arm64: Correctly unmask asynchronous SError interrupts

Arm CPUs have always had an odd feature that allows you to mask not only
true interrupts, but also "external aborts" (memory bus errors from
outside the CPU). CPUs usually have all of these masked after reset,
which we quickly learned was a bad idea back when bringing up the first
arm32 systems in coreboot. Masking external aborts means that if any of
your firmware code does an illegal memory access, you will only see it
once the kernel comes up and unmasks the abort (not when it happens).

Therefore, we always unmask everything in early bootblock assembly code.
When arm64 came around, it had very similar masking bits and we did the
same there, thinking the issue resolved. Unfortunately Arm, in their
ceaseless struggle for more complexity, decided that having a single bit
to control this masking behavior is no longer enough: on AArch64, in
addition to the PSTATE.DAIF bits that are analogous to arm32's CPSR,
there are additional bits in SCR_EL3 that can override the PSTATE
setting for some but not all cases (makes perfect sense, I know...).
When aborts are unmasked in PSTATE, but SCR.EA is not set, then
synchronous external aborts will cause an exception while asynchronous
external aborts will not. It turns out we never intialize SCR in
coreboot and on RK3399 it comes up with all zeroes (even the reserved-1
bits, which is super weird). If you get an asynchronous external abort
in coreboot it will silently hide in the CPU until BL31 enables SCR.EA
before it has its own console handlers registered and silently hangs.

This patch resolves the issue by also initializing SCR to a known good
state early in the bootblock. It also cleans up some bit defintions and
slightly reworks the DAIF unmasking... it doesn't actually make that
much sense to unmask anything before our console and exception handlers
are up. The new code will mask everything until the exception handler is
installed and then unmask it, so that if there was a super early
external abort we could still see it. (Of course there are still dozens
of other processor exceptions that could happen which we have no way to

Change-Id: I5266481a7aaf0b72aca8988accb671d92739af6f
Signed-off-by: Julius Werner <>
Tested-by: build bot (Jenkins) <>
Reviewed-by: Hung-Te Lin <>
5 files changed
tree: f8b652805a320d188ea995bc88bca8837118cfe9
  1. .checkpatch.conf
  2. .clang-format
  3. .editorconfig
  4. .gitignore
  5. .gitmodules
  6. .gitreview
  7. 3rdparty/
  10. Documentation/
  13. Makefile
  16. configs/
  17. gnat.adc
  18. payloads/
  19. src/
  21. util/

coreboot README

coreboot is a Free Software project aimed at replacing the proprietary BIOS (firmware) found in most computers. coreboot performs a little bit of hardware initialization and then executes additional boot logic, called a payload.

With the separation of hardware initialization and later boot logic, coreboot can scale from specialized applications that run directly firmware, run operating systems in flash, load custom bootloaders, or implement firmware standards, like PC BIOS services or UEFI. This allows for systems to only include the features necessary in the target application, reducing the amount of code and flash space required.

coreboot was formerly known as LinuxBIOS.


After the basic initialization of the hardware has been performed, any desired "payload" can be started by coreboot.

See for a list of supported payloads.

Supported Hardware

coreboot supports a wide range of chipsets, devices, and mainboards.

For details please consult:

Build Requirements

  • make
  • gcc / g++ Because Linux distribution compilers tend to use lots of patches. coreboot does lots of "unusual" things in its build system, some of which break due to those patches, sometimes by gcc aborting, sometimes - and that's worse - by generating broken object code. Two options: use our toolchain (eg. make crosstools-i386) or enable the ANY_TOOLCHAIN Kconfig option if you're feeling lucky (no support in this case).
  • iasl (for targets with ACPI support)
  • pkg-config
  • libssl-dev (openssl)


  • doxygen (for generating/viewing documentation)
  • gdb (for better debugging facilities on some targets)
  • ncurses (for make menuconfig and make nconfig)
  • flex and bison (for regenerating parsers)

Building coreboot

Please consult for details.

Testing coreboot Without Modifying Your Hardware

If you want to test coreboot without any risks before you really decide to use it on your hardware, you can use the QEMU system emulator to run coreboot virtually in QEMU.

Please see for details.

Website and Mailing List

Further details on the project, a FAQ, many HOWTOs, news, development guidelines and more can be found on the coreboot website:

You can contact us directly on the coreboot mailing list:

Copyright and License

The copyright on coreboot is owned by quite a large number of individual developers and companies. Please check the individual source files for details.

coreboot is licensed under the terms of the GNU General Public License (GPL). Some files are licensed under the "GPL (version 2, or any later version)", and some files are licensed under the "GPL, version 2". For some parts, which were derived from other projects, other (GPL-compatible) licenses may apply. Please check the individual source files for details.

This makes the resulting coreboot images licensed under the GPL, version 2.