netdev - Re: [PATCH] gianfar: Add I/O barriers when touching buffer descriptor ownership.

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Message-Id: <65f0b79871a670eb595cca7d78e2f4e9@kernel.crashing.org>
Date:	Wed, 2 May 2007 02:54:40 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Scott Wood <scottwood@...escale.com>
Cc:	netdev@...r.kernel.org, linuxppc-dev@...abs.org, jgarzik@...ox.com
Subject: Re: [PATCH] gianfar: Add I/O barriers when touching buffer descriptor ownership.

> The hardware must not see that is given ownership of a buffer until it 
> is
> completely written, and when the driver receives ownership of a buffer,
> it must ensure that any other reads to the buffer reflect its final
> state.  Thus, I/O barriers are added where required.
>
> Without this patch, I have observed GCC reordering the setting of
> bdp->length and bdp->status in gfar_new_skb.

The :::"memory" in the barriers you used prevent GCC
from reordering accesses around the barriers.

AFAICS you need stronger barriers though; {w,r,}mb(),
to prevent _any_ reordering of those memory accesses,
not just the compiler-generated ones.

Segher

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