So I did many more tests today (more than 6h, and flashing around 30 times), with SPD settings hardcoded into raminit, and without the mrc cache interfering.
TLDR: coreboot tries to increase the frequency without increasing the voltage, and that doesn't work for all memory.
Basically, with the problematic ram sticks, I can boot perfectly fine at DDR3-1866 speed, but even the slower setting 11-11-11-31 gives errors on the memtest. This is inconsistent with information I found about my memory from its SPD information, and from other people who overclock this exact same memory.
Even at 10-10-10-27, I still get errors at DDR3-1600 speeds. Far fewer than before, but some errors sill.
After reading more about XMP and SPD, it is my understanding that : - JEDEC specs stop at 1600, and after that XMP is required - even before 1600, XMP also offers profiles, and they are not optional: some memory is otherwise unable to work at its advertised speed - XMP profiles are some kind of overclocking: they usually require adjusting the voltage, to deal with this increased speed - in XMP profile bytes, voltage increase information is given precisely - nowhere in the code I saw anything increasing the voltage, while XMP requires that
I conclude that while there may be errors in selecting the SPD settings, even if the SPD is manually corrected with known-good settings, or if overshooting with very generous latencies, some errors do remain as the ram is being asked to operate outside its voltage specifications (given the frequency)
1.5V is a JEDEC spec, but RAM is advertised based on the information contained in the XMP profiles, which at the moment do not seem fully supported.
I do not know how to adjust the voltage (it should require talking to the IMC of the CPU) but I think that as soon as this is done, stability should improve.
If someone can propose a patch doing that (either using the voltage read from SPD, or by manually entering voltage information), I will be happy to test it.
For now, I urge caution when operating even at DDR-1866 frequencies. Most boards do set up 933 as their max_mem_clock_mhz. It is not very prudent to do that until the voltage situation can be solved.
Charlote
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Charlotte Plusplus