Re: [PATCH kernel] powerpc/powernv: Update comment about shifting IOV BAR

On Thu, 2017-09-14 at 09:27 +0000, David Laight wrote:

You can logically 'hotplug' PCI(e) on any system [1].

The 'problem' is that whatever enumerates the PCI(e) at system
powerup doesn't normally assign extra resources to bridges to allow
for devices that aren't present at boot time.
So you can normally only replace cards with ones that use the same
(or less) resources, or that are not behind any bridges.
This is problematic if you have a docking station connected via
a bridge.

There's also the problem of Max Payload Size. If you can hotplug behind
a bridge then the standard algorithm of finding the max of all devices
behind a host bridge doesn't work anymore and you have to clamp
everybody to 128 bytes.

[1] Apart from some annoying x86 Dell servers we have which generate
an NMI when the PCIe link goes down (when we reprogram the fpga).
They also fail to boot if a link doesn't come up...

        David