Building a software serial driver

Greetings fellow newbies. I'm toying around with a Raspberry PI and trying to create a driver that takes a single GPIO pin and turns it into a read-only TTY device because using the userspace GPIO sysfs API is too slow for me. Here's my current code: https://github.com/tdfischer/gpio_serial It is based off of the tiny_serial example driver in the LDD3 repo: https://github.com/martinezjavier/ldd3/blob/master/tty/tiny_serial.c It builds and loads just fine, and even creates a /dev/ttyGPIO0 device. However, reading from it returns -EIO. Running strace on cat shows that it opens up the device successfully, but fails on the first call to read(). Any idea what I'm missing here? I'm fairly certain it is something obvious.