I have a user running one of my SeaBIOS builds on a Asus Chromebook C302sa (google/cave) who is having trouble with high-capacity SD cards - they aren't detected at all due to timing out during init.  I've tried adjusting the various timeouts, and the only change is longer log files.  

The SD card in question is a Samsung MB-MC128GA U3 Micro SDXC 128GB EVO Plus. Trimmed log below.  The function start indicators were inserted by me as part of debugging.

thanks in advance,
Matt

SeaBIOS (version rel-1.10.0-81-g0fff851-MrChromebox-2017.10.19)
/7a9fe000\ Start thread
|7a9fe000| Searching bootorder for: /pci@i0cf8/*@1e,6
|7a9fe000| sdcard_pci_setup() start
|7a9fe000| sdcard_controller_setup() start
|7a9fe000| sdhci@0xd1138000 ver=1002 cap=7568c881 807
|7a9fe000| sdcard_card_setup() start
|7a9fe000| sdcard_set_frequency 200 400 fa00
|7a9fe000| sdcard_pio cmd 0 0 1ff0000
|7a9fe000| sdcard cmd 0 response 0 0 0 0
|7a9fe000| sdcard_pio cmd 81a 1aa 1ff0000
|7a9fe000| sdcard cmd 81a response 1aa 0 0 0
|7a9fe000| sdcard_pio cmd 371a 0 1ff0000
|7a9fe000| sdcard cmd 371a response 120 0 0 0
|7a9fe000| sdcard_pio cmd 2902 0 1ff0000
|7a9fe000| sdcard cmd 2902 response ff8000 0 0 0
|7a9fe000| sdcard_pio cmd 371a 0 1ff0000
|7a9fe000| sdcard cmd 371a response 120 0 0 0
|7a9fe000| sdcard_pio cmd 2902 40100000 1ff0000
|7a9fe000| sdcard cmd 2902 response ff8000 0 0 0
|7a9fe000| sdcard_pio cmd 371a 0 1ff0000
|7a9fe000| sdcard cmd 371a response 120 0 0 0
|7a9fe000| sdcard_pio cmd 2902 40100000 1ff0000
|7a9fe000| sdcard cmd 2902 response ff8000 0 0 0
<snip ~1s of 371a/2902 sequence repeating>
|7a9fe000| WARNING - Timeout at sdcard_card_setup:432!
\7a9fe000/ End thread
All threads complete.