Pi 3 booting part I: USB mass storage boot beta

When we originally announced the Raspberry Pi 3, we announced that we’d implemented several new boot modes. The first of these is the USB mass storage boot mode, and we’ll explain a little bit about it in this post; stay tuned for the next part on booting over Ethernet tomorrow. We’ve also supplied a boot modes tutorial over on the Raspberry Pi documentation pages.

Note: the new boot modes are still in beta testing and use the “next” branch of the firmware. If you’re unsure about using the new boot modes, it’s probably best to wait until we release it fully.

How did we do this?

Inside the 2835/6/7 devices there’s a small boot ROM, which is an unchanging bit of code used to boot the device. It’s the boot ROM that can read files from SD cards and execute them. Previously, there were two boot modes: SD boot and USB device boot (used for booting the Compute Module). When the Pi is powered up or rebooted, it tries to talk to an attached SD card and looks for a file called bootcode.bin; if it finds it, then it loads it into memory and jumps to it. This piece of code then continues to load up the rest of the Pi system, such as the firmware and ARM kernel.

While squeezing in the Quad A53 processors, I spent a fair amount of time writing some new boot modes. If you’d like to get into a little more detail, there’s more information in the documentation. Needless to say, it’s not easy squeezing SD boot, eMMC boot, SPI boot, NAND flash, FAT filesystem, GUID and MBR partitions, USB device, USB host, Ethernet device, and mass storage device support into a mere 32kB.

What is a mass storage device?

The USB specification allows for a mass storage class which many devices implement, from the humble flash drive to USB attached hard drives. This includes micro SD readers, but generally it refers to anything you can plug into a computer’s USB port and use for file storage.

I’ve tried plugging in a flash drive before and it didn’t do anything. What’s wrong? 

We haven’t enabled this boot mode by default, because we first wanted to check that it worked as expected. The boot modes are enabled in One-Time Programmable (OTP) memory, so you have to enable the boot mode on your Pi 3 first. This is done using a config.txt parameter.

Instructions for implementing the mass storage boot mode, and changing a suitable Raspbian image to boot from a flash drive, can be found here.

Are there any bugs / problems?

There are a couple of known issues:

  1. Some flash drives power up too slowly. There are many spinning disk drives that don’t respond within the allotted two seconds. It’s possible to extend this timeout to five seconds, but there are devices that fail to respond within this period as well, such as the Verbatim PinStripe 64GB.
  2. Some flash drives have a very specific protocol requirement that we don’t handle; as a result of this, we can’t talk to these drives correctly. An example of such a drive would be the Kingston Data Traveller 100 G3 32G.

These bugs exist due to the method used to develop the boot code and squeeze it into 32kB. It simply wasn’t possible to run comprehensive tests.

However, thanks to a thorough search of eBay and some rigorous testing by our awesome work experience student Henry Budden, we’ve found the following devices work perfectly well:

  • Sandisk Cruzer Fit 16GB
  • Sandisk Cruzer Blade 16Gb
  • Samsung 32GB USB 3.0 drive
  • MeCo 16GB USB 3.0

If you find some devices we haven’t been able to test, we’d be grateful if you’d let us know your results in the comments.

Will it be possible to boot a Pi 1 or Pi 2 using MSD?

Unfortunately not. The boot code is stored in the BCM2837 device only, so the Pi 1, Pi 2, and Pi Zero will all require SD cards.

However, I have been able to boot a Pi 1 and Pi 2 using a very special SD card that only contains the single file bootcode.bin. This is useful if you want to boot a Pi from USB, but don’t want the possible unreliability of an SD card. Don’t mount the SD card from Linux, and it will never get corrupted!

My MSD doesn’t work. Is there something else I can do to get it working?

If you can’t boot from the MSD, then there are some steps that you can take to diagnose the problem. Please note, though, this is very much still a work in progress:

  • Format an SD card as FAT32
  • Copy the current next branch bootcode.bin from GitHub onto the SD card
  • Plug it into the Pi and try again

If this still doesn’t work, please open an issue in the firmware repository.