macb: Introduce a few barriers when dealing with DMA descriptors

There were a few theoretical possibilities that the compiler might
optimize away DMA descriptor reads and/or writes and thus cause
synchronization problems with the hardware. Insert barriers where
we depend on reads/writes actually hitting memory.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>

diff --git a/drivers/macb.c b/drivers/macb.c
index 186ab19..43bb878 100644 (file)
--- a/drivers/macb.c
+++ b/drivers/macb.c
@@ -51,6 +51,8 @@
 
 #include "macb.h"
 
+#define barrier() asm volatile("" ::: "memory")
+
 #define CFG_MACB_RX_BUFFER_SIZE                4096
 #define CFG_MACB_RX_RING_SIZE          (CFG_MACB_RX_BUFFER_SIZE / 128)
 #define CFG_MACB_TX_RING_SIZE          16
@@ -185,31 +187,31 @@ static int macb_send(struct eth_device *netdev, volatile void *packet,
 
        macb->tx_ring[tx_head].ctrl = ctrl;
        macb->tx_ring[tx_head].addr = paddr;
+       barrier();
        macb_writel(macb, NCR, MACB_BIT(TE) | MACB_BIT(RE) | MACB_BIT(TSTART));
 
        /*
         * I guess this is necessary because the networking core may
         * re-use the transmit buffer as soon as we return...
         */
-       i = 0;
-       while (!(macb->tx_ring[tx_head].ctrl & TXBUF_USED)) {
-               if (i > CFG_MACB_TX_TIMEOUT) {
-                       printf("%s: TX timeout\n", netdev->name);
+       for (i = 0; i <= CFG_MACB_TX_TIMEOUT; i++) {
+               barrier();
+               ctrl = macb->tx_ring[tx_head].ctrl;
+               if (ctrl & TXBUF_USED)
                        break;
-               }
                udelay(1);
-               i++;
        }
 
        dma_unmap_single(packet, length, paddr);
 
        if (i <= CFG_MACB_TX_TIMEOUT) {
-               ctrl = macb->tx_ring[tx_head].ctrl;
                if (ctrl & TXBUF_UNDERRUN)
                        printf("%s: TX underrun\n", netdev->name);
                if (ctrl & TXBUF_EXHAUSTED)
                        printf("%s: TX buffers exhausted in mid frame\n",
                               netdev->name);
+       } else {
+               printf("%s: TX timeout\n", netdev->name);
        }
 
        /* No one cares anyway */
@@ -234,6 +236,7 @@ static void reclaim_rx_buffers(struct macb_device *macb,
                i++;
        }
 
+       barrier();
        macb->rx_tail = new_tail;
 }
 
@@ -283,6 +286,7 @@ static int macb_recv(struct eth_device *netdev)
                                rx_tail = 0;
                        }
                }
+               barrier();
        }
 
        return 0;