linux-kernel - Re: [PATCH] net: ftgmac100: Ensure tx descriptor updates are visible

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Date:   Thu, 22 Oct 2020 18:41:13 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Arnd Bergmann <arnd@...nel.org>, Joel Stanley <joel@....id.au>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Dylan Hung <dylan_hung@...eedtech.com>,
        Networking <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-aspeed <linux-aspeed@...ts.ozlabs.org>
Subject: Re: [PATCH] net: ftgmac100: Ensure tx descriptor updates are visible

On Wed, 2020-10-21 at 14:40 +0200, Arnd Bergmann wrote:
> On Wed, Oct 21, 2020 at 12:39 PM Joel Stanley <joel@....id.au> wrote:
> 
> > 
> > diff --git a/drivers/net/ethernet/faraday/ftgmac100.c b/drivers/net/ethernet/faraday/ftgmac100.c
> > index 331d4bdd4a67..15cdfeb135b0 100644
> > --- a/drivers/net/ethernet/faraday/ftgmac100.c
> > +++ b/drivers/net/ethernet/faraday/ftgmac100.c
> > @@ -653,6 +653,11 @@ static bool ftgmac100_tx_complete_packet(struct ftgmac100 *priv)
> >         ftgmac100_free_tx_packet(priv, pointer, skb, txdes, ctl_stat);
> >         txdes->txdes0 = cpu_to_le32(ctl_stat & priv->txdes0_edotr_mask);
> > 
> > +       /* Ensure the descriptor config is visible before setting the tx
> > +        * pointer.
> > +        */
> > +       smp_wmb();
> > +
> >         priv->tx_clean_pointer = ftgmac100_next_tx_pointer(priv, pointer);
> > 
> >         return true;
> > @@ -806,6 +811,11 @@ static netdev_tx_t ftgmac100_hard_start_xmit(struct sk_buff *skb,
> >         dma_wmb();
> >         first->txdes0 = cpu_to_le32(f_ctl_stat);
> > 
> > +       /* Ensure the descriptor config is visible before setting the tx
> > +        * pointer.
> > +        */
> > +       smp_wmb();
> > +
> 
> Shouldn't these be paired with smp_rmb() on the reader side?

(Not near the code right now) I *think* the reader already has them
where it matters but I might have overlooked something when I quickly
checked the other day.

Cheers,
Ben.