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Date:   Mon, 19 Oct 2020 12:00:40 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Joel Stanley <joel@jms.id.au>
Cc:     Dylan Hung <dylan_hung@aspeedtech.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        "David S . Miller" <davem@davemloft.net>, netdev@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Po-Yu Chuang <ratbert@faraday-tech.com>,
        linux-aspeed <linux-aspeed@lists.ozlabs.org>,
        OpenBMC Maillist <openbmc@lists.ozlabs.org>,
        BMC-SW <BMC-SW@aspeedtech.com>
Subject: Re: [PATCH] net: ftgmac100: Fix missing TX-poll issue
Message-ID: <20201019120040.3152ea0b@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
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        <CACPK8Xfn+Gn0PHCfhX-vgLTA6e2=RT+D+fnLF67_1j1iwqh7yg@mail.gmail.com>
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On Mon, 19 Oct 2020 08:57:03 +0000 Joel Stanley wrote:
> > diff --git a/drivers/net/ethernet/faraday/ftgmac100.c b/drivers/net/ethernet/faraday/ftgmac100.c
> > index 00024dd41147..9a99a87f29f3 100644
> > --- a/drivers/net/ethernet/faraday/ftgmac100.c
> > +++ b/drivers/net/ethernet/faraday/ftgmac100.c
> > @@ -804,7 +804,8 @@ static netdev_tx_t ftgmac100_hard_start_xmit(struct sk_buff *skb,
> >          * before setting the OWN bit on the first descriptor.
> >          */
> >         dma_wmb();
> > -       first->txdes0 = cpu_to_le32(f_ctl_stat);
> > +       WRITE_ONCE(first->txdes0, cpu_to_le32(f_ctl_stat));
> > +       READ_ONCE(first->txdes0);  
> 
> I understand what you're trying to do here, but I'm not sure that this
> is the correct way to go about it.
> 
> It does cause the compiler to produce a store and then a load.

+1 @first is system memory from dma_alloc_coherent(), right?

You shouldn't have to do this. Is coherent DMA memory broken 
on your platform?