coreboot-kgpe-d16/Documentation/soc/intel/broadwell/blobs.md

Blobs used in Intel Broadwell boards

All Broadwell boards supported by coreboot require two proprietary blobs. In a coreboot image of a Broadwell board, the blobs are named mrc.bin and refcode in CBFS.

mrc.bin is run in romstage to initialize the memory. It is placed at a fixed address in CBFS and is loaded at a fixed address in memory.

refcode is run in ramstage to initialize the system agent and the PCH. It is a relocatable ELF object.

Obtaining the blobs

Both mrc.bin and refcode can be obtained from a coreboot image of a Broadwell board, for example a Purism Librem 13 v1 coreboot image from MrChromebox.

cbfstool coreboot_*.rom extract -f broadwell-mrc.bin -n mrc.bin
cbfstool coreboot_*.rom extract -m x86 -f broadwell-refcode.elf -n fallback/refcode

SPD Addresses

The SPD addresses in Broadwell pei_data struct are similar to Haswell.

Intel GbE support

Unlike Haswell boards, the pei_data struct of Broadwell doesn't have gbe_enable field. For boards with an Intel GbE device, a modification of refcode is needed, otherwise refcode will disable the Intel GbE device and the OS cannot find it in the list of PCI devices.

For the refcode binary extracted from Purism Librem 13 v1 (SHA256: 8a919ffece61ba21664b1028b0ebbfabcd727d90c1ae2f72b48152b8774323a4, .program section starts at file offset 0x2040), we can see the following code sequence:

1e06b:       c6 43 0c 00             movb   $0x0,0xc(%ebx)
1e06f:       c6 83 7e 03 00 00 00    movb   $0x0,0x37e(%ebx)
1e076:       c6 83 70 03 00 00 01    movb   $0x1,0x370(%ebx)
1e07d:       66 89 43 0a             mov    %ax,0xa(%ebx)
1e081:       c6 83 da 01 00 00 01    movb   $0x1,0x1da(%ebx)
1e088:       c6 83 86 03 00 00 01    movb   $0x1,0x386(%ebx)

The code at 0x1e06f sets the field that is to enable the GbE to the hardcoded 0 value. Change the byte at 0x1e075 (file offset 0x200b5) to 0x01 to make the refcode support Intel GbE:

cp refcode.elf refcode_gbe.elf
printf '\x01' | dd of=refcode_gbe.elf bs=1 seek=131253 count=1 conv=notrunc

Use Broadwell SoC code for Haswell ULT boards

Haswell ULT boards can use Broadwell SoC code. To use Broadwell code for Haswell ULT boards, devicetree.cb file and pei_data code need to be ported to Broadwell, and build the code with Broadwell mrc.bin and refcode instead of using Haswell mrc.bin.

Broadwell SoC code doesn't support non-ULT Haswell or non-ULT Broadwell boards.