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

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Date:   Tue, 11 Jun 2019 16:59:54 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Larry Finger <Larry.Finger@...inger.net>,
        Aaro Koskinen <aaro.koskinen@....fi>,
        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, 2019-06-11 at 16:58 +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2019-06-11 at 08:08 +0200, Christoph Hellwig wrote:
> > 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.
> 
>  ... which b43legacy doesn't set to the best of my knowledge ...
> 
> Which makes me wonder how come it didn't work even with your patches ?
> AFAIK, we have less than 1GB of lowmem unless the config has been
> tweaked....

Ah stupid me ... it's dma_set_mask that failed, since it has no idea
that the calling driver is limited to lowmem.

That's also why the "wrong" patch worked.

So yes, a ZONE_DMA at 30-bits will work, though it's somewhat overkill.

Cheers,
Ben.