Subject: Re: [PATCH] arch/ppc/8xx_io/enet.c, version 2

From: Dan Malek <[email-masked]>
Date: Wed, 13 Nov 2002 17:12:00 -0500

Joakim Tjernlund wrote:

> OK, anyone against? Dan?

I'm currently looking at the patches and I'll be integrating something
that hopefully works :-)

This isn't something new that hasn't been tried before.  The problem
in the past with non-coherent processors, incoming DMA, and skbufs is
the buffers would share cache lines with other data which would get
corrupted as the result of the invalidate for the DMA.  Typically,
data that was corrupted were flags and control information for the IP
stack, and under "normal" use you wouldn't notice this.  However,
forwarding/bridging applications would fail to work properly and you
would sometimes see packet retransmits that weren't necessary.

The "trick" is to ensure you allocate a larger than necessary sk buffer
and then align the start and end such that they consume entire cache
lines.  There has been sufficient discussion about this that I hope
the sk buffer mechanism will allow this alignment now, as it didn't
work well in the past.  This is what I want to check out when I
apply and test the patches.

This isn't necessary on the 8260 family due to cache snooping, but it
is required on the 8xx.

Of course, a packet checksum still needs to be performed, and if it
is done as part of the data copy (and if the IP stack doesn't do it
again), it would seem that this implementation rather than DMA would
be more efficient.

Thanks.

	-- Dan

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