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Date:   Tue, 11 Jun 2019 08:08:16 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc:     Larry Finger <Larry.Finger@lwfinger.net>,
        Aaro Koskinen <aaro.koskinen@iki.fi>,
        Christoph Hellwig <hch@lst.de>,
        Christian Zigotzky <chzigotzky@xenosoft.de>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev@lists.ozlabs.org, linux-wireless@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [BISECTED REGRESSION] b43legacy broken on G4 PowerBook
Message-ID: <20190611060816.GA20158@lst.de>
References: <20190605225059.GA9953@darkstar.musicnaut.iki.fi> <73da300c-871c-77ac-8a3a-deac226743ef@lwfinger.net> <7697a9d10777b28ae79fdffdde6d0985555f6310.camel@kernel.crashing.org> <3ed1ccfe-d7ca-11b9-17b3-303d1ae1bb0f@lwfinger.net> <c91ccbddd6a58dbee5705f10ed1d98fb44bd8f8d.camel@kernel.crashing.org>
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On Tue, Jun 11, 2019 at 03:56:33PM +1000, Benjamin Herrenschmidt wrote:
> The reason I think it sort-of-mostly-worked is that to get more than
> 1GB of RAM, those machines use CONFIG_HIGHMEM. And *most* network
> buffers aren't allocated in Highmem.... so you got lucky.
> 
> That said, there is such as thing as no-copy send on network, so I
> wouldn't be surprised if some things would still have failed, just not
> frequent enough for you to notice.

Unless NETIF_F_HIGHDMA is set on a netdev, the core networkign code
will bounce buffer highmem pages for the driver under all circumstances.