The SDIO tool can be used to read or write registers on the card, stress test/benchmark the bus, or print information about the card and controller.
The SDIO tool takes an SDIO function device path as the first argument, e.g. /dev/class/sdio/000. The second argument is the name of the command to invoke (listed below). All read and write commands take a numerical card address as the third argument. The width of this field is 17 bits, so the address must be in the range [0, 0x1ffff]. Numerical fields may be specified in hexadecimal (0x prefix) or decimal (no prefix).
sdio <device> info
Prints everything we know about the controller and the card.
sdio <device> read-byte <address>
Reads one byte from the given address and prints it in hexadecimal.
sdio <device> write-byte <address>
Writes one byte to the given address.
sdio <device> read-stress <address> <size> <loops> [--fifo] [--dma]
Reads loops chunks of data from the given address, where each chunk is size bytes. The data may be read from a fixed address (--fifo) and may be done using DMA (--dma). At the end, statistics about the transfer (total time and throughput) are printed.