[ haswell i7-4600] Why does accessing data in MCHBAR hang the boar?

[Zheng Bao]

After the MRC.bin is loaded and run, the board hang at  accessing data in MCHBAR32. If I move MCHBAR32 backward
before the MRC.bin runs, it still hangs. It seems has nothing to do with MRC.bin, does it?

/* For reference print the System Agent version
     * after executing the UEFI PEI stage.
     */
    post_code(0xb1);
    u32 version = MCHBAR32(0x5034);  //<-------hang here
    post_code(0x1b);
    printk(BIOS_DEBUG, "System Agent Version %d.%d.%d Build %d\n",
        version >> 24 , (version >> 16) & 0xff,
        (version >> 8) & 0xff, version & 0xff);

Zheng

------------------
coreboot-4.6-894-ga26344d-dirty Wed Jan 24 05:38:45 UTC 2018 romstage starting...
Disabling Watchdog reboot... done.
SMBus controller enabled.

dimm: 00.0: 50
00: 92 10 0b 03 03 19 00 01 03 52 01 08 0c 00 3e 00
10: 69 78 69 30 69 11 20 89 00 05 3c 3c 00 f0 83 81
20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
30: 00 00 00 00 00 00 00 00 00 00 00 00 0f 11 21 00
40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
70: 00 00 00 00 00 01 98 04 11 41 4f 1a 2d f8 85 10
80: 48 50 35 37 32 32 39 33 2d 44 38 38 2d 45 4c 43
90: 57 47 00 20 02 fe 00 00 00 00 00 00 00 00 00 00
a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 5a
b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Setting up static northbridge registers... done.
Initializing Graphics...
Back from haswell_early_initialization()
CPU id(40651) ucode:0000001c Intel(R) Core(TM) i7-4600U CPU @ 2.10GHz
AES supported, TXT supported, VT supported
PCH type: LP Premium, device id: 9c43, rev id 4
Starting UEFI PEI System Agent ___
CBFS: 'Master Header Locator' located CBFS at [300100:3fffc0)
CBFS: Locating 'mrc.bin'
CBFS: Found @ offset 9fec0 size 2e6e4
System Agent: Starting up...
System Agent: Initializing PCH
install_ppi: overwrite GUID {ed097352-9041-445a-80b6-b29d509e8845}
install_ppi: overwrite GUID {908c7f8b-5c48-47fb-8357-f5fd4e235276}
System Agent: Initializing PCH (SMBUS)
System Agent: Initializing PCH (USB)
System Agent: Initializing PCH (SA Init)
System Agent: Initializing PCH (Me UMA)
System Agent: Initializing Memory
System Agent: Done.
Sanity checking heap.