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USB Device Boot Code

This is the USB device boot code which supports the Raspberry Pi 1A, 3A+, Compute Module, Compute Module 3, 3+ 4S, and 4, Raspberry Pi Zero and Zero 2 W. N.B. In regards to this document CM4 and CM4S have identical software support.

The default behaviour when run with no arguments is to boot the Raspberry Pi with special firmware so that it emulates USB Mass Storage Device (MSD). The host OS will treat this as a normal USB mass storage device allowing the file system to be accessed. If the storage has not been formatted yet (default for Compute Module) then the Raspberry Pi Imager App can be used to install a new operating system.

Since RPIBOOT is a generic firmware loading interface, it is possible to load other versions of the firmware by passing the -d flag to specify the directory where the firmware should be loaded from. E.g. The firmware in the msd can be replaced with newer/older versions.

From Raspberry Pi5 onwards the MSD firmware has been replaced with a Linux initramfs providing a mass-storage-gadget.

For more information run rpiboot -h.

Building

Linux / Cygwin / WSL

Clone this repository on your Pi or other Linux machine. Make sure that the system date is set correctly, otherwise Git may produce an error.

  • This git repository uses symlinks. For Windows builds clone the repository under Cygwin.
  • Instead of duplicating the EEPROM binaries and tools the rpi-eeprom repository is included as a (git submodule)[https://git-scm.com/book/en/v2/Git-Tools-Submodules]
sudo apt install git libusb-1.0-0-dev pkg-config build-essential
git clone --recurse-submodules --shallow-submodules --depth=1 https://github.com/raspberrypi/usbboot
cd usbboot
make
sudo ./rpiboot

sudo isn't required if you have write permissions for the /dev/bus/usb device.

macOS

From a macOS machine, you can also run usbboot, just follow the same steps:

  1. Clone the usbboot repository
  2. Install libusb (brew install libusb)
  3. Install pkg-config (brew install pkg-config)
  4. (Optional) Export the PKG_CONFIG_PATH so that it includes the directory enclosing libusb-1.0.pc
  5. Build using make
  6. Run the binary
git clone --recurse-submodules --shallow-submodules --depth=1 https://github.com/raspberrypi/usbboot
cd usbboot
brew install libusb
brew install pkg-config
make
sudo ./rpiboot

If the build is unable to find the header file libusb.h then most likely the PKG_CONFIG_PATH is not set properly. This should be set via export PKG_CONFIG_PATH="$(brew --prefix libusb)/lib/pkgconfig".

If the build fails on an ARM-based Mac with a linker error such as ld: warning: ignoring file /usr/local/Cellar/libusb/1.0.26/lib/libusb-1.0.dylib, building for macOS-arm64 but attempting to link with file built for macOS-x86_64 then you may need to build and install libusb-1.0 yourself:

wget https://github.com/libusb/libusb/releases/download/v1.0.26/libusb-1.0.26.tar.bz2
tar -xf libusb-1.0.26.tar.bz2
cd libusb-1.0.26
./configure
make
make check
sudo make install

Running make again should now succeed.

Updating the rpi-eeprom submodule

After updating the usbboot repo (git pull --rebase origin master) update the submodules by running

git submodule update --init

Running

Compute Module 3

Fit the EMMC-DISABLE jumper on the Compute Module IO board before powering on the board or connecting the USB cable.

Compute Module 4

On Compute Module 4 EMMC-DISABLE / nRPIBOOT (GPIO 40) must be fitted to switch the ROM to usbboot mode. Otherwise, the SPI EEPROM bootloader image will be loaded instead.

Raspberry Pi 5

  • Disconnect the USB-C cable. Power must be removed rather than just running "sudo shutdown now"
  • Hold the power button down
  • Connect the USB-C cable (from the RPIBOOT host to the Pi 5)

Compute Module 4 Extensions

In addition to the MSD functionality, there are a number of other utilities that can be loaded via RPIBOOT on Compute Module 4.

Directory Description
recovery Updates the bootloader EEPROM on a Compute Module 4
recovery5 Updates the bootloader EEPROM on a Raspberry Pi 5
rpi-imager-embedded Runs the embedded version of Raspberry Pi Imager on the target device
mass-storage-gadget 32-bit mass storage gadget for BCM2711
mass-storage-gadget64 Mass storage gadget with 64-bit Kernel for BCM2711 and BCM2712
secure-boot-recovery Scripts that extend the recovery process to enable secure-boot, sign images etc
secure-boot-msd DEPRECATED - Scripts for signing the MSD firmware so that it can be used on a secure-boot device.
secure-boot-example Simple Linux initrd with a UART console.

