igb: Clarify idleslope config constraints

By design, the idleslope increments are restricted to 16.384kbps steps.
Add a comment to igb_main.c making that explicit and add one example
that illustrates the impact of that.
Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -1744,6 +1744,20 @@ static void igb_configure_cbs(struct igb_adapter *adapter, int queue,
 		 *     value = idleSlope * 61034
 		 *             -----------------                          (E6)
 		 *                  1000000
+		 *
+		 * NOTE: For i210, given the above, we can see that idleslope
+		 *       is represented in 16.38431 kbps units by the value at
+		 *       the TQAVCC register (1Gbps / 61034), which reduces
+		 *       the granularity for idleslope increments.
+		 *       For instance, if you want to configure a 2576kbps
+		 *       idleslope, the value to be written on the register
+		 *       would have to be 157.23. If rounded down, you end
+		 *       up with less bandwidth available than originally
+		 *       required (~2572 kbps). If rounded up, you end up
+		 *       with a higher bandwidth (~2589 kbps). Below the
+		 *       approach we take is to always round up the
+		 *       calculated value, so the resulting bandwidth might
+		 *       be slightly higher for some configurations.
 		 */
 		value = DIV_ROUND_UP_ULL(idleslope * 61034ULL, 1000000);