Re: [PATCH v5 5/5] PCI: Work around PCIe link training failures

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Nov 04, 2022 at 12:41:11AM +0100, Pali Rohár wrote:
> On Thursday 03 November 2022 18:13:35 Bjorn Helgaas wrote:
> > [+cc Pali]
> > 
> > On Sat, Sep 17, 2022 at 01:03:38PM +0100, Maciej W. Rozycki wrote:
> > > Attempt to handle cases such as with a downstream port of the ASMedia 
> > > ASM2824 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.
> > > 
> > > Instead 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.  Forcibly limiting the target link speed to 
> > > 2.5GT/s with the upstream ASM2824 device however makes the two switches 
> > > communicate correctly.  Removing the speed restriction afterwards makes 
> > > the two devices switch to 5.0GT/s then.
> > > 
> > > Make use of these observations then and detect the inability to train 
> > > the link, by checking for the Data Link Layer Link Active status bit 
> > > being off while the Link Bandwidth Management Status indicating that 
> > > hardware has changed the link speed or width in an attempt to correct 
> > > unreliable link operation.
> > > 
> > > Restrict the speed to 2.5GT/s then with the Target Link Speed field, 
> > > request a retrain and wait 200ms for the data link to go up.  If this 
> > > turns out successful, then lift the restriction, letting the devices 
> > > negotiate a higher speed.
> > > 
> > > Also check for a 2.5GT/s speed restriction the firmware may have already 
> > > arranged and lift it too with ports of devices known to continue working 
> > > afterwards, currently the ASM2824 only, that already report their data 
> > > link being up.
> > 
> > This quirk is run at boot-time and resume-time.  What happens after a
> > Secondary Bus Reset, as is done by pci_reset_secondary_bus()?
> 
> Flipping SBR bit can be done on any PCI-to-PCI bridge device and in this
> topology there are following: PCIe Root Port, ASMedia PCIe Switch
> Upstream Port, ASMedia PCIe Switch Downstream Port, Pericom PCIe Switch
> Upstream Port, Pericom PCIe Switch Downstream Port.
> (Maciej, I hope that this is whole topology and there is not some other
> device of PCI-to-PCI bridge type in your setup; please correct me)
> 
> Bjorn, to make it clear, on which device you mean to issue secondary bus
> reset?

IIUC, the problem is observed on the link between the ASM2824
downstream port and the PI7C9X2G304 upstream port, so my question is
about asserting SBR on the ASM2824 downstream port.  I think that
should cause the link between ASM2824 and PI7C9X2G304 to go down and
back up.

Thanks for the question; I didn't notice before that this quirk
applies to *all* devices.  I'm a little queasy about trying to fix
problems we have not observed.  In this case, I think the hardware is
*supposed* to establish a link at the highest supported speed
automatically.

If we need to work around a hardware bug, that's fine, but I'm not
sure I want to blindly try to help things along.

> Because I would not be surprised if different things happen when issuing
> bus reset on different parts of that topology.
> 
> > PCIe r6.0, sec 7.5.1.3.13, says "setting Secondary Bus Reset triggers
> > a hot reset on the corresponding PCI Express Port".  Sec 4.2.7 says
> > LinkUp is 0 in the LTSSM Hot Reset state, and the Hot Reset state
> > leads to Detect, so it looks like this reset would cause the link to
> > go down and come back up.
> > 
> > Can you tell if that's what happens?  Does the link negotiation fail
> > then, too?
> > 
> > If it does fail then, I don't know how hard we need to work to fix it.
> > Maybe we just accept it?  Or maybe we need a "quirk-after-reset" phase
> > or something?
> > 
> > Bjorn