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

Bjorn Helgaas
2012-05-29T19:23:52Z

On Tue, May 29, 2012 at 12:17 PM, Yinghai Lu <yinghai@kernel.org> wrote:
> On Tue, May 29, 2012 at 10:57 AM, H. Peter Anvin <hpa@zytor.com> wrote:
>> On 05/29/2012 10:55 AM, Yinghai Lu wrote:
>>>
>>> x86 are using 16bits.
>>>
>>> some others use 32 bits.
>>> #define IO_SPACE_LIMIT 0xffffffff
>>>
>>> ia64 and sparc are using 64bits.
>>> #define IO_SPACE_LIMIT               0xffffffffffffffffUL
>>>
>>> but pci only support 16bits and 32bits.
>>>
>>> maybe later we can add
>>> PCI_MAX_RESOURCE_16
>>>
>>> to handle 16bits and 32bit io ports.
>>>
>>
>> Shouldn't this be dealt by root port apertures?
>>
>
> pci bridge could support 16bits and 32bits io port.
> but we did not record if 32bits is supported.
>
> so during allocating, could have allocated above 64k address to non
> 32bit bridge.
>
> but  x86 is ok, because ioport.end always set to 0xffff.
> other arches with IO_SPACE_LIMIT with 0xffffffff or
> 0xffffffffffffffffUL may have problem.

I think current IO_SPACE_LIMIT usage is a little confused. The
"ioport_resource.end = IO_SPACE_LIMIT" in kernel/resource.c refers to
a CPU-side address, not a bus address. Other uses, e.g., in
__pci_read_base(), apply it to bus addresses from BARs, which is
wrong. Host bridges apply I/O port offsets just like they apply
memory offsets. The ia64 IO_SPACE_LIMIT of 0xffffffffffffffffUL means
there's no restriction on CPU-side I/O port addresses, but any given
host bridge will translate its I/O port aperture to bus addresses that
fit in 32 bits.

None of this is really relevant to the question I asked, namely, "why
Yinghai's patch doesn't limit I/O BAR values to 32 bits?" That
constraint is clearly a requirement because I/O BARs are only 32 bits
wide, but I don't think it needs to be enforced in the code here. The
host bridge or upstream P2P bridge apertures should already take care
of that automatically. I don't think the 16- or 32-bitness of P2P
bridge apertures is relevant here, because the I/O resources available
on the secondary bus already reflect that.

After all that discussion, I think my objection here boils down to
"you shouldn't change the I/O BAR constraints in a patch that claims
to allocate 64-bit *memory* BARs above 4GB."

I think the code below is still the clearest way to set the constraints:

if (res->flags & IORESOURCE_MEM_64) {
start = (resource_size_t) (1ULL << 32);
end = PCI_MAX_RESOURCE;
} else {
start = 0;
end = PCI_MAX_RESOURCE_32;
}

It's not strictly necessary to limit I/O BARs to PCI_MAX_RESOURCE_32
because host bridge apertures should already enforce that, but I think
the code above just makes it clearer.