pci: Work around PCIe link training failures

pci: Work around PCIe link training failures

Attempt to handle cases with a downstream port of a PCIe switch where
link training never completes and the link continues switching between
speeds indefinitely with the data link layer never reaching the active
state.

It has been observed with a downstream port of the ASMedia ASM2824 Gen 3
switch wired to the upstream port of the Pericom PI7C9X2G304 Gen 2
switch, using a Delock Riser Card PCI Express x1 > 2 x PCIe x1 device,
P/N 41433, wired to a SiFive HiFive Unmatched board.  In this setup the
switches are supposed to negotiate the link speed of preferably 5.0GT/s,
falling back to 2.5GT/s.

However the link continues oscillating between the two speeds, at the
rate of 34-35 times per second, with link training reported repeatedly
active ~84% of the time, e.g.:

02:03.0 PCI bridge [0604]: ASMedia Technology Inc. ASM2824 PCIe Gen3 Packet Switch [1b21:2824] (rev 01) (prog-if 00 [Normal decode])
[...]
	Bus: primary=02, secondary=05, subordinate=05, sec-latency=0
[...]
	Capabilities: [80] Express (v2) Downstream Port (Slot+), MSI 00
[...]
		LnkSta:	Speed 5GT/s (downgraded), Width x1 (ok)
			TrErr- Train+ SlotClk+ DLActive- BWMgmt+ ABWMgmt-
[...]
		LnkCtl2: Target Link Speed: 8GT/s, EnterCompliance- SpeedDis+, Selectable De-emphasis: -3.5dB
			 Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-
			 Compliance De-emphasis: -6dB
[...]

Forcibly limiting the target link speed to 2.5GT/s with the upstream
ASM2824 device makes the two switches communicate correctly however:

02:03.0 PCI bridge [0604]: ASMedia Technology Inc. ASM2824 PCIe Gen3 Packet Switch [1b21:2824] (rev 01) (prog-if 00 [Normal decode])
[...]
	Bus: primary=02, secondary=05, subordinate=09, sec-latency=0
[...]
	Capabilities: [80] Express (v2) Downstream Port (Slot+), MSI 00
[...]
		LnkSta:	Speed 2.5GT/s (downgraded), Width x1 (ok)
			TrErr- Train- SlotClk+ DLActive+ BWMgmt- ABWMgmt-
[...]
		LnkCtl2: Target Link Speed: 2.5GT/s, EnterCompliance- SpeedDis+, Selectable De-emphasis: -3.5dB
			 Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-
			 Compliance De-emphasis: -6dB
[...]

and then:

05:00.0 PCI bridge [0604]: Pericom Semiconductor PI7C9X2G304 EL/SL PCIe2 3-Port/4-Lane Packet Switch [12d8:2304] (rev 05) (prog-if 00 [Normal decode])
[...]
	Bus: primary=05, secondary=06, subordinate=09, sec-latency=0
[...]
	Capabilities: [c0] Express (v2) Upstream Port, MSI 00
[...]
		LnkSta:	Speed 2.5GT/s (downgraded), Width x1 (downgraded)
			TrErr- Train- SlotClk+ DLActive- BWMgmt- ABWMgmt-
[...]
		LnkCtl2: Target Link Speed: 5GT/s, EnterCompliance- SpeedDis-
			 Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-
			 Compliance De-emphasis: -6dB
[...]

Make use of this observation then and attempt to detect the inability to
negotiate the link speed automatically, and then handle it by hand.  Use
the Data Link Layer Link Active status flag as the primary indicator of
successful link speed negotiation, but given that the flag is optional
by hardware to implement (the ASM2824 does have it though), resort to
checking for the mandatory Link Bandwidth Management Status flag showing
that the link speed or width has been changed in an attempt to correct
unreliable link operation (the ASM2824 does set it too).

If these checks indicate that link may not operate correctly, then poll
the Data Link Layer Link Active status flag along with the Link Training
flag for the duration of 200ms to see if the link has stabilised, that
is either that the Data Link Layer Link Active status flag has been set
or that Link Training has been inactive during at least the second half
of the interval.

If that has indicated failure, restrict the target speed to 2.5GT/s,
request a link retrain and check again if the link has stabilised.  If
that does not work either, then restore the original speed setting and
claim defeat, otherwise we are done.

NB interestingly enough with the ASM2824 vs PI7C9X2G304 configuration
referred above asking the ASM2824 to retrain with a higher target link
speed once the 2.5GT/s speed has been negotiated makes the two devices
successfully negotiate 5.0GT/s.  Lifting the 2.5GT/s speed restriction
would however prevent our workaround from working with an OS that issues
a reset and that is unaware of the problem.  This is because the devices
would then try to negotiate a higher link speed from scratch and fail,
while the sticky property of the Target Link Speed setting will keep the
2.5GT/s speed restriction across a reset.

Keep the 2.5GT/s speed restriction then, conservatively, if functional
once applied.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Reviewed-by: Stefan Roese <sr@denx.de>