This patch fixes a NULL pointer dereference when enabling regmap event
tracing in the presence of a syscon regmap, introduced by commit bdb0066d
("mfd: syscon: Decouple syscon interface from platform devices").
That patch introduced syscon regmaps that have their dev field set to NULL.
The regmap trace events expect it to point to a valid struct device and feed
it to dev_name():

  $ echo 1 > /sys/kernel/debug/tracing/events/regmap/enable

  Unable to handle kernel NULL pointer dereference at virtual address 0000002c
  pgd = 80004000
  [0000002c] *pgd=00000000
  Internal error: Oops: 17 [#1] SMP ARM
  Modules linked in: coda videobuf2_vmalloc
  CPU: 0 PID: 304 Comm: kworker/0:2 Not tainted 4.0.0-rc2+ #9197
  Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
  Workqueue: events_freezable thermal_zone_device_check
  task: 9f25a200 ti: 9f1ee000 task.ti: 9f1ee000
  PC is at ftrace_raw_event_regmap_block+0x3c/0xe4
  LR is at _regmap_raw_read+0x1bc/0x1cc
  pc : [<803636e8>]    lr : [<80365f2c>]    psr: 600f0093
  sp : 9f1efd78  ip : 9f1efdb8  fp : 9f1efdb4
  r10: 00000004  r9 : 00000001  r8 : 00000001
  r7 : 00000180  r6 : 00000000  r5 : 9f00e3c0  r4 : 00000003
  r3 : 00000001  r2 : 00000180  r1 : 00000000  r0 : 9f00e3c0
  Flags: nZCv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
  Control: 10c5387d  Table: 2d91004a  DAC: 00000015
  Process kworker/0:2 (pid: 304, stack limit = 0x9f1ee210)
  Stack: (0x9f1efd78 to 0x9f1f0000)
  fd60:                                                       9f1efda4 9f1efd88
  fd80: 800708c0 805f9510 80927140 800f0013 9f1fc800 9eb2f490 00000000 00000180
  fda0: 808e3840 00000001 9f1efdfc 9f1efdb8 80365f2c 803636b8 805f8958 800708e0
  fdc0: a00f0013 803636ac 9f16de00 00000180 80927140 9f1fc800 9f1fc800 9f1efe6c
  fde0: 9f1efe6c 9f732400 00000000 00000000 9f1efe1c 9f1efe00 80365f70 80365d7c
  fe00: 80365f3c 9f1fc800 9f1fc800 00000180 9f1efe44 9f1efe20 803656a4 80365f48
  fe20: 9f1fc800 00000180 9f1efe6c 9f1efe6c 9f732400 00000000 9f1efe64 9f1efe48
  fe40: 803657bc 80365634 00000001 9e95f910 9f1fc800 9f1efeb4 9f1efe8c 9f1efe68
  fe60: 80452ac0 80365778 9f1efe8c 9f1efe78 9e93d400 9e93d5e8 9f1efeb4 9f72ef40
  fe80: 9f1efeac 9f1efe90 8044e11c 80452998 8045298c 9e93d608 9e93d400 808e1978
  fea0: 9f1efecc 9f1efeb0 8044fd14 8044e0d0 ffffffff 9f25a200 9e93d608 9e481380
  fec0: 9f1efedc 9f1efed0 8044fde8 8044fcec 9f1eff1c 9f1efee0 80038d50 8044fdd8
  fee0: 9f1ee020 9f72ef40 9e481398 00000000 00000008 9f72ef54 9f1ee020 9f72ef40
  ff00: 9e481398 9e481380 00000008 9f72ef40 9f1eff5c 9f1eff20 80039754 80038bfc
  ff20: 00000000 9e481380 80894100 808e1662 00000000 9e4f2ec0 00000000 9e481380
  ff40: 800396f8 00000000 00000000 00000000 9f1effac 9f1eff60 8003e020 80039704
  ff60: ffffffff 00000000 ffffffff 9e481380 00000000 00000000 9f1eff78 9f1eff78
  ff80: 00000000 00000000 9f1eff88 9f1eff88 9e4f2ec0 8003df30 00000000 00000000
  ffa0: 00000000 9f1effb0 8000eb60 8003df3c 00000000 00000000 00000000 00000000
  ffc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
  ffe0: 00000000 00000000 00000000 00000000 00000013 00000000 ffffffff ffffffff
  Backtrace:
  [<803636ac>] (ftrace_raw_event_regmap_block) from [<80365f2c>] (_regmap_raw_read+0x1bc/0x1cc)
   r9:00000001 r8:808e3840 r7:00000180 r6:00000000 r5:9eb2f490 r4:9f1fc800
  [<80365d70>] (_regmap_raw_read) from [<80365f70>] (_regmap_bus_read+0x34/0x6c)
   r10:00000000 r9:00000000 r8:9f732400 r7:9f1efe6c r6:9f1efe6c r5:9f1fc800
   r4:9f1fc800
  [<80365f3c>] (_regmap_bus_read) from [<803656a4>] (_regmap_read+0x7c/0x144)
   r6:00000180 r5:9f1fc800 r4:9f1fc800 r3:80365f3c
  [<80365628>] (_regmap_read) from [<803657bc>] (regmap_read+0x50/0x70)
   r9:00000000 r8:9f732400 r7:9f1efe6c r6:9f1efe6c r5:00000180 r4:9f1fc800
  [<8036576c>] (regmap_read) from [<80452ac0>] (imx_get_temp+0x134/0x1a4)
   r6:9f1efeb4 r5:9f1fc800 r4:9e95f910 r3:00000001
  [<8045298c>] (imx_get_temp) from [<8044e11c>] (thermal_zone_get_temp+0x58/0x74)
   r7:9f72ef40 r6:9f1efeb4 r5:9e93d5e8 r4:9e93d400
  [<8044e0c4>] (thermal_zone_get_temp) from [<8044fd14>] (thermal_zone_device_update+0x34/0xec)
   r6:808e1978 r5:9e93d400 r4:9e93d608 r3:8045298c
  [<8044fce0>] (thermal_zone_device_update) from [<8044fde8>] (thermal_zone_device_check+0x1c/0x20)
   r5:9e481380 r4:9e93d608
  [<8044fdcc>] (thermal_zone_device_check) from [<80038d50>] (process_one_work+0x160/0x3d4)
  [<80038bf0>] (process_one_work) from [<80039754>] (worker_thread+0x5c/0x4f4)
   r10:9f72ef40 r9:00000008 r8:9e481380 r7:9e481398 r6:9f72ef40 r5:9f1ee020
   r4:9f72ef54
  [<800396f8>] (worker_thread) from [<8003e020>] (kthread+0xf0/0x108)
   r10:00000000 r9:00000000 r8:00000000 r7:800396f8 r6:9e481380 r5:00000000
   r4:9e4f2ec0
  [<8003df30>] (kthread) from [<8000eb60>] (ret_from_fork+0x14/0x34)
   r7:00000000 r6:00000000 r5:8003df30 r4:9e4f2ec0
  Code: e3140040 1a00001a e3140020 1a000016 (e596002c)
  ---[ end trace 193c15c2494ec960 ]---

Fixes: bdb0066d (mfd: syscon: Decouple syscon interface from platform devices)
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org