mrc.bin¶
All Haswell boards supported by coreboot currently require a proprietary
blob in order to initialise the DRAM and a few other components. The
blob, named mrc.bin, largely consists of Intel’s memory reference code
(MRC), but it has been tailored specifically for ChromeOS. It is just
under 200 KiB in size. Another name for mrc.bin is the system agent
binary.
Having a replacement for mrc.bin using native coreboot code is very
much desired, but it is not an easy task.
Obtaining mrc.bin¶
Unfortunately, it is not currently possible to distribute mrc.bin as
part of coreboot. Though, it can be obtained from a Haswell Chromebook
firmware image like so, starting in the root of the coreboot directory:
make -C util/cbfstool
cd util/chromeos
./crosfirmware.sh peppy
../cbfstool/cbfstool coreboot-*.bin extract -f mrc.bin -n mrc.bin -r RO_SECTION
Now, place mrc.bin in the root of the coreboot directory.
Alternatively, place mrc.bin anywhere you want, and set MRC_FILE to
its location when building coreboot.
SPD Addresses¶
When porting a board from vendor firmware, the SPD addresses can be obtained
through i2c-tools, which can be found in many GNU/Linux distributions. A more
detailed description of
the procedure and beyond can be found in
Hannu Hartikainen’s blog.
First load the kernel modules:
modprobe i2c-dev
modprobe eeprom
Find the SMBus and the addresses of the DIMM’s EEPROMs (example output):
$ decode-dimms | grep Decoding
Decoding EEPROM: /sys/bus/i2c/drivers/eeprom/7-0050
Decoding EEPROM: /sys/bus/i2c/drivers/eeprom/7-0052
Alternatively, look at the sys filesystem:
$ ls -l /sys/bus/i2c/drivers/eeprom/
total 0
lrwxrwxrwx 1 root root 0 Apr 4 01:46 6-0050 -> ../../../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/i2c-6/6-0050/
lrwxrwxrwx 1 root root 0 Apr 4 01:46 7-0050 -> ../../../../devices/pci0000:00/0000:00:1f.3/i2c-7/7-0050/
lrwxrwxrwx 1 root root 0 Apr 4 01:46 7-0052 -> ../../../../devices/pci0000:00/0000:00:1f.3/i2c-7/7-0052/
--w------- 1 root root 4096 Apr 4 01:47 bind
lrwxrwxrwx 1 root root 0 Apr 4 01:47 module -> ../../../../module/eeprom/
--w------- 1 root root 4096 Apr 4 01:46 uevent
--w------- 1 root root 4096 Apr 4 01:47 unbind
The correct I2C bus is 7 in this case, and the EEPROMs are at 0x50 and 0x52.
Note that the above values are actually hex values.
You can check the correctness of the SMBus and the addresses of the EEPROMs via
i2cdetect:
$ i2cdetect -l
i2c-3 unknown i915 gmbus dpc N/A
i2c-1 unknown i915 gmbus vga N/A
i2c-6 unknown DPDDC-A N/A
i2c-4 unknown i915 gmbus dpb N/A
i2c-2 unknown i915 gmbus panel N/A
i2c-0 unknown i915 gmbus ssc N/A
i2c-7 unknown SMBus I801 adapter at f040 N/A
i2c-5 unknown i915 gmbus dpd N/A
Probing the SMBus:
$ i2cdetect -r 7
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-7 using receive byte commands.
I will probe address range 0x03-0x77.
Continue? [Y/n]
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: 30 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- 44 -- -- -- -- -- -- -- -- -- -- --
50: UU -- UU -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
The SPD addresses need to be left-shifted by 1 for mrc.bin, i.e., multiplied
by 2. For example, if the addresses read through i2c-tools when booted from
vendor firmware are 0x50 and 0x52, the correct values would be 0xa0 and
0xa4. This is because the I2C addresses are 7 bits long.
ECC DRAM¶
When mrc.bin has finished executing, ECC is active on the channels
populated with ECC DIMMs. However, mrc.bin was tailored specifically
for Haswell Chromebooks and Chomeboxes, none of which support ECC DRAM.
While ECC likely functions correctly, it is advised to further validate
the correct operation of ECC if data integrity is absolutely critical.