Laurent Pinchart reported that the Renesas R-Car H2 Lager board (r8a7790)
crashes during suspend tests.  Geert Uytterhoeven managed to reproduce the
issue on an M2-W Koelsch board (r8a7791):

  It occurs when the PME scan runs, once per second.  During PME scan, the
  PCI host bridge (rcar-pci) registers are accessed while its module clock
  has already been disabled, leading to the crash.

One reproducer is to configure s2ram to use "s2idle" instead of "deep"
suspend:

  # echo 0 > /sys/module/printk/parameters/console_suspend
  # echo s2idle > /sys/power/mem_sleep
  # echo mem > /sys/power/state

Another reproducer is to write either "platform" or "processors" to
/sys/power/pm_test.  It does not (or is less likely) to happen during full
system suspend ("core" or "none") because system suspend also disables
timers, and thus the workqueue handling PME scans no longer runs.  Geert
believes the issue may still happen in the small window between disabling
module clocks and disabling timers:

  # echo 0 > /sys/module/printk/parameters/console_suspend
  # echo platform > /sys/power/pm_test    # Or "processors"
  # echo mem > /sys/power/state

(Make sure CONFIG_PCI_RCAR_GEN2 and CONFIG_USB_OHCI_HCD_PCI are enabled.)

Rafael Wysocki agrees that PME scans should be suspended before the host
bridge registers become inaccessible.  To that end, queue the task on a
workqueue that gets frozen before devices suspend.

Rafael notes however that as a result, some wakeup events may be missed if
they are delivered via PME from a device without working IRQ (which hence
must be polled) and occur after the workqueue has been frozen.  If that
turns out to be an issue in practice, it may be possible to solve it by
calling pci_pme_list_scan() once directly from one of the host bridge's
pm_ops callbacks.

Stacktrace for posterity:

  PM: Syncing filesystems ... [   38.566237] done.
  PM: Preparing system for sleep (mem)
  Freezing user space processes ... [   38.579813] (elapsed 0.001 seconds) done.
  Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
  PM: Suspending system (mem)
  PM: suspend of devices complete after 152.456 msecs
  PM: late suspend of devices complete after 2.809 msecs
  PM: noirq suspend of devices complete after 29.863 msecs
  suspend debug: Waiting for 5 second(s).
  Unhandled fault: asynchronous external abort (0x1211) at 0x00000000
  pgd = c0003000
  [00000000] *pgd=80000040004003, *pmd=00000000
  Internal error: : 1211 [#1] SMP ARM
  Modules linked in:
  CPU: 1 PID: 20 Comm: kworker/1:1 Not tainted
  4.9.0-rc1-koelsch-00011-g68db9bc8 #3383
  Hardware name: Generic R8A7791 (Flattened Device Tree)
  Workqueue: events pci_pme_list_scan
  task: eb56e140 task.stack: eb58e000
  PC is at pci_generic_config_read+0x64/0x6c
  LR is at rcar_pci_cfg_base+0x64/0x84
  pc : [<c041d7b4>]    lr : [<c04309a0>]    psr: 600d0093
  sp : eb58fe98  ip : c041d750  fp : 00000008
  r10: c0e2283c  r9 : 00000000  r8 : 600d0013
  r7 : 00000008  r6 : eb58fed6  r5 : 00000002  r4 : eb58feb4
  r3 : 00000000  r2 : 00000044  r1 : 00000008  r0 : 00000000
  Flags: nZCv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment user
  Control: 30c5387d  Table: 6a9f6c80  DAC: 55555555
  Process kworker/1:1 (pid: 20, stack limit = 0xeb58e210)
  Stack: (0xeb58fe98 to 0xeb590000)
  fe80:                                                       00000002 00000044
  fea0: eb6f5800 c041d9b0 eb58feb4 00000008 00000044 00000000 eb78a000 eb78a000
  fec0: 00000044 00000000 eb9aff00 c0424bf0 eb78a000 00000000 eb78a000 c0e22830
  fee0: ea8a6fc0 c0424c5c eaae79c0 c0424ce0 eb55f380 c0e22838 eb9a9800 c0235fbc
  ff00: eb55f380 c0e22838 eb55f380 eb9a9800 eb9a9800 eb58e000 eb9a9824 c0e02100
  ff20: eb55f398 c02366c4 eb56e140 eb5631c0 00000000 eb55f380 c023641c 00000000
  ff40: 00000000 00000000 00000000 c023a928 cd105598 00000000 40506a34 eb55f380
  ff60: 00000000 00000000 dead4ead ffffffff ffffffff eb58ff74 eb58ff74 00000000
  ff80: 00000000 dead4ead ffffffff ffffffff eb58ff90 eb58ff90 eb58ffac eb5631c0
  ffa0: c023a844 00000000 00000000 c0206d68 00000000 00000000 00000000 00000000
  ffc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
  ffe0: 00000000 00000000 00000000 00000000 00000013 00000000 3a81336c 10ccd1dd
  [<c041d7b4>] (pci_generic_config_read) from [<c041d9b0>]
  (pci_bus_read_config_word+0x58/0x80)
  [<c041d9b0>] (pci_bus_read_config_word) from [<c0424bf0>]
  (pci_check_pme_status+0x34/0x78)
  [<c0424bf0>] (pci_check_pme_status) from [<c0424c5c>] (pci_pme_wakeup+0x28/0x54)
  [<c0424c5c>] (pci_pme_wakeup) from [<c0424ce0>] (pci_pme_list_scan+0x58/0xb4)
  [<c0424ce0>] (pci_pme_list_scan) from [<c0235fbc>]
  (process_one_work+0x1bc/0x308)
  [<c0235fbc>] (process_one_work) from [<c02366c4>] (worker_thread+0x2a8/0x3e0)
  [<c02366c4>] (worker_thread) from [<c023a928>] (kthread+0xe4/0xfc)
  [<c023a928>] (kthread) from [<c0206d68>] (ret_from_fork+0x14/0x2c)
  Code: ea000000 e5903000 f57ff04f e3a00000 (e5843000)
  ---[ end trace 667d43ba3aa9e589 ]---

Fixes: df17e62e ("PCI: Add support for polling PME state on suspended legacy PCI devices")
Reported-and-tested-by: NLaurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reported-and-tested-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NLukas Wunner <lukas@wunner.de>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: stable@vger.kernel.org	# 2.6.37+
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Cc: Simon Horman <horms+renesas@verge.net.au>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>