PCI: Work around PCIe link training failures

[Matthew W Carlis <mattc@purestorage.com>]

Hello again.. It looks like there are specific system configurations that are
extremely likely to have issues with this patch & result in undesirable
system behavior..

  Specifically hot-plug systems with side-band presence detection & without
Power Controls (i.e PwrCtrl-) given to config space. It may also be related
to presence on U.2 connectors being first-to-mate/last-to-break, but
I don't have much experience with the different connectors. The main
issue is that the quirk is invoked in at least two common cases when
it is not expected that the link would train. 
  For example, if I power off the slot via an out-of-band vendor specific
mechanism we see the kernel decide that the link should be training,
presumable because it will see PresDet+ in Slot Status. In this case it
decides the link failed to train, writes the Gen1 speed value into TLS
(target link speed) & returns after waiting for the link one more time.
The next time the slot is powered on the link will train to Gen1 due to TLS.
  Another problematic case is when we physically insert a device. In my case
I am using nvme drives with U.2 connectors from several different vendors.
The presence event is often generated before the device is fully powered on
due to U.2 assigning presence as a first-to-mate & power being last-to-mate.
I believe in this case that the kernel is expecting the link to train too
soon & therefore we find that the quirk often applies the Gen1 TLS speed.
Later, when the link comes up it is frequently observed at Gen1. I tried
to unset bit 3 in Slot Control (Presence Detect Changed Enable), but we
still hit the first case I described with powering off slots.
  I should be clear and say that we are able to see devices forced to Gen1
extremely often in the side-band presence configuration. We would really like
to see this "quirk" removed or put behind an opt-in config since it causes
significant regression in common configurations of pcie-hotplug. I have tried
to come up with ideas to modify/improve the quirk, but I am not very
confident that there is a good solution if the kernel cannot know for certain
whether the link is expected to train.

Thanks,
-Matt