void ata_bus_reset (struct ata_port * ap);
This is typically the first time we actually start issuing commands to the ATA channel. We wait for BSY to clear, then issue EXECUTE DEVICE DIAGNOSTIC command, polling for its result. Determine what devices, if any, are on the channel by looking at the device 0/1 error register. Look at the signature stored in each device's taskfile registers, to determine if the device is ATA or ATAPI.
PCI/etc. bus probe sem. Obtains host_set lock.
Sets ATA_FLAG_PORT_DISABLED if bus reset fails.