drm: xlnx: zynqmp: Use switch - case for link rate downshift

Use switch - case to downshift from the current link rate. It's a small
loop now, so fine to be replaced with switch - case. With a loop, it is
confusing and hard to follow as reported below.

The patch d76271d2: "drm: xlnx: DRM/KMS driver for Xilinx ZynqMP
DisplayPort Subsystem" from Jul 7, 2018, leads to the following
static checker warning:

	drivers/gpu/drm/xlnx/zynqmp_dp.c:594 zynqmp_dp_mode_configure()
	error: iterator underflow 'bws' (-1)-2
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1596065445-4630-1-git-send-email-hyun.kwon@xilinx.com

drivers/gpu/drm/xlnx/zynqmp_dp.c

@@ -567,34 +567,37 @@ static int zynqmp_dp_mode_configure(struct zynqmp_dp *dp, int pclock,
 				    u8 current_bw)
 {
 	int max_rate = dp->link_config.max_rate;
-	u8 bws[3] = { DP_LINK_BW_1_62, DP_LINK_BW_2_7, DP_LINK_BW_5_4 };
+	u8 bw_code;
 	u8 max_lanes = dp->link_config.max_lanes;
 	u8 max_link_rate_code = drm_dp_link_rate_to_bw_code(max_rate);
 	u8 bpp = dp->config.bpp;
 	u8 lane_cnt;
-	s8 i;
 
-	if (current_bw == DP_LINK_BW_1_62) {
+	/* Downshift from current bandwidth */
+	switch (current_bw) {
+	case DP_LINK_BW_5_4:
+		bw_code = DP_LINK_BW_2_7;
+		break;
+	case DP_LINK_BW_2_7:
+		bw_code = DP_LINK_BW_1_62;
+		break;
+	case DP_LINK_BW_1_62:
 		dev_err(dp->dev, "can't downshift. already lowest link rate\n");
 		return -EINVAL;
-	}
-
-	for (i = ARRAY_SIZE(bws) - 1; i >= 0; i--) {
-		if (current_bw && bws[i] >= current_bw)
-			continue;
-
-		if (bws[i] <= max_link_rate_code)
-			break;
+	default:
+		/* If not given, start with max supported */
+		bw_code = max_link_rate_code;
+		break;
 	}
 
 	for (lane_cnt = 1; lane_cnt <= max_lanes; lane_cnt <<= 1) {
 		int bw;
 		u32 rate;
 
-		bw = drm_dp_bw_code_to_link_rate(bws[i]);
+		bw = drm_dp_bw_code_to_link_rate(bw_code);
 		rate = zynqmp_dp_max_rate(bw, lane_cnt, bpp);
 		if (pclock <= rate) {
-			dp->mode.bw_code = bws[i];
+			dp->mode.bw_code = bw_code;
 			dp->mode.lane_cnt = lane_cnt;
 			dp->mode.pclock = pclock;
 			return dp->mode.bw_code;