netdev - RE: [EXT] Re: [PATCH v2 net] net: fec: add wmb to ensure correct descriptor values

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Date: Thu, 18 May 2023 13:33:27 +0000
From: Shenwei Wang <shenwei.wang@....com>
To: Heiner Kallweit <hkallweit1@...il.com>, Wei Fang <wei.fang@....com>,
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon
 Horman <horms@...nel.org>, Horatiu Vultur <horatiu.vultur@...rochip.com>
CC: Clark Wang <xiaoning.wang@....com>, dl-linux-imx <linux-imx@....com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, "imx@...ts.linux.dev"
	<imx@...ts.linux.dev>
Subject: RE: [EXT] Re: [PATCH v2 net] net: fec: add wmb to ensure correct
 descriptor values



> -----Original Message-----
> From: Heiner Kallweit <hkallweit1@...il.com>
> Sent: Wednesday, May 17, 2023 2:14 PM
> To: Shenwei Wang <shenwei.wang@....com>; Wei Fang <wei.fang@....com>;
> David S. Miller <davem@...emloft.net>; Eric Dumazet
> <edumazet@...gle.com>; Jakub Kicinski <kuba@...nel.org>; Paolo Abeni
> <pabeni@...hat.com>; Simon Horman <horms@...nel.org>; Horatiu Vultur
> <horatiu.vultur@...rochip.com>
> Cc: Clark Wang <xiaoning.wang@....com>; dl-linux-imx <linux-imx@....com>;
> netdev@...r.kernel.org; imx@...ts.linux.dev
> Subject: [EXT] Re: [PATCH v2 net] net: fec: add wmb to ensure correct descriptor
> values
> 
> Caution: This is an external email. Please take care when clicking links or
> opening attachments. When in doubt, report the message using the 'Report this
> email' button
> 
> 
> On 17.05.2023 18:15, Shenwei Wang wrote:
> > Two wmb() are added in the XDP TX path to ensure proper ordering of
> > descriptor and buffer updates:
> > 1. A wmb() is added after updating the last BD to make sure
> >    the updates to rest of the descriptor are visible before
> >    transferring ownership to FEC.
> > 2. A wmb() is also added after updating the tx_skbuff and bdp
> >    to ensure these updates are visible before updating txq->bd.cur.
> > 3. Start the xmit of the frame immediately right after configuring the
> >    tx descriptor.
> >
> > Fixes: 6d6b39f180b8 ("net: fec: add initial XDP support")
> > Signed-off-by: Shenwei Wang <shenwei.wang@....com>
> > ---
> >  v2:
> >   - update the inline comments for 2nd wmb per Wei Fang's review.
> >
> >  drivers/net/ethernet/freescale/fec_main.c | 17 +++++++++++------
> >  1 file changed, 11 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/freescale/fec_main.c
> > b/drivers/net/ethernet/freescale/fec_main.c
> > index 6d0b46c76924..d210e67a188b 100644
> > --- a/drivers/net/ethernet/freescale/fec_main.c
> > +++ b/drivers/net/ethernet/freescale/fec_main.c
> > @@ -3834,6 +3834,11 @@ static int fec_enet_txq_xmit_frame(struct
> fec_enet_private *fep,
> >       index = fec_enet_get_bd_index(last_bdp, &txq->bd);
> >       txq->tx_skbuff[index] = NULL;
> >
> > +     /* Make sure the updates to rest of the descriptor are performed before
> > +      * transferring ownership.
> > +      */
> > +     wmb();
> > +
> >       /* Send it on its way.  Tell FEC it's ready, interrupt when done,
> >        * it's the last BD of the frame, and to put the CRC on the end.
> >        */
> > @@ -3843,8 +3848,14 @@ static int fec_enet_txq_xmit_frame(struct
> fec_enet_private *fep,
> >       /* If this was the last BD in the ring, start at the beginning again. */
> >       bdp = fec_enet_get_nextdesc(last_bdp, &txq->bd);
> >
> > +     /* Make sure the update to bdp are performed before txq->bd.cur. */
> > +     wmb();
> > +
> 
> Do you really need wmb() here or would a more light-weight smp_wmb() or
> dma_wmb() be sufficient?
> 

In this case, both wmb should be changed to dma_wmb. 

Regards,
Shenwei

> >       txq->bd.cur = bdp;
> >
> > +     /* Trigger transmission start */
> > +     writel(0, txq->bd.reg_desc_active);
> > +
> >       return 0;
> >  }
> >
> > @@ -3873,12 +3884,6 @@ static int fec_enet_xdp_xmit(struct net_device
> *dev,
> >               sent_frames++;
> >       }
> >
> > -     /* Make sure the update to bdp and tx_skbuff are performed. */
> > -     wmb();
> > -
> > -     /* Trigger transmission start */
> > -     writel(0, txq->bd.reg_desc_active);
> > -
> >       __netif_tx_unlock(nq);
> >
> >       return sent_frames;
> > --
> > 2.34.1
> >
> >