netdev - [PATCH v2 8/9] ftgmac100: Fix potential ordering issue in NAPI poll

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Date:   Thu, 13 Apr 2017 14:39:15 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     netdev@...r.kernel.org
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: [PATCH v2 8/9] ftgmac100: Fix potential ordering issue in NAPI poll

We need to ensure the loads from the descriptor are done after the
MMIO store clearing the interrupts has completed, otherwise we
might still miss work.

A read back from the MMIO register will "push" the posted store and
ioread32 has a barrier on weakly aordered architectures that will
order subsequent accesses.

Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
---
 drivers/net/ethernet/faraday/ftgmac100.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/ethernet/faraday/ftgmac100.c b/drivers/net/ethernet/faraday/ftgmac100.c
index 71763e4..9b7a24e 100644
--- a/drivers/net/ethernet/faraday/ftgmac100.c
+++ b/drivers/net/ethernet/faraday/ftgmac100.c
@@ -1347,6 +1347,13 @@ static int ftgmac100_poll(struct napi_struct *napi, int budget)
 		 */
 		iowrite32(FTGMAC100_INT_RXTX,
 			  priv->base + FTGMAC100_OFFSET_ISR);
+
+		/* Push the above (and provides a barrier vs. subsequent
+		 * reads of the descriptor).
+		 */
+		ioread32(priv->base + FTGMAC100_OFFSET_ISR);
+
+		/* Check RX and TX descriptors for more work to do */
 		if (ftgmac100_check_rx(priv) ||
 		    ftgmac100_tx_buf_cleanable(priv))
 			return budget;
-- 
2.9.3