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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Fri, 23 Oct 2020 10:39:32 +0200
Message-ID: <CAK8P3a1JD57NSz+_ffyNwX-ERnDq56taNXryoVyV6ZXzEXft0g@mail.gmail.com>
Subject: Re: [PATCH] net: ftgmac100: Fix missing TX-poll issue
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc:     Arnd Bergmann <arnd@kernel.org>,
        Dylan Hung <dylan_hung@aspeedtech.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Joel Stanley <joel@jms.id.au>,
        "David S . Miller" <davem@davemloft.net>,
        "netdev@vger.kernel.org" <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>
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On Thu, Oct 22, 2020 at 9:41 AM Benjamin Herrenschmidt
<benh@kernel.crashing.org> wrote:
> On Wed, 2020-10-21 at 14:11 +0200, Arnd Bergmann wrote:
>
> > > At the moment, the only chips that need the heavy barrier are
> > > omap4 and mstar_v7, and early l2 cache controllers (not the one
> > > on Cortex-A7) have another synchronization callback that IIRC
> > > is used for streaming mappings.
>
>  .../...
>
> > > Obviously, adding one of these for ast2600 would slow down every
> > > mb() and writel() a lot, but if it is a chip-wide problem rather than
> > > one isolated to the network device, it would be the correct solution,
> > > provided that a correct code sequence can be found.
>
> I'm surprised that problem doesn't already exist on the ast2400 and
> 2500 and I thus worry about the performance impact of such a workaround
> applied generally to every MMIO writes....
>
> But we did kill mmiowb so ... ;-)

The real cost would have to be measured of course, and it depends a
lot on how it's done. The read-from-uncached-memory as in the 1/4
patch here seems fairly expensive, the mstarv7_mb() method (spinning
on an mmio read) seems worse, but the omap4 method (a posted write
to a mmio address in the memory controller to enforce a barrier between
the two ports) doesn't seem that bad and would correspond to what
the chip should be doing in the first place.

       Arnd