[PATCH 3/3] net: macb: add missing barriers when reading buffers

[Anssi Hannula <anssi.hannula@bitwise.fi>]

When reading buffer descriptors on RX or on TX completion, an
RX_USED/TX_USED bit is checked first to ensure that the descriptor has
been populated. However, there are no memory barriers to ensure that the
data protected by the RX_USED/TX_USED bit is up-to-date with respect to
that bit.

Fix that by adding DMA read memory barriers on those paths.

I did not observe any actual issues caused by these being missing,
though.

Tested on a ZynqMP based system.

Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Fixes: 89e5785fc8a6 ("[PATCH] Atmel MACB ethernet driver")
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
---
 drivers/net/ethernet/cadence/macb_main.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 430b7a0f5436..c93baa8621d5 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -861,6 +861,11 @@ static void macb_tx_interrupt(struct macb_queue *queue)
 
 			/* First, update TX stats if needed */
 			if (skb) {
+				/* Ensure all of desc is at least as up-to-date
+				 * as ctrl (TX_USED bit)
+				 */
+				dma_rmb();
+
 				if (gem_ptp_do_txstamp(queue, skb, desc) == 0) {
 					/* skb now belongs to timestamp buffer
 					 * and will be removed later
@@ -1000,12 +1005,16 @@ static int gem_rx(struct macb_queue *queue, int budget)
 		rmb();
 
 		rxused = (desc->addr & MACB_BIT(RX_USED)) ? true : false;
-		addr = macb_get_addr(bp, desc);
-		ctrl = desc->ctrl;
 
 		if (!rxused)
 			break;
 
+		/* Ensure other data is at least as up-to-date as rxused */
+		dma_rmb();
+
+		addr = macb_get_addr(bp, desc);
+		ctrl = desc->ctrl;
+
 		queue->rx_tail++;
 		count++;
 
@@ -1180,11 +1189,14 @@ static int macb_rx(struct macb_queue *queue, int budget)
 		/* Make hw descriptor updates visible to CPU */
 		rmb();
 
-		ctrl = desc->ctrl;
-
 		if (!(desc->addr & MACB_BIT(RX_USED)))
 			break;
 
+		/* Ensure other data is at least as up-to-date as addr */
+		dma_rmb();
+
+		ctrl = desc->ctrl;
+
 		if (ctrl & MACB_BIT(RX_SOF)) {
 			if (first_frag != -1)
 				discard_partial_frame(queue, first_frag, tail);
-- 
2.17.2