The secure-boot-msd, rpi-imager-embedded and mass-storage-gadget extensions require that the 2022-04-26 (or newer) bootloader EEPROM release has already been written to the EEPROM using recovery.bin

Booting Linux

The RPIBOOT protocol provides a virtual file system to the Raspberry Pi bootloader and GPU firmware. It's therefore possible to boot Linux. To do this, you will need to copy all of the files from a Raspberry Pi boot partition plus create your own initramfs. On Raspberry Pi 4 / CM4 the recommended approach is to use a boot.img which is a FAT disk image containing the minimal set of files required from the boot partition.

Troubleshooting

This section describes how to diagnose common rpiboot failures for Compute Modules. Whilst rpiboot is tested on every Compute Module during manufacture the system relies on multiple hardware and software elements. The aim of this guide is to make it easier to identify which component is failing.

Product Information Portal

The Product Information Portal contains the official documentation for hardware revision changes for Raspberry Pi computers. Please check this first to check that the software is up to date.

Hardware

  • Inspect the Compute Module pins and connector for signs of damage and verify that the socket is free from debris.
  • Check that the Compute Module is fully inserted.
  • Check that nRPIBOOT / EMMC disable is pulled low BEFORE powering on the device.
    • On BCM2711, if the USB cable is disconected and the nRPIBOOT jumper is fitted then the green LED should be OFF. If the LED is on then the ROM is detecting that the GPIO for nRPIBOOT is high.
  • Remove any hubs between the Compute Module and the host.
  • Disconnect all other peripherals from the IO board.
  • Verify that the red power LED switches on when the IO board is powered.
  • Use another computer to verify that the USB cable for rpiboot can reliably transfer data. For example, connect it to a Raspberry Pi keyboard with other devices connected to the keyboard USB hub.

Hardware - CM4

  • The CM4 EEPROM supports MMC, USB-MSD, USB 2.0, Network and NVMe boot by default. Try booting to Linux from an alternate boot mode (e.g. network) to verify the nRPIBOOT GPIO can be pulled low and that the USB 2.0 interface is working.
  • If rpiboot is running but the mass storage device does not appear then try running the rpiboot -d mass-storage-gadget because this uses Linux instead of a custom VPU firmware to implement the mass-storage gadget. This also provides a login console on UART and HDMI.

Hardware - Raspberry Pi 5

  • Press, and hold the power button before supplying power to the device.
  • Release the power button immediately after supplying power to the device.
  • Remove any non-essential USB peripherals or HATs.
  • Use a USB-3 port capable of supplying at least 900mA and use a high quality USB-C cable OR supply additional power via the 40-pin header.

Software

The recommended host setup is Raspberry Pi with Raspberry Pi OS. Alternatively, most Linux X86 builds are also suitable. Windows adds some extra complexity for the USB drivers so we recommend debugging on Linux first.

  • Update to the latest software release using apt update rpiboot or download and rebuild this repository from Github.
  • Run rpiboot -v | tee log to capture verbose log output. N.B. This can be very verbose on some systems.

Boot flow

The rpiboot system runs in multiple stages. The ROM, bootcode.bin, the VPU firmware (start.elf) and for the mass-storage-gadget or rpi-imager a Linux initramfs. Each stage disconnects the USB device and presents a different USB descriptor. Each stage will appears as a new USB device connect in the dmesg log.

See also: Raspberry Pi4 Boot Flow

bootcode.bin

Be careful not to overwrite bootcode.bin or bootcode4.bin with the executable from a different subdirectory. The rpiboot process simply looks for a file called bootcode.bin (or bootcode4.bin on BCM2711). However, the file in recovery/secure-boot-recovery directories is actually the recovery.bin EEPROM flashing tool.

Diagnostics

  • Monitor the Linux dmesg output and verify that a BCM boot device is detected immediately after powering on the device. If not, please check the hardware section.
  • Check the green activity LED. On Compute Module 4 this is activated by the software bootloader and should remain on. If not, then it's likely that the initial USB transfer to the ROM failed.
  • On Compute Module 4 connect a HDMI monitor for additional debug output. Flashing the EEPROM using recovery.bin will show a green screen and the mass-storage-gadget enables a console on the HDMI display.
  • If rpiboot starts to download bootcode4.bin but the transfer fails then can indicate a cable issue OR a corrupted file. Check the hash of bootcode.bin file against this repository and check dmesg for USB error.
  • If bootcode.bin or the start.elf detects an error then error-code will be indicated by flashing the green activity LED.
  • Add uart_2ndstage=1 to the config.txt file in msd/ or recovery/ directories to enable UART debug output.

