rpieepromab: docs: Add details to README

rpieepromab/README.md

@@ -1,16 +1,200 @@
-
 # rpi-eeprom-ab
 
-The Raspberry Pi EEPROM AB service is a mailbox based API
+The Raspberry Pi EEPROM AB service is a mailbox-based API
 that allows you to update and manage the AB EEPROM partitions.
 
 Although this service can be used via raw vcmailbox commands the
-recommended API is the command line rpi-eeprom-ab application.
+recommended API is the command line `rpi-eeprom-ab` application.
+
+This service currently only exists on the Raspberry Pi 5 family of
+devices running AB-capable firmware.
 
 ## Build Instructions
 
-- *mkdir build*
-- *cd build*
-- *cmake ..*
-- *make*
-- *sudo make install*
+```
+mkdir build
+cd build
+cmake ..
+make
+sudo make install
+```
+
+If overwriting the system (APT) installed `rpi-eeprom-ab`, set the CMake
+install prefix to `/usr`. Otherwise, there will be a library mismatch because
+the default install prefix is `/usr/local`:
+
+```
+cmake -DCMAKE_INSTALL_PREFIX=/usr ..
+```
+
+## Usage
+
+Display usage instructions for all operations:
+
+```
+rpi-eeprom-ab help
+```
+
+Show the application and library version:
+
+```
+rpi-eeprom-ab version
+```
+
+### EEPROM read and write
+
+Update the opposite AB partition with a new bootloader image. The update file
+must be a valid partition image and must meet the board's minimum bootloader
+version. An update can only be performed when the current partition
+is committed.
+
+```
+rpi-eeprom-ab update pieeprom-ab.bin
+```
+
+Extract a partition image from an AB-capable full pieeprom image:
+
+```
+dd if=pieeprom.bin bs=1K skip=64 count=988 of=pieeprom-ab.bin
+```
+
+Read the currently selected AB partition to a file:
+
+```
+rpi-eeprom-ab read current-partition.bin
+```
+
+Read the entire 2 MiB EEPROM to a file:
+
+```
+rpi-eeprom-ab dump eeprom-full.bin
+```
+
+Get the current status of an EEPROM update:
+
+```
+rpi-eeprom-ab update-status
+```
+
+### Partition info
+
+Get the currently selected AB partition (`A` or `B`):
+
+```
+rpi-eeprom-ab partition
+```
+
+Get whether the current partition is committed (`0` or `1`):
+
+```
+rpi-eeprom-ab committed
+```
+
+Get the committed and valid partition selections and their SHA-256 hashes:
+
+```
+rpi-eeprom-ab partition-status
+```
+
+Get the partition used at boot and whether it was committed at time of boot:
+
+```
+rpi-eeprom-ab status-at-boot
+```
+
+### Partition validation and commit
+
+Mark the uncommitted partition as valid.
+The hash (a 64-character hex string) must match the SHA-256 hash of the update image.
+
+```
+rpi-eeprom-ab mark-partition-valid <hash>
+```
+
+Revert to the committed partition as the valid selection, overwriting a
+previous `mark-partition-valid`. The hash must match the SHA-256 hash of the
+committed partition:
+
+```
+rpi-eeprom-ab revert-to-committed <hash>
+```
+
+Commit the current AB partition:
+
+```
+rpi-eeprom-ab commit
+```
+
+Force commit the opposite partition. When this is used, the partition will not
+automatically be rolled back if there is a failure.
+
+```
+rpi-eeprom-ab force-commit-opposite
+```
+
+### Tryboot
+
+When tryboot is enabled (`1`), the bootloader will attempt to boot from the
+valid but uncommitted partition on the next reboot. When disabled (`0`), only
+the committed partition is used.
+
+Get the current tryboot value:
+
+```
+rpi-eeprom-ab tryboot
+```
+
+Enable or disable tryboot:
+
+```
+rpi-eeprom-ab tryboot 1
+rpi-eeprom-ab tryboot 0
+```
+
+### Typical update workflow
+
+1. Write the new image to the opposite partition:
+   ```
+   rpi-eeprom-ab update pieeprom-ab.bin
+   ```
+2. Mark the updated partition as valid. The hash must match the SHA-256 hash of
+   the image written in step 1:
+   ```
+   export UPDATE_HASH="$(sha256sum pieeprom-ab.bin | awk '{print $1}')"
+   rpi-eeprom-ab mark-partition-valid "$UPDATE_HASH"
+   ```
+3. Enable tryboot and reboot. The tryboot flag is a one-shot flag that is
+   cleared during boot, so a failed boot falls back to the committed
+   partition next time:
+   ```
+   rpi-eeprom-ab tryboot 1
+   sudo reboot
+   ```
+4. After rebooting, verify that the system booted from the new (uncommitted)
+   partition. `committed` returns `0` when running from the uncommitted
+   partition, confirming tryboot succeeded:
+   ```
+   rpi-eeprom-ab committed
+   ```
+5. Commit the partition so it is used for all future boots:
+   ```
+   rpi-eeprom-ab commit
+   ```
+
+#### Forced update alternative
+
+Steps 1 and 2 are the same as above. Then, instead of using tryboot, force
+commit the newly written (opposite) partition so it becomes the partition used
+for all future boots. This skips the tryboot rollback mechanism, so only use it
+when the new image is known to be good:
+
+```
+rpi-eeprom-ab force-commit-opposite
+```
+
+## Error handling and debug
+
+If the firmware reports an error then `rpi-eeprom-ab` prints a descriptive
+message and returns a non-zero exit code.
+
+The firmware logs can be viewed with `sudo vclog -m` for additional debug.