[Bug 192248] New: [vt] panic in vtbuf_fill_locked from vt_upgrade with no vt(4) drivers attached

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=192248

            Bug ID: 192248
           Summary: [vt] panic in vtbuf_fill_locked from vt_upgrade with
                    no vt(4) drivers attached
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: Needs Triage
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: emaste at freebsd.org

** I have no early vt(4) driver:
VT: init without driver.

** My WIP vt(4) fb driver fails to attach:
terasic_mtl0: Failed to attach fbd device

** Later vt_upgrade gets called and panics:
panic: vtbuf_fill_locked begin.tp_row 0 must be < screen height 0
cpuid = 0
KDB: enter: panic
[ thread pid 0 tid 100000 ]
Stopped at      kdb_enter+0x8c: lui     at,0x0
db> bt
Tracing pid 0 tid 100000 td 0xffffffff80729c20
db_trace_thread+40 (?,?,?,?) ra ffffffff80117514 sp 98000000007c5ca0 sz 32
ffffffff80117388+18c (ff0000,?,ffffffff,?) ra ffffffff801165f0 sp
98000000007c5cc0 sz 64
ffffffff80116168+488 (?,?,?,?) ra ffffffff801168bc sp 98000000007c5d00 sz 224
db_command_loop+f4 (?,?,?,?) ra ffffffff8011a2c8 sp 98000000007c5de0 sz 32
ffffffff8011a120+1a8 (?,?,?,?) ra ffffffff802e3e10 sp 98000000007c5e00 sz 832
kdb_trap+1f0 (?,?,?,?) ra ffffffff80515bb0 sp 98000000007c6140 sz 64
trap+10d8 (?,?,?,?) ra ffffffff804ff34c sp 98000000007c6180 sz 320
MipsKernGenException+168 (0,0,80582490,aa) ra ffffffff802e3944 sp
98000000007c62c0 sz 448
kdb_enter+8c (?,?,?,?) ra ffffffff80293800 sp 98000000007c6480 sz 32
ffffffff80293550+2b0 (?,?,?,?) ra ffffffff80293930 sp 98000000007c64a0 sz 32
kassert_panic+c8 (?,0,0,107b3ac) ra ffffffff80186fd4 sp 98000000007c64c0 sz 128
vtbuf_fill_locked+54 (?,?,?,?) ra ffffffff80188ddc sp 98000000007c6540 sz 64
ffffffff80188dc8+14 (?,?,?,?) ra ffffffff802fc28c sp 98000000007c6580 sz 32
terminal_set_winsize_blank+1bc (?,?,?,?) ra ffffffff802fc2e0 sp
98000000007c65a0 sz 96
terminal_set_winsize+28 (?,?,?,?) ra ffffffff80189654 sp 98000000007c6600 sz 32
vt_upgrade+194 (?,?,?,?) ra ffffffff802231dc sp 98000000007c6620 sz 160
mi_startup+20c (?,?,?,?) ra ffffffff8010026c sp 98000000007c66c0 sz 32
_start+dc (?,?,?,?) ra 0 sp 98000000007c66e0 sz 0
pid 0
db>

presumably vt_upgrade needs to detect the case that only the dummy driver
exists, and do nothing

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