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Re: [PATCH] e100: Add missing dma sync for proper operation with non-coh

To: David Daney <ddaney@avtrex.com>
Subject: Re: [PATCH] e100: Add missing dma sync for proper operation with non-coherent caches.
From: David Acker <dacker@roinet.com>
Date: Tue, 26 Aug 2008 14:50:13 -0400
Cc: e1000-devel@lists.sourceforge.net, netdev@vger.kernel.org, linux-mips <linux-mips@linux-mips.org>
In-reply-to: <48B3A8D0.2040108@avtrex.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <48B3A8D0.2040108@avtrex.com>
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David Daney wrote:
I am running the e100 driver on a MIPS 4KEc system (32 bit mips with
non-coherent DMA).  There was a problem where received packets would
get 'stuck' for several seconds at a time and then be released all at
once.

The cause was that if an interrupt were received when no RX packets
were available, the status for the receive buffer would be stuck in
the cache, so when the next interrupt arrived the old status value was
read (indicating no packets available) instead of the new value.

The fix is to call pci_dma_sync_single_for_device on the RX if the
packet is not available to invalidate the cache so that at the next
interrupt valid status is returned.

The driver currently calls pci_dma_sync_single_for_cpu before reading
the status, and this is indeed needed for cases like the R10000 CPU
where the cache can be polluted by speculative execution, but for most
machines it is a nop.

The patch was tested on 2.6.17-rc4 on a MIPS 4KEc.

Signed-off-by: David Daney <ddaney@avtrex.com>
---
 drivers/net/e100.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/drivers/net/e100.c b/drivers/net/e100.c
index 19d32a2..fb8d551 100644
--- a/drivers/net/e100.c
+++ b/drivers/net/e100.c
@@ -1840,6 +1840,11 @@ static int e100_rx_indicate(struct nic *nic, struct rx 
*rx,
if (readb(&nic->csr->scb.status) & rus_no_res)
                                nic->ru_running = RU_SUSPENDED;
+               /* We are done looking at the buffer.  Prepare it for
+                * more DMA.  */
+               pci_dma_sync_single_for_device(nic->pdev, rx->dma_addr,
+                                              sizeof(struct rfd),
+                                              PCI_DMA_FROMDEVICE);
                return -ENODATA;
        }
Should the call to pci_dma_sync_single_for_device be DMA_TO_DEVICE since we are giving the memory back to the device?
-ack


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