At probe time, the musb_am335x driver register its childs by
calling of_platform_populate(), which registers all childs in
the devicetree hierarchy recursively.

On the other side, the driver's remove() function uses of_device_unregister()
to remove each child of musb_am335x's.

However, when musb_dsps is loaded, its devices are attached to the musb_am335x
device as musb_am335x childs. Hence, musb_am335x remove() will attempt to
unregister the devices registered by musb_dsps, which produces a kernel panic.

In other words, the childs in the "struct device" hierarchy are not the same
as the childs in the "devicetree" hierarchy.

Ideally, we should enforce the removal of the devices registered by
musb_am335x *only*, instead of all its child devices. However, because of the
recursive nature of of_platform_populate, this doesn't seem possible.

Therefore, as the only solution at hand, this commit disables musb_am335x
driver removal capability, preventing it from being ever removed. This was
originally suggested by Sebastian Siewior:

https://www.mail-archive.com/linux-omap@vger.kernel.org/msg104946.html

And for reference, here's the panic upon module removal:

musb-hdrc musb-hdrc.0.auto: remove, state 4
usb usb1: USB disconnect, device number 1
musb-hdrc musb-hdrc.0.auto: USB bus 1 deregistered
Unable to handle kernel NULL pointer dereference at virtual address 0000008c
pgd = de11c000
[0000008c] *pgd=9e174831, *pte=00000000, *ppte=00000000
Internal error: Oops: 17 [#1] ARM
Modules linked in: musb_am335x(-) musb_dsps musb_hdrc usbcore usb_common
CPU: 0 PID: 623 Comm: modprobe Not tainted 3.15.0-rc4-00001-g24efd13 #69
task: de1b7500 ti: de122000 task.ti: de122000
PC is at am335x_shutdown+0x10/0x28
LR is at am335x_shutdown+0xc/0x28
pc : [<c0327798>]    lr : [<c0327794>]    psr: a0000013
sp : de123df8  ip : 00000004  fp : 00028f00
r10: 00000000  r9 : de122000  r8 : c000e6c4
r7 : de0e3c10  r6 : de0e3800  r5 : de624010  r4 : de1ec750
r3 : de0e3810  r2 : 00000000  r1 : 00000001  r0 : 00000000
Flags: NzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
Control: 10c5387d  Table: 9e11c019  DAC: 00000015
Process modprobe (pid: 623, stack limit = 0xde122240)
Stack: (0xde123df8 to 0xde124000)
3de0:                                                       de0e3810 bf054488
3e00: bf05444c de624010 60000013 bf043650 000012fc de624010 de0e3810 bf043a20
3e20: de0e3810 bf04b240 c0635b88 c02ca37c c02ca364 c02c8db0 de1b7500 de0e3844
3e40: de0e3810 c02c8e28 c0635b88 de02824c de0e3810 c02c884c de0e3800 de0e3810
3e60: de0e3818 c02c5b20 bf05417c de0e3800 de0e3800 c0635b88 de0f2410 c02ca838
3e80: bf05417c de0e3800 bf055438 c02ca8cc de0e3c10 bf054194 de0e3c10 c02ca37c
3ea0: c02ca364 c02c8db0 de1b7500 de0e3c44 de0e3c10 c02c8e28 c0635b88 de02824c
3ec0: de0e3c10 c02c884c de0e3c10 de0e3c10 de0e3c18 c02c5b20 de0e3c10 de0e3c10
3ee0: 00000000 bf059000 a0000013 c02c5bc0 00000000 bf05900c de0e3c10 c02c5c48
3f00: de0dd0c0 de1ec970 de0f2410 bf05929c de0f2444 bf05902c de0f2410 c02ca37c
