conditional: Document the new expression filter and new Pi5 boot variables

Author: timg236

documentation/asciidoc/computers/config_txt/conditional.adoc

@@ -97,7 +97,7 @@ 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.
+This `partition` filter can 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]
 ----
@@ -123,19 +123,96 @@ 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 `[boot_partition=N]` filter
-The `boot_partition` filter can be used to select alternate OS files (e.g. `cmdline.txt`) to be loaded, depending on which partition `config.txt` was loaded from after processing `autoboot.txt`. This is intended for use with an `A/B` boot-system with `autoboot.txt` where it is desirable to be able to have identical files installed to the boot partition for both the `A` and `B` images.
+=== The expression filter
 
-Example `config.txt` - select the matching root filesystem for the `A/B` boot file-system.
+The expression filter provides support for comparing unsigned integer `boot variables` with constants using a simple set of expressions. It is intended to support OTA update mechanisms, debug and test.
+
+* The `boot variables` are `boot_count`, `boot_arg1`, `boot_partition` and `partition`. 
+* Boot variables in the expressions case must always be lower case.
+* Constants may be written as decimal or as hex.
+* Expression conditional filters have no side-effects e.g. no assignment operators.
+* As with other filter types the expression filter cannot be nested.
+* Use the `[all]` filter to reset expressions and all other conditional filter types.
+
+Syntax:
+[source,ini]
+----
+# ARG is a boot-variable
+# VALUE is an integer constant
+[ARG=VALUE]      # selected if (ARG == VALUE)
+[ARG&MASK]       # selected if ((ARG & VALUE) != 0)
+[ARG&MASK=VALUE] # selected if ((ARG & MASK) == VALUE)
+[ARG<VALUE]      # selected if (ARG < VALUE)
+[ARG>VALUE]      # selected if (ARG > VALUE)
+
+----
+
+=== `boot_arg1`
+Raspberry Pi 5 and newer flagship devices only.
+
+`boot_arg1` is a 32-bit user defined value which stored in a reset-safe register allowing parameters can be passed accross a reboot.
+
+Setting `boot_arg1` to 42 via `config.txt`:
 [source,ini]
 ----
+set_reboot_arg1=42
+----
+
+Setting `boot_arg1` to 42 via vcmailbox:
+[source,console]
+----
+sudo vcmailbox 0x0003808c 8 8 1 42
+----
+
+Reading `boot_arg1` via vcmailbox:
+[source,console]
+----
+sudo vcmailbox 0x0003008c 8 8 1 0
+# Example output - boot_arg1 is 42
+# 0x00000020 0x80000000 0x0003008c 0x00000008 0x80000008 0x00000001 0x0000002a 0x0000000
+
+The value of the `boot_arg1` variable when the OS was started can be read via xref:configuration.adoc#part4[device-tree] at `/proc/device-tree/chosen/bootloader/arg1`
+----
+
+=== `boot_count`
+Raspberry Pi 5 and newer flagship devices only.
+
+`boot_count` variable is an 8-bit value that is incremented (wrapping back to zero at 256) at boot and is reset to zero by a power on reset.
+
+To read `boot_count` via vcmailbox:
+[source,console]
+----
+sudo vcmailbox 0x0003008d 4 4 0
+# Example - boot count is 3
+# 0x0000001c 0x80000000 0x0003008d 0x00000004 0x80000004 0x00000003 0x00000000
+----
+
+Setting/clearing `boot_count` via vcmailbox:
+[source,console]
+----
+# Clear boot_count by setting it to zero.
+sudo vcmailbox 0x0003808d 4 4 0
+----
+The value of `boot_count` at when the OS was started can be read via xref:configuration.adoc#part4[device-tree] at `/proc/device-tree/chosen/bootloader/count`
+
+==== `boot_partition`
+The `boot_partition` variable can be used to select alternate OS files (e.g. `cmdline.txt`) to be loaded, depending on which partition `config.txt` was loaded from after processing `autoboot.txt`. This is intended for use with an `A/B` boot-system with `autoboot.txt` where it is desirable to be able to have identical files installed to the boot partition for both the `A` and `B` images.
+
+The value of the `boot_partition` can be different to the `partition` variable because the required `partition` can be overriden by `autoboot.txt`.
+
+Example `config.txt` - select the matching root filesystem for the `A/B` boot file-system:
+[source,ini]
+----
+# Use different cmdline files to point to different root filesystems based on which partition the system booted from.
 [boot_partition=1]
-cmdline=cmdline_rootfs_a.txt
+cmdline=cmdline_rootfs_a.txt  # Points to root filesystem A
 
 [boot_partition=2]
-cmdline=cmdline_rootfs_b.txt
+cmdline=cmdline_rootfs_b.txt  # Points to root filesystem B
 ----
 
+The value of `boot_partition` i.e. the partition used to boot the OS can be read from device-tree at `/proc/device-tree/chosen/bootloader/partition`
+
 === The `[tryboot]` filter
 
 This filter succeeds if the `tryboot` reboot flag was set.

documentation/asciidoc/computers/configuration/device-tree.adoc

@@ -833,7 +833,14 @@ For a list of supported overlays and parameters, see the https://github.com/rasp
 [[part4]]
 === Firmware parameters
 
-The firmware uses the special https://www.kernel.org/doc/html/latest/devicetree/usage-model.html#runtime-configuration[/chosen] node to pass parameters between the bootloader and/or firmware and the operating system. Each property is stored as a 32-bit integer unless indicated otherwise.
+The firmware uses the special https://www.kernel.org/doc/html/latest/devicetree/usage-model.html#runtime-configuration[/chosen] node to pass parameters between the bootloader and/or firmware and the operating system. Each property is stored as a 32-bit unsigned integer unless indicated otherwise.
+
+Numbers in device-tree are stored in binary and are big-endian.
+[source,console]
+----
+# Example shell script for reading a 32-bit unsigned number from device-tree
+printf "%d" "0x$(od "/proc/device-tree/chosen/bootloader/partition" -v -An -t x1 | tr -d ' ' )"
+----
 
 `overlay_prefix`:: _(string)_ The xref:config_txt.adoc#overlay_prefix[overlay_prefix] string selected by `config.txt`.
 
@@ -859,9 +866,17 @@ Each property is stored as a 32-bit integer unless indicated otherwise.
 
 `tryboot`:: Set to `1` if the `tryboot` flag was set at boot.
 
+==== Boot variables `/chosen/bootloader`
+
+Raspberry Pi 5 only. Each property is stored as a 32-bit unsigned integer unless indicated otherwise.
+
+`arg1`:: The value of the user defined reboot argument from the previous boot. See xref:config_txt.adoc#boot_arg1[boot_arg1]
+
+`count`:: The value of the 8-bit `boot_count` variable when the OS was started. See xref:config_txt.adoc#boot_count[boot_count]
+
 ==== Power supply properties `/chosen/power`
 
-Raspberry Pi 5 only. Each property is stored as a 32-bit integer unless indicated otherwise.
+Raspberry Pi 5 only. Each property is stored as a 32-bit unsigned integer unless indicated otherwise.
 
 `max_current`:: The maximum current in mA that the power supply can supply. The firmware reports the value indicated by the USB-C, USB-PD or PoE interfaces. For bench power supplies (e.g. connected to the GPIO header) define `PSU_MAX_CURRENT` in the bootloader configuration to indicate the power supply current capability.