Re: Some Alphas broken by f75b99d5a77d (PCI: Enforce bus address limits in resource allocation)

[Matt Turner <mattst88@gmail.com>]

On Sat, Feb 22, 2020 at 8:55 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> On Mon, Apr 16, 2018 at 07:33:57AM -0700, Matt Turner wrote:
> > Commit f75b99d5a77d63f20e07bd276d5a427808ac8ef6 (PCI: Enforce bus
> > address limits in resource allocation) broke Alpha systems using
> > CONFIG_ALPHA_NAUTILUS. Alpha is 64-bit, but Nautilus systems use a
> > 32-bit AMD 751/761 chipset. arch/alpha/kernel/sys_nautilus.c maps PCI
> > into the upper addresses just below 4GB.
> >
> > I can get a working kernel by ifdef'ing out the code in
> > drivers/pci/bus.c:pci_bus_alloc_resource. We can't tie
> > PCI_BUS_ADDR_T_64BIT to ALPHA_NAUTILUS without breaking generic
> > kernels.
> >
> > How can we get Nautilus working again?
>
> I don't see a resolution in this thread, so I assume this is still
> broken?  Anybody have any more ideas?

Indeed, still broken.

I can add Kconfig logic to unselect ARCH_DMA_ADDR_T_64BIT if
ALPHA_NAUTILUS, but then generic kernels won't work on Nautilus. It
doesn't look like we have any way of opting out of
ARCH_DMA_ADDR_T_64BIT at runtime, and doing enough plumbing to make
that work is not worth it for such niche hardware. Maybe removing
Nautilus from the generic kernel build is what I should do until such
a time that we really fix this?

Or maybe I could put a hack in pci.c that more or less undoes
d56dbf5bab8c on Nautilus. #if defined CONFIG_ARCH_DMA_ADDR_T_64BIT &&
!defined SYS_NAUTILUS.

Or maybe I just need to take a weekend and try to understand the PCI
code, instead of applying patches I don't understand and praying :)

Thoughts? Other suggestions?