Re: [discuss] Re: Please pull x86-64 bug fixes

Linus Torvalds
2006-10-05T22:46:07Z

On Thu, 5 Oct 2006, Jeff Garzik wrote:

> Andi Kleen wrote:
> > If the choice is between a secret NDA only card with dubious
> > functionality and booting on lots of modern boards I know what to choose.
>
> That's a strawman argument. There is no need to choose. You can clearly boot
> on lots of modern boards with mmconfig just fine. We just need to narrow down
> which ones.

Jeff, _that_ is the strawman argument.

The thing is, nobody has been able to so far come up with a way to narrow
down which ones.

I think Andi's response was quite on the mark: if you have a patch to
narrow it down, please share. Until then, the fact is, we don't know
_how_, and you're barking up the wrong tree.

It is entirely possible that the only reasonable solution is to
potentially _never_ use MMIO as the main config mechanism (where "never"
is "in the next few years" - at some point we may be able to just say
"screw it, we don't care any more"), and that the drivers that want to use
MMIO for config accesses will have to use special operations for that.

In other words, right now we have

int pci_read_config_byte(struct pci_dev *dev, int where, u8 *val)

and maybe we will simply have to add a totally new function like

int pci_read_mmio_config_byte(struct pci_dev *dev, int where, u8 *val)

for drivers that literally _require_ the mmio accesses for one reason or
another.

Alternatively, we could just have a "ioremap()" kind of thing, and use
readl/writel on the end result, ie maybe we could do

void __iomem *cfg = mmiocfg_remap(dev);

if (!cfg) {
printk("MMIO PCI configuration cycles not supported\n");
return -EIO;
}

/* Read the extended error register or some-such crud.. */
val = readl(cfg + 0x180);
..

See? The thign is, at least right now the _advantages_ of MMIOCFG are
basically zero for all regular hardware, and as such the disadvantages (in
the form of machines that hang on device discovery) are way way _way_
bigger, and no way will do start using MMIO config cycles by default just
because some unreleased hardware might want to.

But using them explicitly in drivers - that's another thing.

What do you think about this simple solution?

Linus