Re: [PATCH 1/2] PCI: of: Override 64-bit flag for non-prefetchable memory below 4GB

[Punit Agrawal <punitagrawal@gmail.com>]

Hi Bjorn,

Thanks for taking a look.

Bjorn Helgaas <helgaas@kernel.org> writes:

> On Fri, May 28, 2021 at 12:05:41AM +0900, Punit Agrawal wrote:
>> Some host bridges advertise non-prefetable memory windows that are
>> 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.
>
> Conceptually, I'm not sure why we need IORESOURCE_MEM_64 at all on
> resources we get from DT.  I think the main point of IORESOURCE_MEM_64
> is to convey the information that "this register, e.g., a PCI BAR, has
> space for 64-bit values if you need to write to it."
>
> When we're parsing this from DT, I think we're just getting a fixed
> value and there's no concept of writing anything back, so it doesn't
> seem like we should need to know how wide the hardware register is, or
> even whether there *is* a hardware register.
>
> But I'm sure the PCI resource allocation code probably depends on
> IORESOURCE_MEM_64 in those host bridge windows in very ugly ways.

Thanks for the explanation. From what I can tell, the IORESOURCE_MEM_64
flag is used in pci_bus_alloc_resource() to allocate from high PCI
addresses. Without the flag allocations above 4GB will fail. Not sure
that's legitimate use of the flag though.

>> A PCI-to-PCI bridges cannot forward 64-bit non-prefetchable memory
>> ranges. As a result, the change in behaviour due to the commit causes
>> allocation failure for devices that are connected behind PCI host
>> bridges modelled as PCI-to-PCI bridge and require non-prefetchable bus
>> addresses.
>> 
>> 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 <ardb@kernel.org>
>> Link: https://lore.kernel.org/r/7a1e2ebc-f7d8-8431-d844-41a9c36a8911@arm.com
>> Signed-off-by: Punit Agrawal <punitagrawal@gmail.com>
>> Cc: Bjorn Helgaas <bhelgaas@google.com>
>> Cc: Rob Herring <robh+dt@kernel.org>
>> ---
>>  drivers/pci/of.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
>> index da5b414d585a..b9d0bee5a088 100644
>> --- a/drivers/pci/of.c
>> +++ b/drivers/pci/of.c
>> @@ -565,10 +565,14 @@ static int pci_parse_request_of_pci_ranges(struct device *dev,
>>  		case IORESOURCE_MEM:
>>  			res_valid |= !(res->flags & IORESOURCE_PREFETCH);
>>  
>> -			if (!(res->flags & IORESOURCE_PREFETCH))
>> +			if (!(res->flags & IORESOURCE_PREFETCH)) {
>>  				if (upper_32_bits(resource_size(res)))
>>  					dev_warn(dev, "Memory resource size exceeds max for 32 bits\n");
>> -
>> +				if ((res->flags & IORESOURCE_MEM_64) && !upper_32_bits(res->end)) {
>> +					dev_warn(dev, "Overriding 64-bit flag for non-prefetchable memory below 4GB\n");
>
> Maybe "Clearing 64-bit flag"?
>
> Can you include %pR, so we see the resource in question?

I'll follow your suggestions in the next update.

>
> Unrelated but close by, would be nice if the preceding warning ("size
> exceeds max") also included %pR.

Makes sense. I'll add the resource print to improve the message.

Thanks,
Punit

>
>> +					res->flags &= ~IORESOURCE_MEM_64;
>> +				}
>> +			}
>>  			break;
>>  		}
>>  	}
>> -- 
>> 2.30.2
>> 
>
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