linux-kernel - Re: [PATCH v3 4/6] PCI: generic: Correct, and avoid overflow, in bus

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Date:	Wed, 23 Sep 2015 08:50:45 -0700
From:	David Daney <ddaney@...iumnetworks.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	<linux-arm-kernel@...ts.infradead.org>,
	David Daney <ddaney.cavm@...il.com>,
	<linux-kernel@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	<linux-pci@...r.kernel.org>, Will Deacon <will.deacon@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	<devicetree@...r.kernel.org>, Marc Zyngier <marc.zyngier@....com>,
	David Daney <david.daney@...ium.com>
Subject: Re: [PATCH v3 4/6] PCI: generic: Correct, and avoid overflow, in
 bus_max calculation.

On 09/23/2015 01:01 AM, Arnd Bergmann wrote:
> On Tuesday 22 September 2015 16:49:15 David Daney wrote:
>> From: David Daney <david.daney@...ium.com>
>>
>> There are two problems with the bus_max calculation:
>>
>> 1) The u8 data type can overflow for large config space windows.
>>
>> 2) The calculation is incorrect for a bus range that doesn't start at
>>     zero.
>>
>> Since the configuration space is relative to bus zero, make bus_max
>> just be the size of the config window scaled by bus_shift.  Then clamp
>> it to a maximum of 255, per PCI.  Use a data type of int to avoid
>> overflow problems.
>>
>> Update host-generic-pci.txt to clarify the semantics of the "reg"
>> property with respect to non-zero starting bus numbers.
>>
>> Signed-off-by: David Daney <david.daney@...ium.com>
>
> Not sure about this one
>
>> diff --git a/Documentation/devicetree/bindings/pci/host-generic-pci.txt b/Documentation/devicetree/bindings/pci/host-generic-pci.txt
>> index cf3e205..105a968 100644
>> --- a/Documentation/devicetree/bindings/pci/host-generic-pci.txt
>> +++ b/Documentation/devicetree/bindings/pci/host-generic-pci.txt
>> @@ -34,7 +34,9 @@ Properties of the host controller node:
>>   - #size-cells    : Must be 2.
>>
>>   - reg            : The Configuration Space base address and size, as accessed
>> -                   from the parent bus.
>> +                   from the parent bus.  The base address corresponds to
>> +                   bus zero, even though the "bus-range" property may specify
>> +                   a different starting bus number.
>
> This sounds like very unusual behavior. If you have a system with faked
> bus numbers where the registers only physically exist for a subset of the
> buses, this requires defining a reg property that contains MMIO space
> which is outside of the device and potentially contains other devices.

The pci-host-generic driver only maps the ranges that correspond to the 
"bus-range" buses, so mapping of illegal address ranges should not be a 
problem.

>
> What would break if we instead defined it the expected way and only
> list the registers for the bus numbers in the "bus-range" property?

I'm not sure if we have the luxury of being able to change the 
definition, although the existing code only works with a starting bus 
number of zero.  From this we might conclude that non-zero starting bus 
numbers cannot exist in the wild, so changing the the definition of 
"reg" so that it starts at the starting bus number might be possible.

My reading of:

http://www.o3one.org/hwdocs/openfirmware/pci_supplement_2_1.pdf

Section 3.1.1, does not preclude your interpretation.  Although that is 
for PCI-PCI bridges, and not this pci-host-generic root complex.

If we really want to go with a different definition of what the "reg" 
property means, then actual code has to change, and we risk breaking 
something.

David Daney


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