Re: Help needed in understanding weird PCIe issue on imx6q (PCIe just goes bad)

[Fawad Lateef <fawadlateef@gmail.com>]

Hi Bjorn,

Thanks for your reply. Please see my comments below.

By the way, I have another development kit from "Embedded Artists"
with i.MX6Q SOM. I did similar test quickly (with WLAN attached to
PCIe root-complex _not_ PLX switch). This one also showed same
behavior though I have to confirm this properly (working on it). Then
at-least I can say its not exactly issue of Phytec SOM.

On Thu, 27 Feb 2020 at 00:27, Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> [+cc Richard, Lucas]
>
> On Wed, Feb 26, 2020 at 05:25:52PM -0600, Bjorn Helgaas wrote:
> > On Sat, Feb 22, 2020 at 04:25:41PM +0100, Fawad Lateef wrote:
> > > Hello,
> > >
> > > I am trying to figure-out an issue on our i.MX6Q platform based design
> > > where PCIe interface goes bad.
> > >
> > > We have a Phytec i.MX6Q eMMC SOM, attached to our custom designed
> > > board. PCIe root-complex from i.MX6Q is attached to PLX switch
> > > (PEX8605).
> > >
> > > Linux kernel version is 4.19.9x and also 4.14.134 (from phytec's
> > > linux-mainline repo). Kernel do not have PCIe hot-plug and PNP enabled
> > > in config.
> > >
> > > PLX switch #PERST is attached to a GPIO pin and stays in disable state
> > > until Linux is booted. So at boot time only PCIe root-complex is
> > > initialized by kernel.
> > >
> > > After boot if I do "lspci -v"  and see everything good from PCIe
> > > root-complex (below):
> > >
> > > ~ # lspci -v
> > > 00:00.0 PCI bridge: Synopsys, Inc. Device abcd (rev 01) (prog-if 00
> > > [Normal decode])
> > > Flags: bus master, fast devsel, latency 0, IRQ 295
> > > Memory at 01000000 (32-bit, non-prefetchable) [size=1M]
> > > Bus: primary=00, secondary=01, subordinate=ff, sec-latency=0
> > > I/O behind bridge: None
> > > Memory behind bridge: None
> > > Prefetchable memory behind bridge: None
> > > [virtual] Expansion ROM at 01100000 [disabled] [size=64K]
> > > Capabilities: [40] Power Management version 3
> > > Capabilities: [50] MSI: Enable+ Count=1/1 Maskable+ 64bit+
> > > Capabilities: [70] Express Root Port (Slot-), MSI 00
> > > Capabilities: [100] Advanced Error Reporting
> > > Capabilities: [140] Virtual Channel
> > > Kernel driver in use: pcieport
> > >
> > >
> > > Then I enable the #PERST pin of PLX switch, everything is still good
> > > (no rescan on Linux is done yet)
> > >
> > > ~ # echo 139 > /sys/class/gpio/export
> > > ~ # echo out > /sys/class/gpio/gpio139/direction
> > > ~ # echo 1 > /sys/class/gpio/gpio139/value
> > > ~ # lspci -v
> > > 00:00.0 PCI bridge: Synopsys, Inc. Device abcd (rev 01) (prog-if 00
> > > [Normal decode])
> > > Flags: bus master, fast devsel, latency 0, IRQ 295
> > > Memory at 01000000 (32-bit, non-prefetchable) [size=1M]
> > > Bus: primary=00, secondary=01, subordinate=ff, sec-latency=0
> > > I/O behind bridge: None
> > > Memory behind bridge: None
> > > Prefetchable memory behind bridge: None
> > > [virtual] Expansion ROM at 01100000 [disabled] [size=64K]
> > > Capabilities: [40] Power Management version 3
> > > Capabilities: [50] MSI: Enable+ Count=1/1 Maskable+ 64bit+
> > > Capabilities: [70] Express Root Port (Slot-), MSI 00
> > > Capabilities: [100] Advanced Error Reporting
> > > Capabilities: [140] Virtual Channel
> > > Kernel driver in use: pcieport
> > >
> > >
> > > Now just disable/put-in-reset the PLX switch (Linux don't see the
> > > switch yet, as no rescan on PCIe was done). Now "lspci -v" and
> > > root-complex goes bad.
> > >
> > > ~ # echo 0 > /sys/class/gpio/gpio139/value
> > > ~ # lspci -v
> > > 00:00.0 PCI bridge: Synopsys, Inc. Device abcd (rev 01) (prog-if 00
> > > [Normal decode])
> > > Flags: fast devsel, IRQ 295
> > > Memory at 01000000 (64-bit, prefetchable) [disabled] [size=1M]
> > > Bus: primary=00, secondary=00, subordinate=00, sec-latency=0
> > > I/O behind bridge: 00000000-00000fff [size=4K]
> > > Memory behind bridge: 00000000-000fffff [size=1M]
> > > Prefetchable memory behind bridge: 00000000-000fffff [size=1M]
> > > [virtual] Expansion ROM at 01100000 [disabled] [size=64K]
> > > Capabilities: [40] Power Management version 3
> > > Capabilities: [50] MSI: Enable- Count=1/1 Maskable+ 64bit+
> > > Capabilities: [70] Express Root Port (Slot-), MSI 00
> > > Capabilities: [100] Advanced Error Reporting
> > > Capabilities: [140] Virtual Channel
> > > Kernel driver in use: pcieport
> > >
> > > ~ # uname -a
> > > Linux buildroot-2019.08-imx6 4.14.134-phy2 #1 SMP Thu Feb 20 12:13:33
> > > UTC 2020 armv7l GNU/Linux
> > > ~ #
> > >
> > >
> > > I am really not sure what is going wrong here. Did I am missing
> > > something basic?
> >
> > I agree, it looks like something's wrong, but I really don't have any
> > ideas.
> >
> > I would start by using "lspci -xxxx" to see the actual values we get
> > from config space.  It looks like we're reading zeros from at least
> > the bus and window registers.

Somehow "lspci -xxxx" generate kernel crash ("imprecise external
abort") on both Phytec and Embedded Artists SOMs. lspci with -xxx (3
x) works but not 4 x. Seems like i.MX6 general issue?

> >
> > You could also instrument the i.MX config accessors in case there's
> > something strange going on there.  Maybe try to reproduce this on a
> > current upstream kernel?

I will try to read i.MX PCIe config registers, but I think those will
be read through PCIe interface and when it goes bad, devmem or any
other access to root-complex memory-address hangs the full SOM, not
even sys-rq works.

I was playing with 5.2.xx kernel earlier, but didn't try it on
recently. Will do a clean build with it again and see if I can face
similar situation.

Thanks,

Fawad Lateef

> >
> > Bjorn