS=1
+----
+
 [source,ini]
 ----
 # Boot from partition 2 if GPIO 7 is pulled low