netdev - Re: [PATCH] net: gianfar: fix dma check map error when DMA_API

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Message-ID: <20141031030940.GA14783@pek-khao-d1.corp.ad.wrs.com>
Date:	Fri, 31 Oct 2014 11:09:40 +0800
From:	Kevin Hao <haokexin@...il.com>
To:	Claudiu Manoil <claudiu.manoil@...escale.com>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [PATCH] net: gianfar: fix dma check map error when DMA_API_DEBUG
 is enabled

On Thu, Oct 30, 2014 at 06:28:01PM +0200, Claudiu Manoil wrote:
> On 10/30/2014 12:25 PM, Kevin Hao wrote:
> 
> [...]
> 
> >@@ -2406,6 +2416,25 @@ static int gfar_start_xmit(struct sk_buff *skb, struct net_device *dev)
> >  	spin_unlock_irqrestore(&tx_queue->txlock, flags);
> >
> >  	return NETDEV_TX_OK;
> >+
> >+dma_map_err:
> >+	txbdp = next_txbd(txbdp_start, base, tx_queue->tx_ring_size);
> >+	if (do_tstamp)
> >+		txbdp = next_txbd(txbdp, base, tx_queue->tx_ring_size);
> >+	for (i = 0; i < nr_frags; i++) {
> >+		lstatus = txbdp->lstatus;
> >+		if (!(lstatus & BD_LFLAG(TXBD_READY)))
> >+			break;
> >+
> >+		txbdp->lstatus = lstatus & ~BD_LFLAG(TXBD_READY);
> >+		bufaddr = txbdp->bufPtr;
> >+		dma_unmap_page(priv->dev, bufaddr, txbdp->length,
> >+			       DMA_TO_DEVICE);
> >+		txbdp = next_txbd(txbdp, base, tx_queue->tx_ring_size);
> >+	}
> >+	gfar_wmb();
> 
> Why use the wmb() memory barrier here?

Just want make sure that the update to the txbdp->lstatus is performed before
any possible update to the BD fields later. Maybe this seems a bit redundant.
But this is not in hot path, so we may tolerate this overhead for more safety.

Thanks,
Kevin

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