> On Fri, Feb 28, 2025 at 05:01:51PM -0600, Rob Herring wrote:
> > On Fri, Feb 28, 2025 at 12:27 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> > > On Thu, Nov 14, 2024 at 02:05:08PM +0000, Wannes Bouwen (Nokia)
> wrote:
> > > > Subject: [PATCH 1/1] PCI: of: avoid warning for 4 GiB
> > > > non-prefetchable windows.
> > > >
> > > > According to the PCIe spec, non-prefetchable memory supports only
> > > > 32-bit BAR registers and are hence limited to 4 GiB. In the kernel
> > > > there is a check that prints a warning if a non-prefetchable
> > > > resource exceeds the 32-bit limit.
> > > >
> > > > This check however prints a warning when a 4 GiB window on the
> > > > host bridge is used. This is perfectly possible according to the
> > > > PCIe spec, so in my opinion the warning is a bit too strict. This
> > > > changeset subtracts 1 from the resource_size to avoid printing a
> > > > warning in the case of a 4 GiB non-prefetchable window.
> > > >
> > > > Signed-off-by: Wannes Bouwen <wannes.bouwen@xxxxxxxxx>
> > > > ---
> > > > drivers/pci/of.c | 2 +-
> > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/pci/of.c b/drivers/pci/of.c index
> > > > dacea3fc5128..ccbb1f1c2212 100644
> > > > --- a/drivers/pci/of.c
> > > > +++ b/drivers/pci/of.c
> > > > @@ -622,7 +622,7 @@ static int pci_parse_request_of_pci_ranges(struct
> device *dev,
> > > > res_valid |= !(res->flags & IORESOURCE_PREFETCH);
> > > >
> > > > if (!(res->flags & IORESOURCE_PREFETCH))
> > > > - if (upper_32_bits(resource_size(res)))
> > > > + if (upper_32_bits(resource_size(res) - 1))
> > > > dev_warn(dev, "Memory resource size exceeds
> > > > max for 32 bits\n");
> > >
> > > I guess this relies on the fact that BARs must be a power of two in
> > > size, right? So anything where the upper 32 bits of the size are
> > > non-zero is either 0x1_0000_0000 (4GiB window that we shouldn't warn
> > > about), or 0x2_0000_0000 or bigger (where we *do* want to warn about
> > > it).
> > >
> > > But it looks like this is used for host bridge resources, which are
> > > windows, not BARs, so they don't have to be a power of two size. A
> > > window of size 0x1_8000_0000 is perfectly legal and would fit the
> > > criteria for this warning, but this patch would turn off the warning.
> >
> > 0x1_8000_0000 - 1 = 0x1_7fff_ffff
> >
> > So that would still work. Maybe you read it as the subtract being
> > after upper_32_bits()?
>
> Right, sorry. I guess a better example would be something like this:
>
> [mem 0x2000_0000-0x21ff_ffff] -> [pci 0x0_ff00_0000-0x1_00ff_ffff]
>
> where the size is only 0x0200_0000, so we wouldn't warn about it, but half of
> the window is above 4G on PCI.
>
> > > I don't really understand this warning in the first place, though.
> > > It was added by fede8526cc48 ("PCI: of: Warn if non-prefetchable
> > > memory aperture size is > 32-bit"). But I think the real issue
> > > would be related to the highest address, not the size. For example,
> > > an aperture of 0x0_c000_0000 - 0x1_4000_0000 is only 0x8000_0000 in
> > > size, but the upper half of it it would be invalid for
> > > non-prefetchable 32-bit BARs.
> >
> > Are we talking CPU addresses or PCI addresses? For CPU addresses, it
> > would be perfectly fine to be above 4G as long as PCI addresses are
> > below 4G, right?
>
> Yes, CPU addresses can be above 4G; all that matters for this is the PCI address.
>
> I think what's important is the largest PCI address in the window, not the size.
I agree. The check is I believe in place to make sure the host bridge non-prefetchable
window does not exceed the 4 GiB boundary. This is not possible due to the register
map of PCIe configuration space type 1 (register 0x20). I guess the check would be more
correct if we just check the end address of the resource instead of the size? Something
like this?
@@ -622,7 +622,7 @@ static int pci_parse_request_of_pci_ranges(struct device *dev,
res_valid |= !(res->flags & IORESOURCE_PREFETCH);
if (!(res->flags & IORESOURCE_PREFETCH))
- if (upper_32_bits(resource_size(res)))
+ if (upper_32_bits(res->end))
dev_warn(dev, "Memory resource size exceeds max for 32 bits\n");
break;
