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

["Maciej W. Rozycki" <macro@orcam.me.uk>]

Hi Bjorn,

 Thank you for coming back to this patch series.  I'll try to address your 
concerns, but it may take a little.  The reason is I'm currently on site 
at my lab until the end of the week and barring my day job, etc. I want to 
focus on items to do (and I do have a bunch) that require local hardware 
access.  The issue concerned with this patch series does not, so I'll get 
to looking into it in more depth hopefully from next week.  For the time 
being however please see below.

On Thu, 3 Nov 2022, Bjorn Helgaas wrote:

> > > > 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)

 There is actually a PCIe-to-PCI bridge device further downstream (device 
0000:08:00.0 in the listings below; bus 09 is conventional PCI), but it 
doesn't matter for the issue concerned; the issue triggers whether the 
bridge module has been plugged or not.

> > 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.

 That would be my expectation as well.  Is there a reliable way to request
that however without actually writing a piece of code to do so from inside 
the kernel?  Sadly our documentation is vague on the matter, in particular 
Documentation/ABI/testing/sysfs-bus-pci, but here's what I have obtained:

# lspci -t
-[0000:00]---00.0-[01-0b]----00.0-[02-0b]--+-00.0-[03]--
                                           +-02.0-[04]----00.0
                                           +-03.0-[05-09]----00.0-[06-09]--+-01.0-[07]--+-00.0
                                           |                               |            \-00.3
                                           |                               \-02.0-[08-09]----00.0-[09]--+-01.0
                                           |                                                            \-02.0
                                           +-04.0-[0a]----00.0
                                           \-08.0-[0b]--+-00.0
                                                        \-00.1
# for name in /sys/bus/pci/devices/0000\:??\:??.?/reset_method; do echo "$(basename $(dirname $name)): $(cat $name)"; done
0000:01:00.0: pm bus
0000:02:00.0: pm bus
0000:02:02.0: pm
0000:02:03.0: pm
0000:02:04.0: pm
0000:02:08.0: pm
0000:04:00.0: bus
0000:05:00.0: bus
0000:06:01.0: bus
0000:07:00.0: bus
0000:08:00.0: bus
0000:09:01.0: pm bus
0000:0a:00.0: flr bus
0000:0b:00.0: pm bus
0000:0b:00.1: pm
# 

(mind that the problematic link is between 0000:02:03.0 and 0000:05:00.0), 
and then:

# echo 1 >/sys/bus/pci/devices/0000\:05\:00.0/reset
-sh: echo: write error: Inappropriate ioctl for device
# 

(which I gather is supposed to poke at 0000:02:03.0's SBR) so it doesn't 
seem to be effective.  Needless to say:

# echo 1 >/sys/bus/pci/devices/0000\:02\:03.0/reset
# 

while apparently succeeds doesn't do anything spectacular.  Any 
suggestions?

 In any case I would expect resetting a bus tree to cause all the quirks 
required to be reapplied, preferably after bus re-enumeration even as the 
topology can change afterwards if not for the hardware erratum handled 
here, although indeed I would find it reasonable to only perform the 
actions on the bus subtree reset and not it's parent.  In that case the 
quirk would have to be applied by hand somehow.

> 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.

 I believe hardware is supposed to establish a link at all in the first 
place (i.e. initially at 2.5GT/s, and then possibly upgrade afterwards as 
capabilities have been exchanged between the endpoints), however as this 
case indicates this is not necessarily universally guaranteed.

> 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.

 I have chewed it over before choosing this approach and did this for what 
I think is a very good reason, first for U-Boot and then, when I figured 
out a feasible approach, also for Linux.  Bear in mind that we have a pair 
of devices that completely fail to negotiate link ever by default and we 
do not know whether it's the upstream device (which we can see) or the 
downstream device (which we can't, because the link is down) that is the 
culprit.  With that we have as follows.

1. Both devices in question are PCIe switches, so they can be swapped with 
   each other.  In that case what is now the upstream device will become 
   the downstream device and vice versa.  Now the Pericom/Diodes switch 
   only has 2.5GT/s downstream ports, so the issue by definition cannot 
   trigger, but companies/designers tend to reuse hardware IP blocks, and 
   I am quite sure Pericom/Diodes has switches in its product line that 
   support rates beyond 2.5GT/s in their downstream ports.  So a swapped 
   scenario is in my opinion quite real.  Matching on the vendor:device ID 
   won't work then as the suspect ASMedia part will not be seen due to a 
   failed link.  If it's the Pericom/Diodes switch that is the culprit, 
   then it's even worse, as it is the downstream device already.

2. We don't know really what the ultimate cause is and therefore whether 
   the issue can trigger with other vendor devices (and presumably one of 
   the two involved here).  And the nature of the failure is such, that I 
   think it is very unlikely a user will reach out to our (Linux kernel) 
   community for a workaround.  It does not look like a software problem 
   at all and neither like one that can be solved by software.

   Therefore they will most likely conclude hardware they have is broken 
   and throw it out causing frustration even though the hardware might be 
   (im)perfectly capable of working, just requiring a little poke to get 
   going.

   Just as I almost threw it out even though I dare say I am quite an 
   experienced engineer, reasonably familiar with odd hardware and the 
   Linux kernel and ultimately I was able to come up with a solution.  It 
   is only because I noticed by chance that 0000:02:03.0 device's Link 
   Training bit is flipping, combined with my imagination as to how to 
   possibly deal with such a situation that I succeeded in bringing the 
   link up.  I genuinely thought all was lost and I was exceedingly happy 
   when I overcame it as the PI7C9X2G304 switch module was key to my 
   project with no suitable alternative on the horizon.

3. We apply this quirk to failed links only.  As a non-working link is a
   fatal condition, things cannot get worse by trying to retrain it by 
   hand at 2.5GT/s, as otherwise it doesn't work anyway.  So it's just a 
   last ditch attempt to make it run really.

   Can you think of a scenario where retraining a non-working link would 
   cause more damage than leaving it alone in the non-working state?  

   This quirk only uses generic PCIe features, common to all hardware, and 
   therefore I think it can be applied in a generic manner.  Now there 
   could be broken hardware that does not correctly handle the generic 
   features used here, but if it has a failed link too, then it will just 
   continue not to work.  So it won't be a regression.

 Now if I made any mistake in getting the conditions wrong that are 
supposed to determine that a link is non-working, than I'll be glad to 
have the code corrected.  Such a case shouldn't determine the overall 
validity of the approach I propose here though.

 NB I have included code to lift the 2.5GT/s restriction, matched by the 
downstream port's vendor:device ID, but now that I've been thinking about 
this issue again I suspect it might be too liberal, because we don't 
really know if it is safe to do so if the downstream device is not a 
PI7C9X2G304.  So perhaps we need to match on the vendor:device IDs of both 
endpoints of such a broken link.  What do you think?

 I'll let you chew my reasoning over now too, as I have an issue with a
non-working PCIe parallel port device to sort out (device 0000:07:00.0 
above) and that actually requires flipping rocker switches in my lab, 
which I cannot do remotely.  Thank you for your review.

  Maciej