On Tue, 14 Mar 2017, Bjorn Helgaas wrote:
> On Tue, Mar 14, 2017 at 07:24:14PM -0700, Andi Kleen wrote:
> > > I agree that it should be fairly safe to do ECAM/MMCONFIG without
> > > locking. Can we handle the decision part by adding a "lockless" bit
> > > to struct pci_ops? Old ops don't mention that bit, so it will be
> > > initialized to zero and we'll do locking as today. ECAM/MMCONFIG ops
> > > can set it and we can skip the locking.
> >
> > That's what my other patch already did.
>
> Yes, your 1/4 patch does add the "ll_allowed" bit in struct pci_ops.
>
> What I was wondering, but didn't explain very well, was whether
> instead of setting that bit at run-time in pci_mmcfg_arch_init(), we
> could set it statically in the pci_ops definition, e.g.,
>
> static struct pci_ops ecam_ops = {
> .lockless = 1,
> .read = ecam_read,
> .write = ecam_write,
> };
>
> I think it would be easier to read if the lockless-ness were declared
> right next to the accessors that need it (or don't need it).
>
> But it is a little confusing with all the different paths, at least on
> x86, so maybe it wouldn't be quite that simple.
The pci_ops in x86 are a complete mess. We have
struct pci_ops pci_root_ops = {
.read = pci_read,
.write = pci_write,
};
That's the default and the r/w functions look like this:
pci_read/write()
{
if (domain == 0 && reg && raw_pci_ops)
return raw_pci_ops->read/write();
if (raw_pci_ext_ops)
return raw_pci_ext_ops->read/write();
return -EINVAL;
}
raw_pci_ops and raw_pci_ext_ops are setup through an impenetrable maze of
functions. Some of them overwrite pci_root_ops to something entirely
different.
pci_root_ops is what is finally handed in to pci_scan_root_bus() as ops
argument for any bus segment no matter which type it is.
The locking aspect is interesting as well. The type0/1 functions are having
their own internal locking. Oh, well.
What we really want is to differentiate bus segments. That means a PCIe
segment takes mmconfig ops and a PCI segment the type0/1 ops. That way we
can do what you suggested above, i.e. marking the ecam/mmconfig ops as
lockless.
Sure that's more work than just whacking a sloppy quirk into the code, but
the right thing to do.
Thanks,
tglx
