libata: Check PCI sub-class code before disabling AHCI

This patch adds functionality to check the PCI sub-class code of an
AHCI capable device before disabling AHCI.  It fixes a bug where an
ICH7 sata controller is being setup by the BIOS as sub-class 1 (ide)
and the AHCI control registers weren't being initialized, thus causing
an IO error in piix_disable_ahci().
Signed-off-by: Gregory Felix <greg.felix@gmail.com>

drivers/scsi/ata_piix.c

@@ -38,6 +38,7 @@ enum {
 	PIIX_IOCFG		= 0x54, /* IDE I/O configuration register */
 	ICH5_PMR		= 0x90, /* port mapping register */
 	ICH5_PCS		= 0x92,	/* port control and status */
+	PIIX_SCC		= 0x0A, /* sub-class code register */
 
 	PIIX_FLAG_AHCI		= (1 << 28), /* AHCI possible */
 	PIIX_FLAG_CHECKINTR	= (1 << 29), /* make sure PCI INTx enabled */
@@ -62,6 +63,8 @@ enum {
 	ich6_sata_rm		= 4,
 	ich7_sata		= 5,
 	esb2_sata		= 6,
+
+	PIIX_AHCI_DEVICE	= 6,
 };
 
 static int piix_init_one (struct pci_dev *pdev,
@@ -626,10 +629,14 @@ static int piix_init_one (struct pci_dev *pdev, const struct pci_device_id *ent)
 	port_info[1] = NULL;
 
 	if (port_info[0]->host_flags & PIIX_FLAG_AHCI) {
+               u8 tmp;
+               pci_read_config_byte(pdev, PIIX_SCC, &tmp);
+               if (tmp == PIIX_AHCI_DEVICE) {
                        int rc = piix_disable_ahci(pdev);
                        if (rc)
                            return rc;
                }
+	}
 
 	if (port_info[0]->host_flags & PIIX_FLAG_COMBINED) {
 		u8 tmp;