netdev - RE: [PATCH net-next v20 07/13] rtase: Implement a function to receive packets

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Date: Tue, 18 Jun 2024 03:40:34 +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>,
        "Ping-Ke
 Shih" <pkshih@...ltek.com>,
        Larry Chiu <larry.chiu@...ltek.com>
Subject: RE: [PATCH net-next v20 07/13] rtase: Implement a function to receive packets

> On Mon, 17 Jun 2024 06:44:55 +0000 Justin Lai wrote:
> > > > +             /* make sure discriptor has been updated */
> > > > +             rmb();
> > >
> > > Barriers are between things. What is this barrier between?
> >
> > At the end of this do while loop, it fetches the next descriptor. This
> > barrier is mainly used between fetching the next descriptor and using
> > the next descriptor, to ensure that the content of the next descriptor
> > is completely fetched before using it.
> 
> What does it mean to "fetch the next descriptor"? The prefetch?
> Prefetches are not ordered at all.

Let me explain again, at the end of the do while loop in rx_handler, we
have changed the address pointed by desc to the next descriptor.
Therefore, the main purpose of this barrier is to ensure that both desc
and cur_rx have been updated to the next entry.