libata: Power off empty ports

Give users the option of completely powering off unoccupied
SATA ports using the existing min_power link_power_management_policy
option.  When the use selects this option on an empty port, we
will power the port off by setting DET to off.  For occupied ports,
behavior is unchanged.
Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>

drivers/ata/libata-core.c

@@ -3619,8 +3619,14 @@ int sata_link_scr_lpm(struct ata_link *link, enum ata_lpm_policy policy,
 		scontrol |= (0x2 << 8);
 		break;
 	case ATA_LPM_MIN_POWER:
+		if (ata_link_nr_enabled(link) > 0)
 			/* no restrictions on LPM transitions */
 			scontrol &= ~(0x3 << 8);
+		else {
+			/* empty port, power off */
+			scontrol &= ~0xf;
+			scontrol |= (0x1 << 2);
+		}
 		break;
 	default:
 		WARN_ON(1);

drivers/ata/libata-eh.c

@@ -3423,7 +3423,7 @@ static int ata_eh_set_lpm(struct ata_link *link, enum ata_lpm_policy policy,
 	return rc;
 }
 
-static int ata_link_nr_enabled(struct ata_link *link)
+int ata_link_nr_enabled(struct ata_link *link)
 {
 	struct ata_device *dev;
 	int cnt = 0;

include/linux/libata.h

@@ -1151,6 +1151,7 @@ extern void ata_do_eh(struct ata_port *ap, ata_prereset_fn_t prereset,
 		      ata_reset_fn_t softreset, ata_reset_fn_t hardreset,
 		      ata_postreset_fn_t postreset);
 extern void ata_std_error_handler(struct ata_port *ap);
+extern int ata_link_nr_enabled(struct ata_link *link);
 
 /*
  * Base operations to inherit from and initializers for sht