linux-kernel - Help needed in understanding weird PCIe issue on imx6q (PCIe just goes bad)

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Date:   Sat, 22 Feb 2020 16:25:41 +0100
From:   Fawad Lateef <fawadlateef@...il.com>
To:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci@...r.kernel.org
Subject: Help needed in understanding weird PCIe issue on imx6q (PCIe just
 goes bad)

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?

Thanks in advance,

-- Fawad Lateef