U-Boot

Important

Make sure you have read Adding new device types first.

U-Boot is a very common sight on embedded devices. It is a very capable piece of Open Source software that supports a diverse range of devices out of the box, and it is easily configurable and modifiable to add support for new devices too. Comprehensive documentation is available on the U-Boot website.

If your new device includes U-Boot then this can be a useful beginning. However, the build of U-Boot on the device may potentially hinder integration due to the wide range of configuration options and behavioural changes available inside a patched U-Boot build.

See also

Vendor builds

Generally, the more components of U-Boot that are disabled or removed from a vendor build, the harder it will be to integrate. If you are able to fully script a U-Boot process from interrupting the bootloader to booting a kernel of your own choice, this will greatly assist in integrating the device into LAVA.

To start the device integration of a device using U-Boot, you must at least have a complete list of all the commands which are necessary to interrupt U-Boot and manually boot the device.

Common U-Boot support

The following elements are common to all U-Boot integration projects. Some information may need to be obtained from the default vendor configuration as shipped on the device. interrupt U-Boot and take a copy of all the settings before you start integration work. Some elements need to be considered during the development or updating of the build of U-Boot on the device itself. Sometimes there might be limitations in the hardware itself. Most devices which can use mainline U-Boot can be integrated into LAVA. Where mainline support is available, it is recommended to replace the vendor-supplied version of U-Boot with a mainline build.

  • Make sure that the U-Boot environment can be modified and that changes are persistent across reboots.
  • Read and understand how base-uboot.jinja2 operates to follow how the various values will be used to create the U-Boot boot commands. Not all of the logic in the base-uboot.jinja2 template is covered in this section.
  • Create a unit test and ensure that the final rendered device configuration matches the needs of the device.

Configuration

Ensure that the U-Boot build supports a string which can be used to interrupt U-Boot and that once interrupted, the prompt is set to a usable string like => or uboot# etc. Make sure that the configuration supports TFTP using commands sent over the serial port. The timeout for interrupting the boot process must be configurable. Typically, the timeout should be at least 5 seconds in the default configuration. There is no need to configure a working automatic boot once the timeout expires. Whilst having a default boot is useful during development, it can be simpler in automation to simply drop to the bootloader prompt if the bootloader could not be interrupted successfully within the timeout. This avoids wasting time watching the device boot into a system which is not the system intended by the test writer. (The default boot would likely fail to support the correct auto-login or, worse, could allow login and then fail to run any tests as the overlay will not have been deployed to that system. Either option is confusing when trying to debug why the test job failed as the actual error is close to the top of the LAVA test logs, not at the end after all of the boot messages have been sent.)

Prompts

Two prompts are required in the device configuration:

interrupt_prompt: {{ interrupt_prompt|default('Hit any key to stop autoboot') }}
bootloader_prompt: {{ bootloader_prompt|default('=>') }}

The interrupt_prompt is seen first as the device boots and then waits for the timeout before attempting to boot automatically.

The bootloader_prompt is seen after the bootloader has been interrupted and after each command is sent to the bootloader.

Many U-Boot configurations use the same prompt strings as the defaults in base-uboot.jinja2, as shown above.

Load addresses

U-Boot typically requires the load addresses to be specified in the commands used to load and execute the downloaded kernel, ramdisk and DTB. The initial load addresses can be obtained from the device in uEnv.txt or in the saved environment of the default U-Boot configuration (via printenv). The load addresses may need changes later to support larger ramdisks or kernels. Some U-Boot devices use different load addresses according to the kernel to be booted, so each address can be specified separately or mapped to an existing value.

