usb: musb: "(null)" in sysfs mode file after disabling a gadget (and at other times, system hangs)

[Merlijn Wajer <merlijn@wizzup.org>]

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Hi,

I found that the "mode" file in musb sys node will return "(null)" when
one would expect it show b_idle.

Wrong /mode file:

[Fresh boot, cable not connected yet]
root@n900devuan:~# modprobe g_nokia
[Insert cable connected to PC]
root@n900devuan:~# cat
/sys/devices/platform/68000000.ocp/480ab000.usb_otg_hs/musb-hdrc.0.auto/mode
b_peripheral
[Remove cable connected to PC]
root@n900devuan:~# cat
/sys/devices/platform/68000000.ocp/480ab000.usb_otg_hs/musb-hdrc.0.auto/mode
b_idle
root@n900devuan:~# rmmod g_nokia
root@n900devuan:~# cat
/sys/devices/platform/68000000.ocp/480ab000.usb_otg_hs/musb-hdrc.0.auto/mode
(null)
[Insert cable connected to PC]
root@n900devuan:~# cat
/sys/devices/platform/68000000.ocp/480ab000.usb_otg_hs/musb-hdrc.0.auto/mode
b_idle

I would expect it to state "b_idle" instead of "(null)".
I have also been able to reproduce this "(null)" state using only
configfs (and not deprecated g_nokia module), but this example is more
cumbersome to write up)


Sometimes it crashes the entire system (reproducibly):

[Fresh boot, USB cable connect to PC at all times]
root@n900devuan:~# cat
/sys/devices/platform/68000000.ocp/480ab000.usb_otg_hs/musb-hdrc.0.auto/mode
b_idle
root@n900devuan:~# modprobe g_nokia
root@n900devuan:~# cat
/sys/devices/platform/68000000.ocp/480ab000.usb_otg_hs/musb-hdrc.0.auto/mode
b_peripheral
root@n900devuan:~# rmmod g_nokia
(Hangs, reboots after a few seconds, probably due to watchdog)


This occurs on Nokia N900 (RX-51). Kernel config has:

CONFIG_USB_LIBCOMPOSITE=m
CONFIG_USB_F_*=m
CONFIG_USB_G_*=m

I don't have a trace because I don't have a serial on my Nokia N900. I
have not yet tested this on other musb devices, but I can do that if
that helps reproducing the issue.

Cheers,
Merlijn


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