dm: core: Add a clarifying comment on struct udevice's seq member

The sequence number is unique within the uclass, so state this clearly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>

doc/driver-model/README.txt

@@ -359,7 +359,9 @@ Device Sequence Numbers
 U-Boot numbers devices from 0 in many situations, such as in the command
 line for I2C and SPI buses, and the device names for serial ports (serial0,
 serial1, ...). Driver model supports this numbering and permits devices
-to be locating by their 'sequence'.
+to be locating by their 'sequence'. This numbering unique identifies a
+device in its uclass, so no two devices within a particular uclass can have
+the same sequence number.
 
 Sequence numbers start from 0 but gaps are permitted. For example, a board
 may have I2C buses 0, 1, 4, 5 but no 2 or 3. The choice of how devices are

include/dm/device.h

@@ -57,7 +57,8 @@ struct driver_info;
  * @sibling_node: Next device in list of all devices
  * @flags: Flags for this device DM_FLAG_...
  * @req_seq: Requested sequence number for this device (-1 = any)
- * @seq: Allocated sequence number for this device (-1 = none)
+ * @seq: Allocated sequence number for this device (-1 = none). This is set up
+ * when the device is probed and will be unique within the device's uclass.
  */
 struct udevice {
 	struct driver *driver;