linux-kernel - RE: [PATCH net-next v26 07/13] rtase: Implement a function to receive packets

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Message-ID: <1f846891e9264b94bf00cb934dd83d2a@realtek.com>
Date: Fri, 9 Aug 2024 06:43:38 +0000
From: Justin Lai <justinlai0215@...ltek.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com"
	<edumazet@...gle.com>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "andrew@...n.ch"
	<andrew@...n.ch>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "horms@...nel.org"
	<horms@...nel.org>,
        "rkannoth@...vell.com" <rkannoth@...vell.com>,
        "jdamato@...tly.com" <jdamato@...tly.com>,
        Ping-Ke Shih <pkshih@...ltek.com>, Larry Chiu <larry.chiu@...ltek.com>
Subject: RE: [PATCH net-next v26 07/13] rtase: Implement a function to receive packets

> On Wed, 7 Aug 2024 11:37:17 +0800 Justin Lai wrote:
> > +static int rx_handler(struct rtase_ring *ring, int budget) {
> > +     union rtase_rx_desc *desc_base = ring->desc;
> > +     u32 pkt_size, cur_rx, delta, entry, status;
> > +     struct rtase_private *tp = ring->ivec->tp;
> > +     struct net_device *dev = tp->dev;
> > +     union rtase_rx_desc *desc;
> > +     struct sk_buff *skb;
> > +     int workdone = 0;
> > +
> > +     cur_rx = ring->cur_idx;
> > +     entry = cur_rx % RTASE_NUM_DESC;
> > +     desc = &desc_base[entry];
> > +
> > +     do {
> > +             /* make sure discriptor has been updated */
> > +             dma_rmb();
> 
> I think I already asked, but this still makes no sense to me.
> dma barriers are between accesses to descriptors.
> dma_rmb() ensures ordering of two reads.
> Because modern CPUs can perform reads out of order.
> What two reads is this barrier ordering?
> 
> Please read the doc on memory barriers relating to dma.
> I think this is in the wrong place.
> 
> r8169 gets it right.

Thank you for your reply. I will modify it.

Thanks
Justin
> 
> > +             status = le32_to_cpu(desc->desc_status.opts1);
> > +
> > +             if (status & RTASE_DESC_OWN)
> > +                     break;
> > +