linux-kernel - Re: [BISECTED REGRESSION] b43legacy broken on G4 PowerBook

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Date:   Tue, 11 Jun 2019 08:08:16 +0200
From:   Christoph Hellwig <hch@....de>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Larry Finger <Larry.Finger@...inger.net>,
        Aaro Koskinen <aaro.koskinen@....fi>,
        Christoph Hellwig <hch@....de>,
        Christian Zigotzky <chzigotzky@...osoft.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-wireless@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [BISECTED REGRESSION] b43legacy broken on G4 PowerBook

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.