Enumerating an empty bus hangs the entire system

[Mason]

Hello,

My driver works reasonably well on revision 1 of the PCIe controller.
(For lax enough values of "reasonably well"...)

So I wanted to try it out on revision 2 of the controller.

Turns out the system hangs if I boot with no card inserted in the PCIe
slot. (This does not happen on revision 1.) If I log all config space
accesses, this is what I see:

...
[    2.966402] tango_config_read: bus=0 devfn=0 where=128 size=2
[    2.972284] tango_config_read: bus=0 devfn=0 where=140 size=4
[    2.978167] tango_config_read: bus=0 devfn=0 where=146 size=2
[    2.984144] pci_bus 0000:01: busn_res: can not insert [bus 01-ff] under [bus 00-3f] (conflicts with (null) [bus 00-3f])
[    2.995105] tango_config_write: bus=0 devfn=0 where=24 size=4 val=0xff0100
[    3.002134] pci_bus 0000:01: scanning bus
[    3.006274] tango_config_read: bus=1 devfn=0 where=0 size=4

Basically, the PCI framework tries to read vendor and device IDs
of the non-existent device on bus 1, which hangs the system,
because the read never completes :-(

I had the same problem with the legacy driver for 3.4 but I was
hoping I would magically avoid it in a recent kernel.

The only work-around I see is: assuming the first access to a
bus will be to register 0, check the PHY for an active link
before sending an actual read request to register 0.

Is that reasonable?

Is it compliant for the PCIe controller to hang like that,
or should it handle some kind of time out?

Liviu suggested: "The PCIe controller probably generates (or propagates)
a bus abort that it should actually trap in HW.	Check if there is a SW
configurable way to recover that."

But I unmasked all system/misc errors, and I don't see any
interrupts firing.

Regards.