NVMe: Fix START_STOP_UNIT Scsi->NVMe translation.

This patch contains several fixes for Scsi START_STOP_UNIT. The previous
code did not account for signed vs. unsigned arithmetic which resulted
in an invalid lowest power state caculation when the device only supports
1 power state.

The code for Power Condition == 2 (Idle) was not following the spec. The
spec calls for setting the device to specific power states, depending
upon Power Condition Modifier, without accounting for the number of
power states supported by the device.

The code for Power Condition == 3 (Standby) was using a hard-coded '0'
which is replaced with the macro POWER_STATE_0.
Signed-off-by: Dan McLeran <daniel.mcleran@intel.com>
Reviewed-by: Vishal Verma <vishal.l.verma@linux.intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>

drivers/block/nvme-scsi.c

@@ -1476,7 +1476,7 @@ static int nvme_trans_power_state(struct nvme_ns *ns, struct sg_io_hdr *hdr,
 		goto out_dma;
 	}
 	id_ctrl = mem;
-	lowest_pow_st = id_ctrl->npss - 1;
+	lowest_pow_st = max(POWER_STATE_0, (int)(id_ctrl->npss - 1));
 
 	switch (pc) {
 	case NVME_POWER_STATE_START_VALID:
@@ -1493,20 +1493,19 @@ static int nvme_trans_power_state(struct nvme_ns *ns, struct sg_io_hdr *hdr,
 		break;
 	case NVME_POWER_STATE_IDLE:
 		/* Action unspecified if POWER CONDITION MODIFIER != [0,1,2] */
-		/* min of desired state and (lps-1) because lps is STOP */
 		if (pcmod == 0x0)
-			ps_desired = min(POWER_STATE_1, (lowest_pow_st - 1));
+			ps_desired = POWER_STATE_1;
 		else if (pcmod == 0x1)
-			ps_desired = min(POWER_STATE_2, (lowest_pow_st - 1));
+			ps_desired = POWER_STATE_2;
 		else if (pcmod == 0x2)
-			ps_desired = min(POWER_STATE_3, (lowest_pow_st - 1));
+			ps_desired = POWER_STATE_3;
 		break;
 	case NVME_POWER_STATE_STANDBY:
 		/* Action unspecified if POWER CONDITION MODIFIER != [0,1] */
 		if (pcmod == 0x0)
-			ps_desired = max(0, (lowest_pow_st - 2));
+			ps_desired = max(POWER_STATE_0, (lowest_pow_st - 2));
 		else if (pcmod == 0x1)
-			ps_desired = max(0, (lowest_pow_st - 1));
+			ps_desired = max(POWER_STATE_0, (lowest_pow_st - 1));
 		break;
 	case NVME_POWER_STATE_LU_CONTROL:
 	default: