Mainline EDK2 used to carry support for a number of ARM development platforms, such as TC2 and Juno (both of which are based on Versatile Express). These have been moved to OpenPlatformPkg, a separate platforms tree that is intended to complement the EDK2 mainline tree, and carries support for a number of platforms based on the ARM architecture (although non-ARM platforms are more than welcome as well).
Recently, EDK2 has gone back to only supporting various emulators (custom emulators built for Windows or X11, but also the QEMU system emulator in X86 or ARM mode) in its mainline tree, but the intention is to merge the entirety of OpenPlatformPkg back into EDK2 once a reorganization of the directory structure is completed. Until then, OpenPlatformPkg could be considered ‘upstream’ for all intents and purposes, as far as bare metal ARM platforms are concerned.
Upstream support for AMD Overdrive in EDK2
AMD is widely recognized for its efforts in open source, and as one of the founding members of the Linaro Enterprise Group (LEG), it has put its weight behind the work Linaro is doing to improve support for ARMv8 based servers in the enterprise.
As part of this effort, the UEFI engineers in LEG have been collaborating with AMD engineers to get support for AMD’s Overdrive platform into the EDK2 upstream. Due to its similarity to Overdrive, UEFI support for the upcoming LeMaker Celloboard is now public as well.
Unlike the Linux kernel community, which has a strict, GPL-based open source policy, the EDK2 community is lax about mixing open and closed source modules, and the fact that the EDK2 upstream by itself can only run on emulators attests to that. However, there is another way to combine the open source core components of EDK2 with closed source special sauce, by combining sources and binaries at the module level.
Binary modules in EDK2
The snippet below was taken from AmdSataInitLib.inf, showing how a static library that was built separately from the platform has been included as a binary module in the Overdrive build. The Binaries section appears instead of the usual Sources section, and contains the static library that makes up the module. (The .lib file in question was simply taken from a build that includes the module in source form, i.e., a .inf file containing the various sources in a Sources section, and a .dsc file that lists the .inf in a Components section.) The trailing asterisk means that the same file should be used for DEBUG and RELEASE builds.
Note the FixedPcd section: a static EDK2 library will contain symbol references to the exact name/type combinations of these PCDs, and so it is recommended to use a strict match here (FixedPcd rather than Pcd)
In a similar way, complete PEI or DXE phase PE/COFF executables can be distributed in binary form as well, with the caveat that dynamic PCDs should be avoided (they simply don’t work)
Taking Gionb.inf as another example,
[Binaries.AARCH64] PE32|Gionb.efi|* PEI_DEPEX|Gionb.depex|*
the first thing that stands out is the PEI_DEPEX line. The .depex file it refers to was taken from the same build that produced the .efi file, and is required by the runtime dispatcher to decide when the Gionb PEI module can be dispatched.
What is especially interesting about this module is the non-standard looking PCD references in the PatchPcd section. It lists the patchable PCDs that are referenced by the module, their default values, and their offsets into the binary (the .efi file from the Binaries section). If this module is incorporated into a platform .DSC that uses different values for these PCDs, the EDK2 build system will patch the desired values into a copy of the binary before incorporating it into the final firmware image. This is an especially powerful feature that allows us to share the Gionb module, which performs the PCIe link training, between the Overdrive and Cello platforms, which have different PCIe slot configurations.
In addition to AmdSataInitLib and Gionb, there are a few other modules that are distributed as binaries: IscpPei and IscpDxe, which produce the protocols to communicate with the SCP, and SnpDxePort0 and SnpDxePort1, which drive the two 10GigE ports.
The Overdrive platform is based on the AMD Seattle SOC, which combines a 32-bit ARM Cortex-A5 based System Control Processor (SCP) with up to 8 64-bit Cortex-A57 cores. The firmware that runs on the A5, and the secure world (EL3) firmware that runs on the A57s has not been published as source code, and is incorporated into the firmware image as a single binary blob. This means that only the code that executes in the same context as UEFI (EL2) has been released (modulo the binary modules mentioned above)
Call for collaboration
Apart from the pieces described above, the Overdrive UEFI firmware is completely open, and can be built and studied by anyone who is interested. This means anyone can upgrade their EDK2 core components if they want to enable things like new hardening features or HTTP boot. It also means people can contribute improvements and enhancements to the existing platform. One thing that is particularly high on my wish list is support for the Overdrive/Cello SD slot, which is simply an SD slot wired to an ARM standard PL022 SPI controller (and the Linux kernel already supports it). If anyone is interested in contributing that, please contact me with a proposal, and I will try to arrange support for it.