Coreboot¶

Build Instructions for U-Boot as coreboot payload¶

Building U-Boot as a coreboot payload is just like building U-Boot for targets on other architectures, like below:

$ make coreboot_defconfig
$ make all

Test with coreboot¶

For testing U-Boot as the coreboot payload, there are things that need be paid attention to. coreboot supports loading an ELF executable and a 32-bit plain binary, as well as other supported payloads. With the default configuration, U-Boot is set up to use a separate Device Tree Blob (dtb). As of today, the generated u-boot-dtb.bin needs to be packaged by the cbfstool utility (a tool provided by coreboot) manually as coreboot’s ‘make menuconfig’ does not provide this capability yet. The command is as follows:

# in the coreboot root directory
$ ./build/util/cbfstool/cbfstool build/coreboot.rom add-flat-binary \
  -f u-boot-dtb.bin -n fallback/payload -c lzma -l 0x1110000 -e 0x1110000

Make sure 0x1110000 matches CONFIG_TEXT_BASE, which is the symbol address of _x86boot_start (in arch/x86/cpu/start.S).

If you want to use ELF as the coreboot payload, change U-Boot configuration to use CONFIG_OF_EMBED instead of CONFIG_OF_SEPARATE.

To enable video you must enable these options in coreboot:

Set framebuffer graphics resolution (1280x1024 32k-color (1:5:5))

Keep VESA framebuffer

At present it seems that for Minnowboard Max, coreboot does not pass through the video information correctly (it always says the resolution is 0x0). This works correctly for link though.

64-bit U-Boot¶

In addition to the 32-bit ‘coreboot’ build there is a ‘coreboot64’ build. This produces an image which can be booted from coreboot (32-bit). Internally it works by using a 32-bit SPL binary to switch to 64-bit for running U-Boot. It can be useful for running UEFI applications, for example.

This has only been lightly tested.

Memory map¶

Address

Region at that address

ffffffff

Top of ROM (and last byte of 32-bit address space)

7a9fd000

Typical top of memory available to U-Boot (use cbsysinfo to see where memory range ‘table’ starts)

10000000

Memory reserved by coreboot for mapping PCI devices (typical size 2151000, includes framebuffer)

1920000

CONFIG_SYS_CAR_ADDR, fake Cache-as-RAM memory, used during startup

1110000

CONFIG_TEXT_BASE (start address of U-Boot code, before reloc)

110000

CONFIG_BLOBLIST_ADDR (before being relocated)

100000

CONFIG_PRE_CON_BUF_ADDR

f0000

ACPI tables set up by U-Boot (typically redirects to 7ab10030 or similar)

500

Location of coreboot sysinfo table, used during startup

Debug UART¶

It is possible to enable the debug UART with coreboot. To do this, use the info from the cbsysinfo command to locate the UART base. For example:

=> cbsysinfo
...
Serial I/O port: 00000000
   base        : 00000000
   pointer     : 767b51bc
   type        : 2
   base        : fe03e000
   baud        : 0d115200
   regwidth    : 4
   input_hz    : 0d1843200
   PCI addr    : 00000010
...

Here you can see that the UART base is fe03e000, regwidth is 4 (1 << 2) and the input clock is 1843200. So you can add the following CONFIG options:

CONFIG_DEBUG_UART=y
CONFIG_DEBUG_UART_BASE=fe03e000
CONFIG_DEBUG_UART_CLOCK=1843200
CONFIG_DEBUG_UART_SHIFT=2
CONFIG_DEBUG_UART_ANNOUNCE=y