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SubjectRe: [PATCH] PCI: pciehp: Do not turn off slot if presence comes up after link
On Tue, Feb 05, 2019 at 03:06:56PM -0600, Alexandru Gagniuc wrote:
> According to PCIe 3.0, the presence detect state is a logical OR of
> in-band and out-of-band presence.

Since Bjorn asked for a spec reference:

PCIe r4.0 sec 7.8.11: "Presence Detect State - This bit indicates the
presence of an adapter in the slot, reflected by the logical OR of the
Physical Layer in-band presence detect mechanism and, if present, any
out-of-band presence detect mechanism defined for the slot's
corresponding form factor."

Table 4-14 in sec 4.2.6 is also important because it shows the difference
between Link Up and in-band presence.


> With this, we'd expect the presence
> state to always be asserted when the link comes up.
>
> Not all hardware follows this, and it is possible for the presence to
> come up after the link. In this case, the PCIe device would be
> erroneously disabled and re-probed. It is possible to distinguish
> between a delayed presence and a card swap by looking at the DLL state
> changed bit -- The link has to come down if the card is removed.

So in reality, PDC and DLLSC events rarely come in simultaneously and
we do allow some leeway in pcie_wait_for_link(), it's just not enough
in your particular case due to the way presence is detected by the
platform.

I think in this case instead of changing the behavior for everyone,
it's more appropriate to add a quirk for your hardware. Two ideas:

* Amend pcie_init() to set slot_cap |= PCI_EXP_SLTCAP_ABP. Insert
this as a quirk right below the existing Thunderbolt quirk and
use PCI vendor/device IDs and/or DMI checks to identify your
particular hardware. If the ABP bit is set, PDC events are not
enabled by pcie_enable_notification(), so you will solely rely on
DLLSC events. Problem solved. (Hopefully.)

* Alternatively, add a DECLARE_PCI_FIXUP_FINAL() quirk which sets
pdev->link_active_reporting = false. This causes pcie_wait_for_link()
to wait 1100 msec before the hot-added device's config space is
accessed for the first time.

Would this work for you?

Thanks,

Lukas

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