Arm PCIe hotplugging

[Jason Gunthorpe]

On Fri, Oct 04, 2013 at 02:14:34PM +0100, Matthew Minter wrote:

> I have been doing some work with an Armada XP board (DB-MV784MP-GP)
> with PCIe support, along with using a PEX 8614 (AA) PCIe switch. The
> switch in question is said to have hot plug support with the PCIe
> downstream slots.

PCI-E hot plug has several layers.

The first is the PCI-E serial negotiation and bring up. This is just
supported out of the box, and works fine with any compliant
device. You don't need drivers, /sys/bus/pci/rescan is enough. This is
what I'm doing on kirkwood.

The next is actual *physical* card removal/insertion. For in-box card
edge connectors this requires power management. You must remove all
power to the slot before removing/inserting a card. AFAIK, this is
what the PCI hot plug items in the kernel are all doing, and they do
this with help from firmware (eg ACPI on x86).

If you have a mechanical system that has staged power connectors (and
possibly inrush current limiters on the cards) then you don't really
need to worry much at this point, just rely on the serial hot plug.

PLX (AFIAK) has some 'GPIOs' on their switches dedicated to hot plug
functions so if the board is properly wired they can control slot
power/etc through their internal registers. I suspect it shouldn't be
too hard to get this sort of system to work.. The DT can be augmented
with slot chassis name properties so you have stable labels work with.

If you have board-specific controls, then you need to start augmenting
the DT with things like phandles to power regulators and other stuff.

> Looking closer it seems that in the ARM kernel the PCI hotplug code is
> all disabled. Am I just out of luck and should wait until there is a

I doubt you'll see anything like this emerge in the near future. Fancy
ARM64 servers with hot plug PCI might motivate the work, but they
might also do it with ACPI...

BTW,