{% set bootm_kernel_addr = '0x40007000' %}
{% set bootm_ramdisk_addr = '0x45000000' %}
{% set bootm_dtb_addr = '0x41f00000' %}
{% set bootz_kernel_addr = bootm_kernel_addr %}
{% set bootz_ramdisk_addr = bootm_ramdisk_addr %}
{% set bootz_dtb_addr = bootm_dtb_addr %}

Required configuration

At a minimum, any new U-Boot device requires the following pieces of configuration:

  • console device - There seems to be no standard or default here, so every request needs to specify the argument to pass to console= on the kernel command line, including baud rate.
  • load addresses - Kernel, ramdisk and DTB load addresses.
  • mkimage arch - the architecture value to pass to mkimage when preparing modified uImage or uboot headers.
  • MAC address - if the MAC address is not pre-configured as a guaranteed unique address, a way of setting a fixed and unique MAC address must be provided.
  • boot methods - booti, bootz and bootm - which ones are supported on this device?
  • prompts - What is the configured U-Boot prompt on the required build of U-Boot for the device. Has the autoboot prompt been modified and if so, what is the autoboot prompt?

Booting the kernel

When this goes wrong, the infamous Bad Linux magic error can be seen. Retrieve the available boot methods from the existing U-Boot configuration, typically one or more of bootz, booti or bootm.

If booti_kernel_addr is set, image parameters will be set for the ramdisk and the DTB.

If bootm_kernel_addr is set, uimage parameters will be set for the ramdisk and the DTB.

If bootz_kernel_addr is set, zimage parameters will be set for the ramdisk and the DTB.

U-Boot bootargs

U-Boot uses the bootargs (“boot arguments”) variable to specify the command line when booting a Linux kernel. This can be critical in determining whether a device boots at all or whether particular hardware is available in the booted system. Equally, some bootargs settings can be entirely cosmetic and simply add (or silence) messages during the boot process. Experiment with your board to work out which bootargs are mandatory for all boots, which are useful as defaults but which can be omitted for some test jobs and which are entirely optional.

Mandatory bootargs need to be put into the template as hard-coded strings. Useful bootargs can be set as the default value of {{base_kernel_args}}. Optional bootargs can be left as comments for test writers to supply via the job context and then added to the bootargs using {{extra_kernel_args}}.

Using mkimage

U-Boot typically requires use of the mkimage binary in various ways. Most commonly, a test job which only boots a ramdisk needs to have the LAVA overlay added to the ramdisk. Many devices then require a U-Boot header to be added to the ramdisk, requiring the add-header flag in the test job submission. mkimage creates a different header for arm than for arm64. The uboot_mkimage_arch value will need to be set according to the requirements of the device.

Note

Most ARMv7 devices will use arm as the architecture and most ARMv8 devices will use arm64, but this is not always the case. For example, the APM Mustang is an arm64 device but the U-Boot build on the Mustang pre-dates arm64 support in mainline U-Boot. It uses {% set uboot_mkimage_arch = 'arm' %}

Vendor builds

Not all devices have mainline U-Boot support and the configurability of the U-Boot source code means that some vendor-supplied builds of U-Boot may behave very differently to those found on other U-Boot devices. Do not assume that options and commands in existing U-Boot devices will always have any equivalent in a vendor build of U-Boot.

Network support

Network support in U-Boot is essential for any useful automation. Specifically, TFTP support in U-Boot needs to work to use any of the existing U-Boot support in LAVA V2.

Additional U-Boot support

Some developers integrating new U-Boot devices may need to consider more elements of U-Boot behaviour and configuration.

Filesystem support

Filesystem support in U-Boot is optional, but will be required for secondary media. Check if U-Boot on the device supports the filesystems you want to use, fat or ext2|3|4. Check if your U-Boot has limits on the sizes of the filesystems it supports. In some cases, it may be necessary to use a separate small /boot partition to make things work.

Interface names

Some configurations of U-Boot may change how interfaces like SATA are accessed by U-Boot. For secondary media support or to read files from an attached storage device, you will need to find out how the U-Boot describes that storage interface (e.g. sata, scsi, usb, mmc).

Initialising subsystems

Some U-Boot devices will not enable some of the onboard storage or peripheral devices without explicitly initialising them first. Some may need other subsystems to be initialised first - for example the Panda needs usb start before networking will work, as the onboard network interface is attached via USB.

Appending the DTB

Some U-Boot configurations support loading a DTB for the device separately, but not all. If your U-Boot does not support this, you will need to append the DTB to the kernel instead. This will obviously affect the commands used to boot your device (e.g. tftp, loadm or bootm), but also remember that you will need to generate this combined image file ready for use on the device.