Re: [PATCH] pci: Work around ASMedia ASM2824 PCIe link training failures

From: Maciej W. Rozycki
Date: Sun Nov 21 2021 - 19:20:53 EST

On Sat, 20 Nov 2021, Maciej W. Rozycki wrote:

> One case that has been nurturing me though is the reverse scenario, that
> is where the Pericom PI7C9X2G304 switch is upstream while the ASMedia
> ASM2824 switch is downstream. Presumably the same situation will happen,
> so matching on the ASM2824 ID only would be a problem. Unfortunately the
> other device does not implement the Data Link Layer Link Active status
> bit, so a more complex approach, such as clearing and then checking for
> the Link Bandwidth Management Status having been set again might be an
> option. Unlike U-Boot we cannot do aggressive polling of the Link
> Training bit.
> Option hardware with M.2 slots is commercially available with the ASM2824
> onboard, so a test environment can be in principle arranged, though I'm
> not sure if just for the sake of such an experiment I'm willing to spend
> money that will ultimately go to a manufacturer that cannot be bothered to
> take responsibility for their faults and at the very least respond to a
> problem report. And without verifying the actual problem exists I'm
> reluctant to try and implement a workaround. On the other hand the
> problem with the Unmatched board is real and this change addresses it, at
> least for me.

NB I have realised the reverse scenario cannot actually be reproduced
with the Delock device as the downstream ports of the PI7C9X2G304 chip
appear to have been strapped for 2.5GT/s operation. Or so it seems, as
the Pericom datasheet seems unclear about such an option; all it says is:



Indicates the maximum speed of the Express link.
3:0 Maximum Link RO 0001b: 2.5 Gb/s
Speed 0010b: 5.0 Gb/s
Reset to 0010b.

Indicates the maximum width of the given PCIe Link.
9:4 Maximum Link RO Reset to 000010b (x2) for Port 0.
Reset to 000001b (x1) for Port 1.
Reset to 000001b (x1) for Port 2.

and then:



3:0 Target Link Speed RWS Reset to 0010b.

so it does tell the upstream port (port 0) and the downstream ports (port
1 & 2) apart where appropriate (link width), but says nothing about any
ports capable of being strapped for 2.5GT/s operation. However the actual
device reports:

06:01.0 PCI bridge [0604]: Pericom Semiconductor PI7C9X2G304 EL/SL PCIe2 3-Port/4-Lane Packet Switch [12d8:2304] (rev 05) (prog-if 00 [Normal decode])
Capabilities: [c0] Express (v2) Downstream Port (Slot+), MSI 00
LnkCap: Port #1, Speed 2.5GT/s, Width x1, ASPM not supported
ClockPM- Surprise- LLActRep- BwNot+ ASPMOptComp-
LnkSta: Speed 2.5GT/s (ok), Width x1 (ok)
TrErr- Train- SlotClk+ DLActive- BWMgmt- ABWMgmt-
LnkCtl2: Target Link Speed: 5GT/s, EnterCompliance- SpeedDis-, Selectable De-emphasis: -3.5dB
Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-
Compliance De-emphasis: -6dB

-- note the discrepancy between the Maximum Link Speed and the Target Link
Speed, possibly a device erratum. Also there's no usable Supported Speeds
Vector to confirm either way:



31:0 Link Capabilities 2 RO Reset to 0000_0000h.

It could be possible to verify the reverse scenario perhaps with another
manufactured device featuring the PI7C9X2G304 chip though, as they seem
somewhat common.

Also I got distracted mid-way through my submission and then forgot to
mention that I was unable to verify the resume part of this workaround as
the Unmatched hardware does not provide such a capability (it's out of
scope for this kind of a development/evaluation board).

And I have only minimally checked hot-plug operation as I dared not live
unplug and replug hardware which may not be prepared for that (I did that
once with an ISA board, not exactly on purpose as I didn't realise power
was still applied, and all the pieces survived, but the system had to be
power-cycled). Removing and rescanning buses behind the ASM2824 switch
did work (and triggered rather a surprising oops/panic with the nicstar
driver handling an ATM board downstream, which I then had to remove from
my configuration for the purpose of this verification), but did not cause
the offending link to go down, so the workaround didn't (have to) trigger.

Last but not least I have chosen the timeout for retraining somewhat
heuristically, so I'll appreciate advice from someone more experienced
with PCIe than I am -- most of my involvement in this area so far has been
with conventional PCI.