DSi SD card init hangs forever on ACMD51 (GET_SCR) with some cards #8

Bug Report

What's the issue you encountered?

sdmmcCardInit never returns for my SanDisk Extreme 16 GB SDHC UHS-I. this could affect other sd cards too.

How can the issue be reproduced?

launching ftpd v3.2.1 in a nintendo dsi xl in dsi mode, built against calico 1.0.0, using the SanDisk Extreme 16 GB SDHC UHS-I. you will encounter a white screen. using another sd card with identical contents fixes this

every command up to and including ACMD6 succeeds and then the ACMD51 (GET_SCR) transaction just doesn't complete. i ran some tests on a custom ARM7 and may PR later if I can find a fix

Evidence

tested the command sequence in source/dev/sdmmc.c and recorded each tmioTransact call in shared RAM, which the ARM9 prints:

polls 3 ocr C0FF8000
c00 00000000>00000000 e00000000
c08 000001AA>000001AA e00000000
c55 00000000>00000120 e00000000
A41 40100000>40FF8000 e00000000
c02 00000000>5E2E00E8 e00000000
c03 00000000>AAAA0520 e00000000
c09 AAAA0000>0A404000 e00000000
c07 AAAA0000>00000700 e00000000
c16 00000200>00000900 e00000000
c55 AAAA0000>00000920 e00000000
A42 00000000>00000920 e00000000
c55 AAAA0000>00000920 e00000000
A06 00000000>00000920 e00000000
c55 AAAA0000>00000920 e00000000
A51 00000000>

the last line stays pending after more than 60 seconds, so fault is likely around there