Re: [RFC PATCH] PCI MMCONFIG: add validation against ACPI motherboard resources

[Robert Hancock <hancockr@shaw.ca>]

Jesse Barnes wrote:
> On Tuesday, May 22, 2007, Robert Hancock wrote:
>> Eww. I don't see where we disable the decode at all while we probe the
>> BARs on the device. That seems like a bad thing, especially with the way
>> we probe 64-bit BARs (do the low 32 bits first and then the high 32
>> bits). This means the base address effectively gets set to 0xfffffff0
>> momentarily, which might cause some issues.
> 
> I'm a bit shocked that things work as well as they do without the 
> disabling...
> 
>> I'd try adding some code inside pci_setup_device (drivers/pci/probe.c)
>> to disable PCI_COMMAND_IO and PCI_COMMAND_MEMORY on the device when
>> probing devices with the standard header type and then restoring the
>> previous command bits afterwards, and see what effect that has. It'll be
>>   interesting if it does, since obviously it seems to work as it is with
>> non-MMCONFIG access methods. Maybe the base address being set like that
>> interferes with MMCONFIG access itself somehow?
> 
> I tried that, and it seems to get past probing the graphics device at 
> least, but it hangs a bit later.  It could be that the enable/disable I 
> added wasn't correct though, I didn't check to see which one I should 
> disable in the command word, which may be a problem (just disabled them 
> both every probe).  I'll try again with more precise enable/disable 
> semantics.

There was a big discussion about this back in 2002, in which Linus 
wasn't overly enthused about disabling the decode during probing due to 
risk of causing problems with some devices:

http://lkml.org/lkml/2002/12/19/145

In this particular case (64-bit BAR) we might be able to avoid the 
problem by changing the order in which we probe the two halves of the 
address, i.e. change the top half to 0xffffffff before messing with the 
bottom half and then change it back last. That way, we end up mapping it 
way to the top of 64-bit address space, which hopefully is less likely 
to conflict..

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/