The old floppy drive edge connector makes for a nice and simple SD socket - the card fits tightly into it. I used the SPI pins on the header for the SPI interface, despite the fact that the actual SPI hardware support isn't used. The spi-gpio and mmc-spi drivers need to be configured in the kernel configuration. I used the 3.1.9 kernel, and here's a patch with my changes.
This is what the Linux kernel reports:
Here's a simple speed test with hdparm -t:[ 5.818182] mmc_spi spi0.0: ASSUMING 3.2-3.4 V slot power
[ 5.827827] mmc_spi spi0.0: SD/MMC host mmc1, no DMA, no WP, no poweroff
[ 6.152809] mmc1: host does not support reading read-only switch. assuming write-enable.
[ 6.182565] mmc1: new SD card on SPI
[ 6.205942] mmcblk1: mmc1:0000 SD01G 972 MiB
The hardware SPI support should make this a lot faster, maybe I'll try again with the 3.2 kernel./dev/mmcblk1:
Timing buffered disk reads: 2 MB in 11.50 seconds = 178.07 kB/sec