netdev - Re: [PATCH net-next 1/4] via-rhine: commit receive buffer address before descriptor status update.

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Message-Id: <20150407.172748.700634713750750146.davem@davemloft.net>
Date:	Tue, 07 Apr 2015 17:27:48 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	romieu@...zoreil.com
Cc:	netdev@...r.kernel.org, nix@...eri.org.uk, rl@...lgate.ch,
	gurligebis@...too.org, alexander.h.duyck@...hat.com
Subject: Re: [PATCH net-next 1/4] via-rhine: commit receive buffer address
 before descriptor status update.

From: Francois Romieu <romieu@...zoreil.com>
Date: Tue, 7 Apr 2015 23:02:48 +0200

> David Miller <davem@...emloft.net> :
>> From: Francois Romieu <romieu@...zoreil.com>
> [...]
>> > @@ -2063,6 +2063,7 @@ static int rhine_rx(struct net_device *dev, int limit)
>> >  				break;
>> >  			}
>> >  			rp->rx_ring[entry].addr = cpu_to_le32(rp->rx_skbuff_dma[entry]);
>> > +			wmb();
>> 
>> dma_wmb() perhaps?  I think this is exactly the situation that interface was
>> added for.
> 
> I need the buffer address to be written in the receive descriptor before
> the descriptor status is. The cpu does W1, W2 and the nic mustn't see W2, W1.

That's exactly dma_wmb().

It barriers cpu writes so that the device sees things in a certain
order.

It's what all the most common ethernet chip drivers use in their
descriptor handling routines now.
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