From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Jakub Kicinski <kuba@kernel.org>, Joel Stanley <joel@jms.id.au>
Cc: BMC-SW <BMC-SW@aspeedtech.com>,
Po-Yu Chuang <ratbert@faraday-tech.com>,
linux-aspeed <linux-aspeed@lists.ozlabs.org>,
netdev@vger.kernel.org,
OpenBMC Maillist <openbmc@lists.ozlabs.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Dylan Hung <dylan_hung@aspeedtech.com>,
"David S . Miller" <davem@davemloft.net>
Subject: Re: [PATCH] net: ftgmac100: Fix missing TX-poll issue
Date: Tue, 20 Oct 2020 10:23:41 +1100 [thread overview]
Message-ID: <1a02e57b6b7d425a19dc59f84091c38ca4edcf47.camel@kernel.crashing.org> (raw)
In-Reply-To: <20201019120040.3152ea0b@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
On Mon, 2020-10-19 at 12:00 -0700, Jakub Kicinski wrote:
> 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?
I suspect the problem is that the HW (and yes this would be a HW bug)
doesn't order the CPU -> memory and the CPU -> MMIO path.
What I think happens is that the store to txde0 is potentially still in
a buffer somewhere on its way to memory, gets bypassed by the store to
MMIO, causing the MAC to try to read the descriptor, and getting the
"old" data from memory.
It's ... fishy, but that isn't the first time an Aspeed chip has that
type of bug (there's a similar one in the USB device controler iirc).
Cheers,
Ben.
next prev parent reply other threads:[~2020-10-19 23:25 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-19 7:39 [PATCH] net: ftgmac100: Fix missing TX-poll issue Dylan Hung
2020-10-19 8:57 ` Joel Stanley
2020-10-19 9:19 ` Dylan Hung
2020-10-19 19:00 ` Jakub Kicinski
2020-10-19 23:23 ` Benjamin Herrenschmidt [this message]
2020-10-20 2:57 ` Jakub Kicinski
2020-10-20 6:15 ` Benjamin Herrenschmidt
2020-10-20 17:24 ` Jakub Kicinski
2020-10-20 6:14 ` Dylan Hung
2020-10-20 13:15 ` David Laight
2020-10-20 22:05 ` Benjamin Herrenschmidt
2020-10-20 19:49 ` Arnd Bergmann
2020-10-20 22:10 ` Benjamin Herrenschmidt
2020-10-20 22:25 ` Andrew Jeffery
2020-10-23 13:08 ` Dylan Hung
2020-10-26 22:21 ` Benjamin Herrenschmidt
2020-10-27 2:18 ` Joel Stanley
2020-10-21 7:16 ` Arnd Bergmann
2020-10-21 12:11 ` Arnd Bergmann
2020-10-22 7:40 ` Benjamin Herrenschmidt
2020-10-23 8:39 ` Arnd Bergmann
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