Re: [BISECTED REGRESSION] b43legacy broken on G4 PowerBook

From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Christoph Hellwig <hch@lst.de>
Cc: Larry Finger <Larry.Finger@lwfinger.net>,
	Aaro Koskinen <aaro.koskinen@iki.fi>,
	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
Date: Tue, 11 Jun 2019 16:59:54 +1000	[thread overview]
Message-ID: <b30ced162fa96d0ca63b8b9629d6fe9bc5c78746.camel@kernel.crashing.org> (raw)
In-Reply-To: <fdfc817d1dcdc83f5bc45f0ab12cbce0c61e6702.camel@kernel.crashing.org>

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.