Re: [PATCH 4/4] PCI: fix the io resource alignment calculation in pbus_size_io()

[Bjorn Helgaas <bhelgaas@google.com>]

On Tue, Aug 06, 2013 at 11:22:27AM +0800, Gavin Shan wrote:
> On Mon, Aug 05, 2013 at 04:21:35PM -0600, Bjorn Helgaas wrote:
> >On Mon, Aug 05, 2013 at 02:09:27PM -0700, Yinghai Lu wrote:
> >> On Mon, Aug 5, 2013 at 1:59 PM, Bjorn Helgaas <bhelgaas@google.com> wrote:
> >> >> then, we should drop that 4k capping.
> >> >> I was thinking there could be strange or wild res with bigger than 4k.
> >> >
> >> > If there *were* an I/O BAR larger than 4KB, how should it be handled?
> >> > I don't think capping the alignment to 4KB sounds like the best way.
> >> > For example, a 16KB I/O BAR would still need to be aligned on 16KB.
> >> >
> >> > And I think capping to 4KB as you did above will break the powerpc
> >> > pcibios_window_alignment() implementation.  For example, if
> >> > pcibios_window_alignment() returned 16KB, and we later capped it to
> >> > 4KB, we're going to allocate space for the bridge window with the
> >> > wrong alignment.
> >> 
> >> Agree.
> >
> >OK.  Can you guys try this out and see whether it fixes the problem?
> >I don't know what the actual problem *is*, so I can't tell whether
> >this is a possible fix.
> >
> 
> The code looks simpler, but it potentially breaks PowerNV platforms.
> Lets have inline description if I understand everything here.
> 
> >diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> >index d4f1ad9..8333c92 100644
> >--- a/drivers/pci/setup-bus.c
> >+++ b/drivers/pci/setup-bus.c
> >@@ -749,12 +749,12 @@ static void pbus_size_io(struct pci_bus *bus, resource_size_t min_size,
> > 	struct resource *b_res = find_free_bus_resource(bus, IORESOURCE_IO);
> > 	resource_size_t size = 0, size0 = 0, size1 = 0;
> > 	resource_size_t children_add_size = 0;
> >-	resource_size_t min_align, io_align, align;
> >+	resource_size_t min_align, align;
> >
> > 	if (!b_res)
> >  		return;
> >
> >-	io_align = min_align = window_alignment(bus, IORESOURCE_IO);
> >+	min_align = window_alignment(bus, IORESOURCE_IO);
> > 	list_for_each_entry(dev, &bus->devices, bus_list) {
> > 		int i;
> >
> 
> "min_align" here indicates the IO segment size on PowerNV platform.
> On PowerNV platform, the IO range of specific PHB (PCI controller)
> is divided evenly and each piece of that is called IO segment. For
> example, the IO segment size ("min_align") is 64K initially. If one
> of specific IO BAR has size of 96K (it's possible?), "min_align"
> becomes 96K eventually, which isn't IO segment aligned.

96K is not a valid size for any PCI BAR.  BARs are always a power-of-2
size.  If PowerNV has an I/O segment size of 64K and we have an I/O BAR
of 128K (illegal per spec, but assume we did for the sake of argument),
we would allocate a 128K-aligned area, which does satisfy the PowerNV
alignment restriction.

Bjorn