I'm trying to hook up a Wifi chip via SDIO to the SD1 interface (GPIO22-27, ALT3) on the compute module. I've enabled the sdio-overlay and see linux attempt to initialize mmc1 but it fails repeating this message several times:
[ 2.283907] mmc1: host does not support reading read-only switch, assuming write-enable
[ 2.300090] mmc1: error -110 whilst initialising SD card
I tried compiling both a stable rpi-4.1.y kernel as well the most recent rpi-4.2.y branch with no noticeable difference in behaviour. I see reasonably clean signals @ 3.3V on the interface and, with a logic analyser, I can see what looks like a good MMC initialization sequence (CLK@400kHz, CMD signalling, and, eventually activity on DAT1). To rule out hardware initialization problems with the Wifi chip, I also tried wiring up a microSD card but with no success.
1. What might I be missing here? Should I be able to use this interface for SD (not SD via SPI)? These posts [1, 2] hinted to me that this should be possible so I'm probably doing something silly.
2. Does this SD interface support the 1.8V signalling mode?
Many thanks for any insights.
(My first post! Please excuse me if this is not the right forum for this question.)