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

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Date:   Thu, 22 Oct 2020 06:35:27 +0000
From:   Joel Stanley <joel@....id.au>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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>,
        Andrew Jeffery <andrew@...id.au>
Subject: Re: [PATCH] net: ftgmac100: Ensure tx descriptor updates are visible

On Wed, 21 Oct 2020 at 12:40, Arnd Bergmann <arnd@...nel.org> 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?

Now that I've read memory-barriers.txt, yes, they should.

On my clarification of the description of the patch, I thought it was
about making sure that the txdes0 store (and   thus setting of the
ownership bit) happens before the reader side checks that bit.

But we're not, we're making sure all of the writes to the descriptor
happen before updating the pointer, so when the reader side loads the
ownership bit in txdes0, it sees that store to txdes0 at that pointer.

I'll add in the read side barrier(s) and send a v2.

Cheers,

Joel