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Subject: Re: [PATCH] PCI: Check whether bridges allow access to extended
config space
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In-Reply-To: <5AEC8002.5000309@kontron.com>

[+cc Fred, Sinan]

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
>
> Suggestion : maybe change the pci_info() string to have :
> pci_bus 0000:xx: extended config space not accessible
> instead of
> pci_bus 0000:xx: extended config space not accessible on secondary bus
> as xx is already the number of the secondary bus

Oops, when I wrote that I was thinking it would be printed for the
bridge device (not the bus). I changed it as you suggest.

Interesting, I didn't think about the fact that pci_info() would work
on a struct pci_bus * as well as on a struct pci_dev *, since it's a
macro and they both have a "dev" member.

> 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

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