Re: alpha iommu fixes

[Andi Kleen]

On Mon, May 21, 2001 at 03:55:15AM -0700, David S. Miller wrote:
> The original claim is that the situation was not handled at all.  All
> I'm trying to say is simply that the net stack does check via
> illegal_highdma() the condition you stated was not being checked at
> all.  To me it sounded like you were claiming that HIGHMEM pages went
> totally unchecked through device transmit, and that is totally untrue.

Yes I was wrong on this one, but after looking at the acenic I think
the current solution is not very nice to driver authors.

> 
> If you were trying to point out the problem with what the Acenic
> driver is doind, just state that next time ok? :-)

It's more a generic problem with PCI DMA API, acenic just happens to
be the clearest victim of it @)

> Plainly, I'm going to be highly reluctant to make changes to the PCI
> dma API in 2.4.x  It is already hard enough to get all the PCI drivers
> in line and using it.  Suggesting this kind of change is similar to
> saying "let's change the arguments to request_irq()".  We would do it
> to fix a true "people actually hit this" kind of bug, of course.  Yet
> we would avoid it at all possible costs due to the disruption this
> would cause.

How about a new function (pci_nonrepresentable_address() or whatever) 
that returns true when page cache contains pages that are not representable
physically as void *. On IA32 it would return true only if CONFIG_PAE is 
true and there is memory >4GB. 

Then the network driver could do

	if (!pci_nonrepresentable_address()) 
		dev->features |= NETIF_F_HIGHDMA;

[in theory it would be possible to push it up into dev.c to avoid
bounces for highmem memory <4GB, but I don't want to teach such details
to the generic layer]

DAC support would be the next step, not necessarily one for 2.4, but perhaps
one that could be backported from 2.5 at some point.

-Andi