ata_mmio_devchk — PATA device presence detection
unsigned int ata_mmio_devchk (struct ata_port * ap, unsigned int device);
This technique was originally described in Hale Landis's ATADRVR (www.ata-atapi.com), and later found its way into the ATA/ATAPI spec.
Write a pattern to the ATA shadow registers, and if a device is present, it will respond by correctly storing and echoing back the ATA shadow register contents.