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Re: [E1000-devel] [PATCH] e100: Add missing dma sync for proper operatio

To: "Kok, Auke" <>
Subject: Re: [E1000-devel] [PATCH] e100: Add missing dma sync for proper operation with non-coherent caches.
From: David Daney <>
Date: Tue, 26 Aug 2008 09:54:47 -0700
Cc:,, linux-mips <>
In-reply-to: <>
Original-recipient: rfc822;
References: <> <>
User-agent: Thunderbird (X11/20080723)
Kok, Auke wrote:
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

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.

lol, that's a bit old :)

It was a typo.  The real version is 2.6.27-rc4 (aka the HEAD).

The bug is present on the HEAD.  Sorry for the confusion.

David Daney.

A LOT of work has gone into 2.6.26+'s version of e100 that addresses 
non-coherent DMA machines, including a patch that looks very close to what you
write below here.

can you test the latest git tree and see if you still need (a modified or 
version) of your patch?

I know for sure that the guys maintaining e100 do not have anything close to 
system, so they can't test that.



Signed-off-by: David Daney <>
 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 
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;

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