Subject: Re: [PATCH v2 1/4] PCI: of: Override 64-bit flag for non-prefetchable memory below 4GB

From: Punit Agrawal <[email-masked]>
Date: Wed, 02 Jun 2021 22:38:03 +0900

Hi Ard,

Thanks for the comments.

Ard Biesheuvel <[email-masked]> writes:

> Hi Punit,
>
> On Tue, 1 Jun 2021 at 00:11, Punit Agrawal <[email-masked]> wrote:
>>
>> Some host bridges advertise non-prefetable memory windows that are
>
> typo ^

Oops. Fixed locally.

>> entirely located below 4GB but are marked as 64-bit address memory.
>>
>> Since commit 9d57e61bf723 ("of/pci: Add IORESOURCE_MEM_64 to resource
>> flags for 64-bit memory addresses"), the OF PCI range parser takes a
>> stricter view and treats 64-bit address ranges as advertised while
>> before such ranges were treated as 32-bit.
>>
>> A PCI host bridge that is modelled as PCI-to-PCI bridge cannot forward
>
> It is the root port which is modeled as a P2P bridge. The root port(s)
> together with the host bridge is what makes up the root complex.
>
>
>> 64-bit non-prefetchable memory ranges. As a result, the change in
>> behaviour due to the commit causes allocation failure for devices that
>> require non-prefetchable bus addresses.
>>
>
> AIUI, the problem is not that the device requires a non-prefetchable
> bus address, but that it fails to allocate a 32-bit BAR from a 64-bit
> non-prefetchable window.

That's right. Is the below (borrowed from your explanation) clearer?

    A PCI root port modelled as a PCI-to-PCI bridge cannot forward 64-bit
    non-prefetchable memory ranges. As a result, the change in behaviour due
    to the commit causes failure to allocate 32-bit BAR from a 64-bit
    non-prefetchable window.

If there are no further comments on the set, I'll send an update in a
couple of days.

Thanks,
Punit

>> In order to not break platforms, override the 64-bit flag for
>> non-prefetchable memory ranges that lie entirely below 4GB.
>>
>> Suggested-by: Ard Biesheuvel <[email-masked]>
>> Link: https://lore.kernel.org/r/[email-masked]
>> Signed-off-by: Punit Agrawal <[email-masked]>
>> Cc: Bjorn Helgaas <[email-masked]>
>> Cc: Rob Herring <[email-masked]>
>>
>> Signed-off-by: Punit Agrawal <[email-masked]>
>> ---
>>  drivers/pci/of.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
>> index da5b414d585a..e2e64c5c55cb 100644
>> --- a/drivers/pci/of.c
>> +++ b/drivers/pci/of.c
>> @@ -346,6 +346,14 @@ static int devm_of_pci_get_host_bridge_resources(struct device *dev,
>>                                 dev_warn(dev, "More than one I/O resource converted for %pOF. CPU base address for old range lost!\n",
>>                                          dev_node);
>>                         *io_base = range.cpu_addr;
>> +               } else if (resource_type(res) == IORESOURCE_MEM) {
>> +                       if (!(res->flags & IORESOURCE_PREFETCH)) {
>> +                               if (res->flags & IORESOURCE_MEM_64)
>> +                                       if (!upper_32_bits(range.pci_addr + range.size - 1)) {
>> +                                               dev_warn(dev, "Clearing 64-bit flag for non-prefetchable memory below 4GB\n");
>> +                                               res->flags &= ~IORESOURCE_MEM_64;
>> +                                       }
>> +                       }
>>                 }
>>
>>                 pci_add_resource_offset(resources, res, res->start - range.pci_addr);
>> --
>> 2.30.2
>>
>
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