netdev - [PATCH net 1/2] sfc: use 64-bit writes for PIO.

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Message-ID: <537B08BE.4080807@solarflare.com>
Date:	Tue, 20 May 2014 08:48:14 +0100
From:	Shradha Shah <sshah@...arflare.com>
To:	David Miller <davem@...emloft.net>
CC:	<netdev@...r.kernel.org>, <linux-net-drivers@...arflare.com>
Subject: [PATCH net 1/2] sfc: use 64-bit writes for PIO.

From: Jon Cooper <jcooper@...arflare.com>

The linux net driver uses memcpy_toio() in order to copy into
the PIO buffers.
Even on a 64bit machine this causes 32bit accesses to a write-
combined memory region.
There are hardware limitations that mean that only
64bit naturally aligned accesses are safe in all cases. Due to being
write-combined memory region two 32bit accesses may be coalesced to
form a 64bit non 64bit aligned access.
Solution was to open-code the memory copy routines using pointers
and to only enable PIO for x86_64 machines.

Patch to open-code the memory copy routines.
32bit writes over the PCI bus causes data corruption.

Fixes:ee45fd92c739db5b7950163d91dfe5f016af6d24

orig-hg-hash: 853f313def1e5e9c733f980f2b4e6330a7d063ef
Signed-off-by: Shradha Shah <sshah@...arflare.com>
---
 drivers/net/ethernet/sfc/tx.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/drivers/net/ethernet/sfc/tx.c b/drivers/net/ethernet/sfc/tx.c
index fa94753..bd6a4b8 100644
--- a/drivers/net/ethernet/sfc/tx.c
+++ b/drivers/net/ethernet/sfc/tx.c
@@ -189,6 +189,20 @@ struct efx_short_copy_buffer {
        u8 buf[L1_CACHE_BYTES];
 };

+/* Copy in explicit 64-bit writes. */
+static void efx_memcpy_64(void *dest, void *src, size_t len)
+{
+       uint64_t *src64 = src, *dest64 = dest;
+       size_t i, l64 = len / 8;
+
+       WARN_ON_ONCE(len % 8 != 0);
+       WARN_ON_ONCE(((u8 *)dest - (u8 *) 0) % 8 != 0);
+       BUILD_BUG_ON(sizeof(uint64_t) != 8);
+
+       for(i = 0; i < l64; ++i)
+               dest64[i] = src64[i];
+}
+
 /* Copy to PIO, respecting that writes to PIO buffers must be dword aligned.
  * Advances piobuf pointer. Leaves additional data in the copy buffer.
  */
@@ -198,7 +212,7 @@ static void efx_memcpy_toio_aligned(struct efx_nic *efx, u8 __iomem **piobuf,
 {
        int block_len = len & ~(sizeof(copy_buf->buf) - 1);

-       memcpy_toio(*piobuf, data, block_len);
+       efx_memcpy_64(*piobuf, data, block_len);
        *piobuf += block_len;
        len -= block_len;

@@ -230,7 +244,7 @@ static void efx_memcpy_toio_aligned_cb(struct efx_nic *efx, u8 __iomem **piobuf,
                if (copy_buf->used < sizeof(copy_buf->buf))
                        return;

-               memcpy_toio(*piobuf, copy_buf->buf, sizeof(copy_buf->buf));
+               efx_memcpy_64(*piobuf, copy_buf->buf, sizeof(copy_buf->buf));
                *piobuf += sizeof(copy_buf->buf);
                data += copy_to_buf;
                len -= copy_to_buf;
@@ -245,7 +259,7 @@ static void efx_flush_copy_buffer(struct efx_nic *efx, u8 __iomem *piobuf,
 {
        /* if there's anything in it, write the whole buffer, including junk */
        if (copy_buf->used)
-               memcpy_toio(piobuf, copy_buf->buf, sizeof(copy_buf->buf));
+               efx_memcpy_64(piobuf, copy_buf->buf, sizeof(copy_buf->buf));
 }

 /* Traverse skb structure and copy fragments in to PIO buffer.
@@ -304,8 +318,8 @@ efx_enqueue_skb_pio(struct efx_tx_queue *tx_queue, struct sk_buff *skb)
                 */
                BUILD_BUG_ON(L1_CACHE_BYTES >
                             SKB_DATA_ALIGN(sizeof(struct skb_shared_info)));
-               memcpy_toio(tx_queue->piobuf, skb->data,
-                           ALIGN(skb->len, L1_CACHE_BYTES));
+               efx_memcpy_64(tx_queue->piobuf, skb->data,
+                             ALIGN(skb->len, L1_CACHE_BYTES));
        }

        EFX_POPULATE_QWORD_5(buffer->option,

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