3f20: c02ca364 c02c8db0 bf05929c de0f2410 bf05929c c02c94c8 bf05929c 00000000
3f40: 00000800 c02c8ab4 bf0592e0 c007fc40 c00dd820 6273756d 336d615f 00783533
3f60: c064a0ac de1b7500 de122000 de1b7500 c000e590 00000001 c000e6c4 c0060160
3f80: 00028e70 00028e70 00028ea4 00000081 60000010 00028e70 00028e70 00028ea4
3fa0: 00000081 c000e500 00028e70 00028e70 00028ea4 00000800 becb59f8 00027608
3fc0: 00028e70 00028e70 00028ea4 00000081 00000001 00000001 00000000 00028f00
3fe0: b6e6b6f0 becb59d4 000160e8 b6e6b6fc 60000010 00028ea4 00000000 00000000
[<c0327798>] (am335x_shutdown) from [<bf054488>] (dsps_musb_exit+0x3c/0x4c [musb_dsps])
[<bf054488>] (dsps_musb_exit [musb_dsps]) from [<bf043650>] (musb_shutdown+0x80/0x90 [musb_hdrc])
[<bf043650>] (musb_shutdown [musb_hdrc]) from [<bf043a20>] (musb_remove+0x24/0x68 [musb_hdrc])
[<bf043a20>] (musb_remove [musb_hdrc]) from [<c02ca37c>] (platform_drv_remove+0x18/0x1c)
[<c02ca37c>] (platform_drv_remove) from [<c02c8db0>] (__device_release_driver+0x70/0xc8)
[<c02c8db0>] (__device_release_driver) from [<c02c8e28>] (device_release_driver+0x20/0x2c)
[<c02c8e28>] (device_release_driver) from [<c02c884c>] (bus_remove_device+0xdc/0x10c)
[<c02c884c>] (bus_remove_device) from [<c02c5b20>] (device_del+0x104/0x198)
[<c02c5b20>] (device_del) from [<c02ca838>] (platform_device_del+0x14/0x9c)
[<c02ca838>] (platform_device_del) from [<c02ca8cc>] (platform_device_unregister+0xc/0x20)
[<c02ca8cc>] (platform_device_unregister) from [<bf054194>] (dsps_remove+0x18/0x38 [musb_dsps])
[<bf054194>] (dsps_remove [musb_dsps]) from [<c02ca37c>] (platform_drv_remove+0x18/0x1c)
[<c02ca37c>] (platform_drv_remove) from [<c02c8db0>] (__device_release_driver+0x70/0xc8)
[<c02c8db0>] (__device_release_driver) from [<c02c8e28>] (device_release_driver+0x20/0x2c)
[<c02c8e28>] (device_release_driver) from [<c02c884c>] (bus_remove_device+0xdc/0x10c)
[<c02c884c>] (bus_remove_device) from [<c02c5b20>] (device_del+0x104/0x198)
[<c02c5b20>] (device_del) from [<c02c5bc0>] (device_unregister+0xc/0x20)
[<c02c5bc0>] (device_unregister) from [<bf05900c>] (of_remove_populated_child+0xc/0x14 [musb_am335x])
[<bf05900c>] (of_remove_populated_child [musb_am335x]) from [<c02c5c48>] (device_for_each_child+0x44/0x70)
[<c02c5c48>] (device_for_each_child) from [<bf05902c>] (am335x_child_remove+0x18/0x30 [musb_am335x])
[<bf05902c>] (am335x_child_remove [musb_am335x]) from [<c02ca37c>] (platform_drv_remove+0x18/0x1c)
[<c02ca37c>] (platform_drv_remove) from [<c02c8db0>] (__device_release_driver+0x70/0xc8)
[<c02c8db0>] (__device_release_driver) from [<c02c94c8>] (driver_detach+0xb4/0xb8)
[<c02c94c8>] (driver_detach) from [<c02c8ab4>] (bus_remove_driver+0x4c/0xa0)
[<c02c8ab4>] (bus_remove_driver) from [<c007fc40>] (SyS_delete_module+0x128/0x1cc)
[<c007fc40>] (SyS_delete_module) from [<c000e500>] (ret_fast_syscall+0x0/0x48)

Fixes: 97238b35 ("usb: musb: dsps: use proper child nodes")
Cc: <stable@vger.kernel.org> # v3.12+
Acked-by: George Cherian <george.cherian@ti.com>
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Felipe Balbi <balbi@ti.com>