igb: Update DMA Coalescing threshold calculation.

This patch updates the DMA Coalescing feature parameters to account for
larger MTUs. Previously, sufficient space may not have been allocated in
the receive buffer, causing packet drop.
Signed-off-by: Matthew Vick <matthew.vick@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

@@ -7061,15 +7061,28 @@ static void igb_init_dmac(struct igb_adapter *adapter, u32 pba)
 			wr32(E1000_DMCTXTH, 0);
 
 			/*
-			 * DMA Coalescing high water mark needs to be higher
-			 * than the RX threshold. set hwm to PBA -  2 * max
-			 * frame size
+			 * DMA Coalescing high water mark needs to be greater
+			 * than the Rx threshold. Set hwm to PBA - max frame
+			 * size in 16B units, capping it at PBA - 6KB.
 			 */
-			hwm = pba - (2 * adapter->max_frame_size);
+			hwm = 64 * pba - adapter->max_frame_size / 16;
+			if (hwm < 64 * (pba - 6))
+				hwm = 64 * (pba - 6);
+			reg = rd32(E1000_FCRTC);
+			reg &= ~E1000_FCRTC_RTH_COAL_MASK;
+			reg |= ((hwm << E1000_FCRTC_RTH_COAL_SHIFT)
+				& E1000_FCRTC_RTH_COAL_MASK);
+			wr32(E1000_FCRTC, reg);
+
+			/*
+			 * Set the DMA Coalescing Rx threshold to PBA - 2 * max
+			 * frame size, capping it at PBA - 10KB.
+			 */
+			dmac_thr = pba - adapter->max_frame_size / 512;
+			if (dmac_thr < pba - 10)
+				dmac_thr = pba - 10;
 			reg = rd32(E1000_DMACR);
 			reg &= ~E1000_DMACR_DMACTHR_MASK;
-			dmac_thr = pba - 4;
-
 			reg |= ((dmac_thr << E1000_DMACR_DMACTHR_SHIFT)
 				& E1000_DMACR_DMACTHR_MASK);
 
@@ -7085,7 +7098,6 @@ static void igb_init_dmac(struct igb_adapter *adapter, u32 pba)
 			 * coalescing(smart fifb)-UTRESH=0
 			 */
 			wr32(E1000_DMCRTRH, 0);
-			wr32(E1000_FCRTC, hwm);
 
 			reg = (IGB_DMCTLX_DCFLUSH_DIS | 0x4);