Re: [PATCH] PCI: Check whether bridges allow access to extended config space

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, May 04, 2018 at 03:06:00PM -0500, Bjorn Helgaas wrote:
> On Fri, May 04, 2018 at 03:45:07PM +0000, Gilles Buloz wrote:
> > Le 04/05/2018 00:31, Bjorn Helgaas a écrit :
> > > [+cc LKML]
> > >
> > > On Thu, May 03, 2018 at 12:40:27PM +0000, Gilles Buloz wrote:
> > >> Subject:    [PATCH] For exception at PCI probe due to bridge reporting UR
> > >>
> > >> Even if a device supports extended config access, no such access must be
> > >> done to this device If there's a bridge not supporting that in the path
> > >> to this device. Doing such access with UR reporting enabled on the root
> > >> bridge leads to an exception.
> > >>
> > >> This is the case on a LS1043A CPU (NXP QorIQ Layerscape) platform with
> > >> the following bus topology :
> > >>    LS1043 PCIe root
> > >>      -> PEX8112 PCIe-to-PCI bridge (not supporting ext cfg on PCI side)
> > >>        -> PMC slot connector (for legacy PMC modules)
> > >> With a PMC module topology as follows :
> > >>    PMC connector
> > >>      -> PCI-to-PCIe bridge
> > >>        -> PCIe switch (4 ports)
> > >>          -> 4 PCIe devices (one on each port)
> > >> In this case all devices behind the PEX8112 are supporting extended config
> > >> access but this is prohibited by the PEX8112. Without this patch, an
> > >> exception (synchronous abort) occurs in pci_cfg_space_size_ext().
> > >>
> > >> This patch checks the parent bridge of each allocated child bus to know if
> > >> extended config access is supported on the child bus, and sets a flag in
> > >> child->bus_flags if not supported. This  flag is inherited by all children
> > >> buses of this child bus and then is checked to avoid this unsupported
> > >> accesses to every device on these buses.
> > > Hi Gilles,
> > >
> > > Thanks for the patch!  I reworked it a little bit to simplify the code
> > > in pci_alloc_child_bus().  Can you test it and make sure I didn't
> > > break anything?
> > >
> > Hi Bjorn,
> > 
> > Your rework works as expected. Tested on LS1043A platform with kernel
> > 4.17-rc1, and with some backport on kernel 4.1.35

Thanks for testing it!  I applied it to pci/enumeration for v4.18.

I think the ASPM issue below is unrelated.  But I would like to figure out
what's going on there, too, if you have any more information.

> > Info : with kernel 4.17-rc1, it turns out I need pcie_aspm=off to
> > have the PMC devices behind the PCI-to-PCIe bridge of the PMC safely
> > detected/configured. But this is not caused by the patch.
> 
> > Without pcie_aspm=off I saw this at one boot :
> >     "pci 0000:02:0e.0: ASPM: Could not configure common clock" for this bridge, but devices
> >     correctly detected/configured
> > but at most boots I get :
> >     no ASPM message but "pci 0000:04:02.0: bridge configuration invalid ([bus ff-ff]), reconfiguring "
> >     instead, and some devices are missing. Also lspci show "rev ff" for some devices.
> > I don't see this problem on 4.1.35 with the same backported patch.
> 
> This is interesting, especially since you have this unusual topology
> of a path to the device that is PCIe, then conventional PCI, then PCIe
> again.  We *should* be able to use ASPM on the PCIe links, but it's
> definitely not a well-tested scenario.
> 
> Can you tell if something is actually broken?  Sinan's recent change,
> 04875177dbe0 ("PCI/ASPM: Don't warn if already in common clock mode"),
> which appeared in v4.17-rc1, turns off the message in some cases.
> 
> The "bridge configuration invalid" message just means the firmware
> didn't configure the bridge.  We *should* still set it up correctly,
> but please report a bug if we don't.
> 
> lspci showing "ff" for some devices might be a symptom of the devices
> being powered off.  In that case config reads normally return ~0 data
> (though on your platform maybe it would cause exceptions).  I've seen
> this in other situations and wondered if it would be worth adding a
> hint to lspci so it could say "device may be powered off".
> 
> Anyway, if you are seeing something broken (more than just the
> messages), please start a new thread about each one.  If you do, could
> you please:
> 
>   - open a report at https://bugzilla.kernel.org/, in the Drivers/PCI
>     component (open a separate bug for each issue you see)
> 
>   - use kernel version 4.17-rc1 and mark it as a regression if
>     appropriate
> 
>   - attach (don't paste inline) the complete dmesg log and "lspci -vv"
>     output (as root) to the bug
> 
>   - post a note to linux-pci@vger.kernel.org, cc Fred, Sinan, and me,
>     and include the link to the bugzilla
> 
> Bjorn