# Boot Device Selection

## ARM firmware

As already mentioned before, there is unfortunately no standard for system firmware on ARM systems. Even the behaviour of different systems which use nominally the same firmware can be quite different. This results from the fact that a large part of the devices using the ARM architecture are embedded systems, for which the manufacturers usually build heavily customized firmware versions and include device-specific patches. Unfortunately the manufacturers often do not submit their changes and extensions back to the mainline firmware developers, so their changes are not integrated into newer versions of the original firmware.

As a result even newly sold systems often use a firmware that is based on a years-old manufacturer-modified version of a firmware whose mainline codebase has evolved a lot further in the meantime and offers additional features or shows different behaviour in certain aspects. In addition to that, the naming of onboard devices is not consistent between different manufacturer-modified versions of the same firmware, therefore it is nearly impossible to provide usable product-independend instructions for ARM-based systems.

## Debian-provided U-Boot (system firmware) images

Debian provides U-Boot images for various armhf systems that can load their U-Boot from an SD card at &armmp-uboot-img;. The U-Boot builds are offered in two formats: raw U-Boot components and a ready-made card image that can easily be written onto an SD card. The raw U-Boot components are provided for advanced users; the generally recommended way is to use one of the ready-made SD card images. They are named <system-type>.sdcard.img.gz and can be written to a card e.g. with

    zcat <system-type>.sdcard.img.gz > /dev/SD_CARD_DEVICE

As with all images, please be aware that writing the image to an SD card wipes all previous contents of the card!

If Debian provides a U-Boot image for your system, it is recommended that you use this image instead of the vendor-provided U-Boot, as the version in Debian is usually newer and has more features.

## Setting the ethernet MAC address in U-Boot

The MAC address of every ethernet interface should normally be globally unique, and it technically has to be unique within its ethernet broadcast domain. To achieve this, the manufacturer usually allocates a block of MAC addresses from a centrally-administered pool (for which a fee has to be paid) and preconfigures one of these addresses on each item sold.

In the case of development boards, sometimes the manufacturer wants to avoid paying these fees and therefore provides no globally unique addresses. In these cases the users themselves have to define MAC addresses for their systems. When no MAC address is defined for an ethernet interface, some network drivers generate a random MAC address that can change on every boot, and if this happens, network access would be possible even when the user has not manually set an address, but e.g. assigning semi-static IP addresses by DHCP based on the MAC address of the requesting client would obviously not work reliably.

To avoid conflicts with existing officially-assigned MAC addresses, there is an address pool which is reserved for so-called locally administered addresses. It is defined by the value of two specific bits in the first byte of the address (the article MAC address in the English language Wikipedia gives a good explanation). In practice this means that e.g. any address starting with hexadecimal ca (such as ca:ff:ee:12:34:56) can be used as a locally administered address.

On systems using U-Boot as system firmware, the ethernet MAC address is placed in the ethaddr environment variable. It can be checked at the U-Boot command prompt with the command printenv ethaddr and can be set with the command setenv ethaddr ca:ff:ee:12:34:56. After setting the value, the command saveenv makes the assignment permanent.

## Kernel/Initrd/Device-Tree relocation issues in U-Boot

On some systems with older U-Boot versions there can be problems with properly relocating the Linux kernel, the initial ramdisk and the device-tree blob in memory during the boot process. In this case, U-Boot shows the message Starting kernel ..., but the system freezes afterwards without further output. These issues have been solved with newer U-Boot versions from v2014.07 onwards.

If the system has originally used a U-Boot version older than v2014.07 and has been upgraded to a newer version later, the problem might still occur even after upgrading U-Boot. Upgrading U-Boot usually does not modify the existing U-Boot environment variables and the fix requires an additional environment variable (bootm_size) to be set, which U-Boot does automatically only on fresh installations without existing environment data. It is possible to manually set bootm_size to the new U-Boot's default value by running the command env default bootm_size; saveenv at the U-Boot prompt.

Another possibility to circumvent relocation-related problems is to run the command setenv fdt_high ffffffff; setenv initrd_high 0xffffffff; saveenv at the U-Boot prompt to completely disable the relocation of the initial ramdisk and the device-tree blob.