USB mass-storage drivers for Compute Module 4

This directory provides a bootloader image that loads a Linux initramfs that exports common block devices (EMMC, NVMe) as USB mass storage devices using the Linux gadget-fs drivers.

This allows Raspberry Pi Imager to be run on the host computer and write OS images to the Compute Module block devices.

secure-boot

If secure-boot mode has been locked (via OTP) then both the bootloader and rpiboot bootcode4.bin will only load boot.img files signed with the customer's private key. Therefore, access to rpiboot mass storage mode is disabled.

Mass storage mode can be re-enabled by signing a boot image containing the firmware mass storage drivers.

N.B. The signed image should normally be kept secure because can be used on any device signed with the same customer key.

To sign the mass storage mode boot image run:-

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

Running

To run load the USB MSD device drivers via RPIBOOT run

../rpiboot -d .

The activity LED will flash three times once the USB MSD devices have been initialised. A message will also be printed to the HDMI console.

The UART console is also enabled (user 'root' no password) so that the user can login and debug the Compute Module.

N.B. This takes a few seconds longer to initialise than the previous mass storage implementation. However, the write speed should be much faster now that all of the file-system code is running on the ARM processors.

Source code

The buildroot configuration and supporting patches is available on the mass-storage-gadget branch of the Raspberry Pi buildroot repo.

Building

git clone --branch mass-storage-gadget git@github.com:raspberrypi/buildroot.git
cd buildroot
make raspberrypicm4io_initrd_defconfig
make

The output is written to output/target/images/sdcard.img and can be copied to boot.img

Optional: Load time can be reduced by optmizing the size of the sdcard.img as follows (run as root).

mkdir -p source-files
kpartx -av sdcard.img
mount /dev/mapper/loop0p1 source-files/
usbboot/tools/make-boot-image -d source-files/ -o boot.img
umount source-files
kpartx -dv sdcard.img
dmsetup remove /dev/mapper/loop0p1
losetup -D