[BISECTED REGRESSION] b43legacy broken on G4 PowerBook

[Benjamin Herrenschmidt]

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.