usb: storage: add a macro for the upper limit of max LUN

The meaning of this value is already used in several places,
but with constant values and comments to explain it separately.
It's better to have a central place to do this then use the macro
in those places for better readability.

Signed-off-by: Dingyan Li <[e-mail]>
Reviewed-by: Alan Stern <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/function/f_tcm.c

···
 			pr_err("No LUNs configured?\n");
 			return -EINVAL;
 		}
-		/*
-		 * If 4 LUNs are present we return 3 i.e. LUN 0..3 can be
-		 * accessed. The upper limit is 0xf
-		 */
 		luns--;
-		if (luns > 0xf) {
+		if (luns > US_BULK_MAX_LUN_LIMIT) {
 			pr_info_once("Limiting the number of luns to 16\n");
-			luns = 0xf;
+			luns = US_BULK_MAX_LUN_LIMIT;
 		}
 		ret_lun = cdev->req->buf;
 		*ret_lun = luns;

drivers/usb/gadget/function/storage_common.h

···
 #define FSG_BUFLEN	((u32)16384)
 
 /* Maximal number of LUNs supported in mass storage function */
-#define FSG_MAX_LUNS	16
+#define FSG_MAX_LUNS	(US_BULK_MAX_LUN_LIMIT + 1)
 
 enum fsg_buffer_state {
 	BUF_STATE_SENDING = -2,

drivers/usb/storage/transport.c

···
 	usb_stor_dbg(us, "GetMaxLUN command result is %d, data is %d\n",
 		     result, us->iobuf[0]);
 
-	/*
-	 * If we have a successful request, return the result if valid. The
-	 * CBW LUN field is 4 bits wide, so the value reported by the device
-	 * should fit into that.
-	 */
+	/* If we have a successful request, return the result if valid. */
 	if (result > 0) {
-		if (us->iobuf[0] < 16) {
+		if (us->iobuf[0] <= US_BULK_MAX_LUN_LIMIT) {
 			return us->iobuf[0];
 		} else {
 			dev_info(&us->pusb_intf->dev,

include/linux/usb/storage.h

···
 #define US_BULK_RESET_REQUEST   0xff
 #define US_BULK_GET_MAX_LUN     0xfe
 
+/*
+ * If 4 LUNs are supported then the LUNs would be
+ * numbered from 0 to 3, and the return value for
+ * US_BULK_GET_MAX_LUN request would be 3. The valid
+ * LUN field is 4 bits wide, the upper limit is 0x0f.
+ */
+#define US_BULK_MAX_LUN_LIMIT   0x0f
+
 #endif