Re: [patch 2.5] 2-pass PCI probing, generic part

[Ivan Kokshaysky]

On Thu, Jan 09, 2003 at 03:35:32PM -0800, Linus Torvalds wrote:
> 
> On Fri, 10 Jan 2003, Ivan Kokshaysky wrote:
> > 
> > PCI-PCI, PCI-ISA bridges - probably, but not host bridges. On x86 they
> > often have quite a few BARs, like AGP window, AGP MMIO, power management
> > etc., which we cannot ignore.
> 
> Oh, but we _can_ ignore it.
> 
> All those things are stuff that if the kernel doesn't use them, the kernel 
> doesn't even need to know they are there. 

It does - even if we don't use it, this stuff occupies regions in the bus
address space. If we ignore it we'll end up with incomplete resource
tree and pci_assign_resource() won't work anymore. Especially with AGP
aperture window, which tends to be large (32-128M), we'd have very good
chances to allocate a new resource (either by hotplug drivers or just
working around BIOS allocation bugs) in the middle of this window. Ouch.

> Sure, if we support AGP, we need to see the aperture size etc, but then 
> we'd have the AGP driver just do the "pci_enable_dev()" thing to work it 
> out.
> 
> The only real reason to worry about BAR sizing is really to do resource
> discovery in order to make sure that out bridges have sufficiently big
> windows for the IO regions. Agreed?

Yes, and as Grant already pointed out, to deal with overlapping (or just
incorrectly allocated) regions.

> And that should be a non-issue especially on a host bridge, since we 
> almost certainly don't want to reprogram the bridge windows there anyway.

[Hmm, on alpha we're doing that for years. :-)]
But even if we don't reprogram the bridge, we need to know about the
ranges that it decodes as we don't want to step on them accidentally.

> So I'd like to make the _default_ be to probe the minimal possible, 
> _especially_ for host bridges. Then, the PCI quirks could be used to 
> expand on that default.

I agree in general. Reasonable probing policy certainly is a good thing.
"Don't probe by default" would work wonderfully for PCI and ISA bridges -
I've yet to see one with normal BARs.
I'm just afraid that with the default "don't probe northbridges" we'll end
up with a lot more quirks than with "probe by default".

In either case, all of this can be trivially implemented on the top of
that patch. I'd suggest another pci_dev bit - "force_probe", to override
default policy:

static inline int default_probing_policy(struct pci_dev *dev)
{
	switch (dev->class >> 8) {
	case PCI_CLASS_BRIDGE_ISA:
	case PCI_CLASS_BRIDGE_EISA:
	case PCI_CLASS_BRIDGE_PCI:
		return 0;	/* Don't probe */
	case PCI_CLASS_BRIDGE_HOST:
		return ???
	default:
		return 1;	/* Do probe */
	}
}

static void pci_read_bases(struct pci_dev *dev, unsigned int howmany, int rom)
{
	...

	if (dev->skip_probe ||
	    (!dev->force_probe && !default_probing_policy(dev)))
		return;

	/* Disable I/O & memory decoding while we size the BARs. */
	pci_read_config_word(dev, PCI_COMMAND, &cmd);
	...

Ivan.