[Bug 216123] ofwfb: r269278 broke booting on Power Mac G4

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=216123

            Bug ID: 216123
           Summary: ofwfb: r269278 broke booting on Power Mac G4
           Product: Base System
           Version: CURRENT
          Hardware: powerpc
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: tgl at sss.pgh.pa.us

I've been trying to install FreeBSD on a Power Mac G4 ("PowerMac 3,6",
a/k/a Mirrored Drive Doors 2003).  10.3 works fine, but neither
11-STABLE nor CURRENT are able to boot.  I've bisected the problem
and can state confidently that this commit broke it:

  r269278 | nwhitehorn | 2014-07-29 19:11:05 -0400 (Tue, 29 Jul 2014) | 6 lines

  Make mmap() of the console device when using ofwfb work like other supported
  framebuffer drivers. This lets ofwfb work with xf86-video-scfb and makes
  the driver much more generic and less PCI-centric. This changes some
  user-visible behavior and will require updates to the xorg-server port
  on PowerPC when using ATI graphics cards.

The immediately preceding commit boots fine, with or without kernel debug
options.  (Well, I have to back-patch r269365 and r305901, but then it
boots fine.)  With r269278, the behavior is one of these:

* kernel debug options enabled: seems to freeze immediately at boot.
After the boot loader says it's booting the kernel, the screen goes
black as expected, but no kernel output text ever appears.

* kernel debug options disabled: fatal kernel trap after a few dozen
lines of dmesg output.  I've transcribed a screen shot:

...
gem0: 10kB RX FIFO, 4kB TX FIFO
gem0: Ethernet address: (machine's MAC address here)
cryptosoft0: <software crypto> on nexus0

fatal kernel trap:

   exception  = 0xd0004840 (unknown)
   srr0       = 0x0
   srr1       = 0x0
   lr         = 0x861104
   curthread  = 0

panic: unknown trap
cpuid = 0
KDB: stack backtrace:
#0 0x4a93ac at panic+0x16c
#1 0x861308 at trap_fatal+0x1bc
#2 0x8622e4 at trap+0xfb4
#3 0x84fef0 at powerpc_interrupt+0x170
Uptime: 1s
(reboots)


Furthermore, on the way to isolating when the problem was introduced,
I discovered that the non-debug behavior changed from a trap to a silent
freeze at r279750 ("Make 32-bit PowerPC kernels, like 64-bit PowerPC
kernels, position-independent executables").  It seems to occur at the
same place though, right after reporting cryptosoft0, so I suppose that
it's the same bug manifesting differently.

These behaviors of freeze at boot with debug options, or freeze after
"cryptosoft0" without, are still present in CURRENT and 11-STABLE.
I gather from recent reports on the freebsd-ppc list that a number of
other people see these same behaviors, but not everyone does.  I am unable
to offer a theory as to just what the triggering difference is, or how the
bug might be fixed.  But I'd be happy to run further testing if given some
guidance.

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