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Re: [PATCH 1/2] netdev: octeon_mgmt: Correct tx IFG workaround.

To: Joe Perches <joe@perches.com>
Subject: Re: [PATCH 1/2] netdev: octeon_mgmt: Correct tx IFG workaround.
From: David Daney <ddaney@caviumnetworks.com>
Date: Wed, 19 Jun 2013 18:47:34 -0700
Cc: David Daney <ddaney.cavm@gmail.com>, <netdev@vger.kernel.org>, "David S. Miller" <davem@davemloft.net>, <linux-mips@linux-mips.org>, David Daney <david.daney@cavium.com>
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On 06/19/2013 06:37 PM, Joe Perches wrote:
On Wed, 2013-06-19 at 18:28 -0700, David Daney wrote:
On 06/19/2013 06:08 PM, Joe Perches wrote:
On Wed, 2013-06-19 at 17:40 -0700, David Daney wrote:
From: David Daney <david.daney@cavium.com>

The previous fix was still too agressive to meet ieee specs.  Increase
to (14, 10).
[]
diff --git a/drivers/net/ethernet/octeon/octeon_mgmt.c 
b/drivers/net/ethernet/octeon/octeon_mgmt.c
[]
@@ -1141,10 +1141,13 @@ static int octeon_mgmt_open(struct net_device *netdev)
                /* For compensation state to lock. */
                ndelay(1040 * NS_PER_PHY_CLK);

-               /* Some Ethernet switches cannot handle standard
-                * Interframe Gap, increase to 16 bytes.
+               /* Default Interframe Gaps are too small.  Recommended
+                * workaround is.
+                *
+                * AGL_GMX_TX_IFG[IFG1]=14
+                * AGL_GMX_TX_IFG[IFG2]=10

Why isn't the TX IFG just 96 bit times?

I don't have a full understanding of how the transistors are wired up on
the chip, so I cannot accurately answer your question.  But I can say
that after I empirically found the previous values to get the thing to
work, the hardware designers independently found that the values
supplied in this patch are required to achieve industry standard IFGs
with this hardware.

For one specific chip or for the Octeon entire family?


You will notice, if you look at the code, that there is an if statement that controls which chips get the special IFG treatment.

But to summarize: Only chips that have 1Gig MII ports are affected. Older versions (that only support 10M and 100M) do not get the adjustment.

David Daney


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