It seems some standard combinations as recommended by the manual do not work with coreboot while some non-standard combinations do, so one cannot really refer to the ASUS manual for memory installation if using coreboot.
However, it seems memory is not the main issue here as I just tried using 4 x 8GB Micron DDR3-1600 ECC Unbuffered sticks for 32GB, and it did not make any difference compared to 2 x 16GB Samsung DDR3-1600 ECC REG VLP sticks that I previously used.
The issue I'm having is more likely USB-related and it seems the issue is with libpayload. The function "dma_memalign" might be the culprit, as the error message "Not enough memory creating EHCI periodic frame list" is to happen if dma_memalign returned null.
Reference: payloads/libpayload/drivers/usb/ehci.c (from line 809): /* Initialize periodic frame list */ /* 1024 32-bit pointers, 4kb aligned */ u32 *const periodic_list = (u32 *)dma_memalign(4096, 1024 * sizeof(u32)); if (!periodic_list) fatal("Not enough memory creating EHCI periodic frame list.\n");
By the way, I'm using two Opteron 6386 SE CPUs. Not sure if 6300 family requires something else other than microcode (which has been generated from tree).