kern/50453: "no console" panic after pci ext. conf change

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

>Number:         50453
>Category:       kern
>Synopsis:       "no console" panic after pci ext. conf change
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Nov 20 14:45:00 +0000 2015
>Originator:     Patrick Welche
>Release:        NetBSD 7.99.21
>Organization:
>Environment:
-current/amd64
>Description:
As per http://mail-index.netbsd.org/current-users/2015/10/29/msg028298.html

after updating to a -current/amd64 kernel with

commit 5ba1445273b0f70468889b5dda99d8c1cbfa5e3c
Author: msaitoh <msaitoh>
Date:   Fri Oct 2 05:22:49 2015 +0000

    PCI Extended Configuration stuff written by nonaka@:
    - Add PCI Extended Configuration Space support into x86.
    - Check register offset of pci_conf_read() in MD part. It returns (pcireg_t)
-1
      if it isn't accessible.
    - Decode Extended Capability in PCI Extended Configuration Space.
...

a pineview (i915drmkms) desktop panics on boot with

  panic: cnopen: no console device

If one attaches a serial console, all is well (but no X).

Working dmesg extracts:
pci0 at mainbus0 bus 0: configuration mode 1
pci0: i/o space, memory space enabled, rd/line, rd/mult, wr/inv ok
pchb0 at pci0 dev 0 function 0: vendor 8086 product a000 (rev. 0x02)
agp0 at pchb0: i915-family chipset
agp0: detected 7932k stolen memory
agp0: BAR size 100000 mismatches detected GTT size 40000; trusting BAR
agp0: aperture at 0xd0000000, size 0x10000000
i915drmkms0 at pci0 dev 2 function 0: vendor 8086 product a001 (rev. 0x02)
intelfb0 at i915drmkms0

Failing dmesg extracts:
pci0 at mainbus0 bus 0: configuration mode 1
acpi0: MCFG: 000:02:0: Ok (cfg[0x100]=0x00000000)
acpi0: MCFG: 000:02:1: Ok (cfg[0x100]=0x00000000)
pci0: i/o space, memory space enabled, rd/line, rd/mult, wr/inv ok
pchb0 at pci0 dev 0 function 0: vendor 8086 product a000 (rev. 0x02)
agp0 at pchb0: i915-family chipset
agp0: can't set up chipset flush page: 35
i915drmkms0 at pci0 dev 2 function 0: vendor 8086 product a001 (rev. 0x02)
DRM debug in intel_detect_pch: No PCH found.
DRM error in i915_gmch_probe: failed to set up gmch
i915drmkms0: unable to attach drm: 5
vendor 8086 product a002 (miscellaneous display, revision 0x02) at pci0 dev 2 function 1 not configured

The PCI Extended Configuration commit is huge, so it hard to see where to
look... "No PCH found" seems like a clue...

>How-To-Repeat:
Boot a -current/amd64 kernel on this pineview box
>Fix:

>Unformatted:
 -current post 2 Oct 2015