The current code in __mdiobus_register() doesn't properly handle
failures returned by the devm_gpiod_get_optional() call: it returns
immediately, without unregistering the device that was added by the
call to device_register() earlier in the function.

This leaves a stale device, which then causes a NULL pointer
dereference in the code that handles deferred probing:

[    1.489982] Unable to handle kernel NULL pointer dereference at virtual address 00000074
[    1.498110] pgd = (ptrval)
[    1.500838] [00000074] *pgd=00000000
[    1.504432] Internal error: Oops: 17 [#1] SMP ARM
[    1.509133] Modules linked in:
[    1.512192] CPU: 1 PID: 51 Comm: kworker/1:3 Not tainted 4.20.0-00039-g3b73a4cc8b3e-dirty #99
[    1.520708] Hardware name: Xilinx Zynq Platform
[    1.525261] Workqueue: events deferred_probe_work_func
[    1.530403] PC is at klist_next+0x10/0xfc
[    1.534403] LR is at device_for_each_child+0x40/0x94
[    1.539361] pc : [<c0683fbc>]    lr : [<c0455d90>]    psr: 200e0013
[    1.545628] sp : ceeefe68  ip : 00000001  fp : ffffe000
[    1.550863] r10: 00000000  r9 : c0c66790  r8 : 00000000
[    1.556079] r7 : c0457d44  r6 : 00000000  r5 : ceeefe8c  r4 : cfa2ec78
[    1.562604] r3 : 00000064  r2 : c0457d44  r1 : ceeefe8c  r0 : 00000064
[    1.569129] Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
[    1.576263] Control: 18c5387d  Table: 0ed7804a  DAC: 00000051
[    1.582013] Process kworker/1:3 (pid: 51, stack limit = 0x(ptrval))
[    1.588280] Stack: (0xceeefe68 to 0xceef0000)
[    1.592630] fe60:                   cfa2ec78 c0c03c08 00000000 c0457d44 00000000 c0c66790
[    1.600814] fe80: 00000000 c0455d90 ceeefeac 00000064 00000000 0d7a542e cee9d494 cfa2ec78
[    1.608998] fea0: cfa2ec78 00000000 c0457d44 c0457d7c cee9d494 c0c03c08 00000000 c0455dac
[    1.617182] fec0: cf98ba44 cf926a00 cee9d494 0d7a542e 00000000 cf935a10 cf935a10 cf935a10
[    1.625366] fee0: c0c4e9b8 c0457d7c c0c4e80c 00000001 cf935a10 c0457df4 cf935a10 c0c4e99c
[    1.633550] ff00: c0c4e99c c045a27c c0c4e9c4 ced63f80 cfde8a80 cfdebc00 00000000 c013893c
[    1.641734] ff20: cfde8a80 cfde8a80 c07bd354 ced63f80 ced63f94 cfde8a80 00000008 c0c02d00
[    1.649936] ff40: cfde8a98 cfde8a80 ffffe000 c0139a30 ffffe000 c0c6624a c07bd354 00000000
[    1.658120] ff60: ffffe000 cee9e780 ceebfe00 00000000 ceeee000 ced63f80 c0139788 cf8cdea4
[    1.666304] ff80: cee9e79c c013e598 00000001 ceebfe00 c013e44c 00000000 00000000 00000000
[    1.674488] ffa0: 00000000 00000000 00000000 c01010e8 00000000 00000000 00000000 00000000
[    1.682671] ffc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[    1.690855] ffe0: 00000000 00000000 00000000 00000000 00000013 00000000 00000000 00000000
[    1.699058] [<c0683fbc>] (klist_next) from [<c0455d90>] (device_for_each_child+0x40/0x94)
[    1.707241] [<c0455d90>] (device_for_each_child) from [<c0457d7c>] (device_reorder_to_tail+0x38/0x88)
[    1.716476] [<c0457d7c>] (device_reorder_to_tail) from [<c0455dac>] (device_for_each_child+0x5c/0x94)
[    1.725692] [<c0455dac>] (device_for_each_child) from [<c0457d7c>] (device_reorder_to_tail+0x38/0x88)
[    1.734927] [<c0457d7c>] (device_reorder_to_tail) from [<c0457df4>] (device_pm_move_to_tail+0x28/0x40)
[    1.744235] [<c0457df4>] (device_pm_move_to_tail) from [<c045a27c>] (deferred_probe_work_func+0x58/0x8c)
[    1.753746] [<c045a27c>] (deferred_probe_work_func) from [<c013893c>] (process_one_work+0x210/0x4fc)
[    1.762888] [<c013893c>] (process_one_work) from [<c0139a30>] (worker_thread+0x2a8/0x5c0)
[    1.771072] [<c0139a30>] (worker_thread) from [<c013e598>] (kthread+0x14c/0x154)
[    1.778482] [<c013e598>] (kthread) from [<c01010e8>] (ret_from_fork+0x14/0x2c)
[    1.785689] Exception stack(0xceeeffb0 to 0xceeefff8)
[    1.790739] ffa0:                                     00000000 00000000 00000000 00000000
[    1.798923] ffc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[    1.807107] ffe0: 00000000 00000000 00000000 00000000 00000013 00000000
[    1.813724] Code: e92d47f0 e1a05000 e8900048 e1a00003 (e5937010)
[    1.819844] ---[ end trace 3c2c0c8b65399ec9 ]---

The actual error that we had from devm_gpiod_get_optional() was
-EPROBE_DEFER, due to the GPIO being provided by a driver that is
probed later than the Ethernet controller driver.

To fix this, we simply add the missing device_del() invocation in the
error path.

Fixes: 69226896 ("mdio_bus: Issue GPIO RESET to PHYs")
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>