Re: [PATCH 02/11] PCI: Try to allocate mem64 above 4G at first

Bjorn Helgaas
2012-05-25T19:37:22Z

On Fri, May 25, 2012 at 11:39:26AM -0700, Yinghai Lu wrote:
> On Fri, May 25, 2012 at 10:53 AM, Yinghai Lu <yinghai@kernel.org> wrote:
> >> I don't really like the dependency on PCIBIOS_MAX_MEM_32 + 1ULL
> >> overflowing to zero -- that means the reader has to know what the
> >> value of PCIBIOS_MAX_MEM_32 is, and things would break in non-obvious
> >> ways if we changed it.
> >>
>
> please check if attached one is more clear.
>
> make max and bottom is only related to _MEM and not default one.
>
> - if (!(res->flags & IORESOURCE_MEM_64))
> - max = PCIBIOS_MAX_MEM_32;
> + if (res->flags & IORESOURCE_MEM) {
> + if (!(res->flags & IORESOURCE_MEM_64))
> + max = PCIBIOS_MAX_MEM_32;
> + else if (PCIBIOS_MAX_MEM_32 != -1)
> + bottom = (resource_size_t)(1ULL<<32);
> + }
>
> will still not affect to other arches.

That's goofy. You're proposing to make only x86_64 and x86-PAE try to put
64-bit BARs above 4GB. Why should this be specific to x86? I acknowledge
that there's risk in doing this, but if it's a good idea for x86_64, it
should also be a good idea for other 64-bit architectures.

And testing for "is this x86_32 without PAE?" with
"PCIBIOS_MAX_MEM_32 == -1" is just plain obtuse and hides an
important bit of arch-specific behavior.

Tangential question about allocate_resource(): Is its "max" argument
really necessary? We'll obviously only allocate from inside the root
resource, so "max" is just a way to artificially avoid the end of
that resource. Is there really a case where that's required?

"min" makes sense because in a case like this, it's valid to allocate from
anywhere in the root resource, but we want to try to allocate from the >4GB
part first, then fall back to allocating from the whole resource. I'm not
sure there's a corresponding case for "max."

Getting back to this patch, I don't think we should need to adjust "max" at
all. For example, this:

commit cb1c8e46244cfd84a1a2fe91be860a74c1cf4e25
Author: Bjorn Helgaas <bhelgaas@google.com>
Date: Thu May 24 22:15:26 2012 -0600

PCI: try to allocate 64-bit mem resources above 4GB

If we have a 64-bit mem resource, try to allocate it above 4GB first. If
that fails, we'll fall back to allocating space below 4GB.

diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
index 4ce5ef2..075e5b1 100644
--- a/drivers/pci/bus.c
+++ b/drivers/pci/bus.c
@@ -121,14 +121,16 @@ pci_bus_alloc_resource(struct pci_bus *bus, struct resource *res,
{
int i, ret = -ENOMEM;
struct resource *r;
- resource_size_t max = -1;
+ resource_size_t start = 0;
+ resource_size_t end = MAX_RESOURCE;

type_mask |= IORESOURCE_IO | IORESOURCE_MEM;

- /* don't allocate too high if the pref mem doesn't support 64bit*/
- if (!(res->flags & IORESOURCE_MEM_64))
- max = PCIBIOS_MAX_MEM_32;
+ /* If this is a 64-bit mem resource, try above 4GB first */
+ if (res->flags & IORESOURCE_MEM_64)
+ start = (resource_size_t) (1ULL << 32);

+again:
pci_bus_for_each_resource(bus, r, i) {
if (!r)
continue;
@@ -145,12 +147,18 @@ pci_bus_alloc_resource(struct pci_bus *bus, struct resource *res,

/* Ok, try it out.. */
ret = allocate_resource(r, res, size,
- r->start ? : min,
- max, align,
+ max(start, r->start ? : min),
+ end, align,
alignf, alignf_data);
if (ret == 0)
- break;
+ return 0;
+ }
+
+ if (start != 0) {
+ start = 0;
+ goto again;
}
+
return ret;
}