Secure Boot

Secure Boot requires a recent bootloader stable image e.g. the version in this repository.

NOTE: Secure Boot is not currently supported on Raspberry Pi 5.

Tutorial

Creating a secure boot system from scratch can be quite complex. The secure boot tutorial uses a minimal example OS image to demonstrate how the Raspberry Pi-specific aspects of secure boot work.

Additional documentation

Host Setup

Secure boot require a 2048 bit RSA asymmetric keypair and the Python pycrytodomex module to sign the bootloader EEPROM config and boot image.

Install Python Crypto Support (the pycryptodomex module)

sudo apt install python3-pycryptodome

Create an RSA key-pair using OpenSSL. Must be 2048 bits

cd $HOME
openssl genrsa 2048 > private.pem

Secure Boot - configuration

Secure Boot - image creation

Secure Boot requires self-contained ramdisk (boot.img) FAT image to be created containing the GPU firmware, kernel and any other dependencies that would normally be loaded from the boot partition.

This plus a signature file (boot.sig) must be placed in the boot partition of the Raspberry Pi or network download location.

The boot.img file should contain:-

  • The kernel
  • Device tree overlays
  • GPU firmware (start.elf and fixup.dat)
  • Linux initramfs containing the application OR scripts to mount/create an encrypted file-system.

Disk encryption

Secure-boot is responsible for loading the Kernel + initramfs and loads all of the data from a single boot.img file stored on an unencrypted FAT/EFI partition.

There is no support in the ROM or firmware for full-disk encryption.

If a custom OS image needs to use an encrypted file-system then this would normally be implemented via scripts within the initramfs.

Raspberry Pi computers do not have a secure enclave, however, it's possible to store a 256 bit device specific private key in OTP. The key is accessible to any process with access to /dev/vcio (vcmailbox), therefore, the secure-boot OS must ensure that access to this interface is restricted.

It is not possible to prevent code running in ARM supervisor mode (e.g. kernel code) from accessing OTP hardware directly

See also:-

The secure boot tutorial contains a boot.img that supports cryptsetup and a simple example.

Building boot.img using buildroot

The secure-boot-example directory contains a simple boot.img example with working HDMI, network, UART console and common tools in an initramfs.

This was generated from the raspberrypi-signed-boot buildroot config. Whilst not a generic fully featured configuration it should be relatively straightforward to cherry-pick the raspberrypi-secure-boot package and helper scripts into other buildroot configurations.

Minimum firmware version

The firmware must be new enough to support secure boot. The latest firmware APT package supports secure boot. To download the firmware files directly.

git clone --depth 1 --branch stable https://github.com/raspberrypi/firmware

To check the version information within a start4.elf firmware file run

strings start4.elf | grep VC_BUILD_

Verifying the contents of a boot.img file

To verify that the boot image has been created correctly use losetup to mount the .img file.

sudo su
mkdir -p boot-mount
LOOP=$(losetup -f)
losetup -f boot.img
mount ${LOOP} boot-mount/

 echo boot.img contains
find boot-mount/

umount boot-mount
losetup -d ${LOOP}
rmdir boot-mount

Signing the boot image

For secure-boot, rpi-eeprom-digest extends the current .sig format of sha256 + timestamp to include an hex format RSA bit PKCS#1 v1.5 signature. The key length must be 2048 bits.

../tools/rpi-eeprom-digest -i boot.img -o boot.sig -k "${KEY_FILE}"

To verify the signature of an existing image set the PUBLIC_KEY_FILE environment variable to the path of the public key file in PEM format.

../tools/rpi-eeprom-digest -i boot.img -k "${PUBLIC_KEY_FILE}" -v boot.sig

Hardware security modules

rpi-eeprom-digest is a shell script that wraps a call to openssl dgst -sign. If the private key is stored within a hardware security module instead of a .PEM file the openssl command will need to be replaced with the appropriate call to the HSM.

rpi-eeprom-digest called by update-pieeprom.sh to sign the EEPROM config file.

The RSA public key must be stored within the EEPROM so that it can be used by the bootloader. By default, the RSA public key is automatically extracted from the private key PEM file. Alternatively, the public key may be specified separately via the -p argument to update-pieeprom.sh and rpi-eeprom-config.

To extract the public key in PEM format from a private key PEM file, run:

openssl rsa -in private.pem -pubout -out public.pem

Copy the secure boot image to the boot partition on the Raspberry Pi.

Copy boot.img and boot.sig to the boot filesystem. Secure boot images can be loaded from any of the normal boot modes (e.g. SD, USB, Network).