Detecting host connection with gadget serial driver, and WARNING backtraces

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I am trying to get a system working with the gadget serial driver, I need userland apps to change behaviour when connected to a PC (and enumerated).

The driver detects this event and prints a message, but no hotplug event fires. The tty is created at driver load time (i.e. on boot).

Is there some way I can / should be detecting this event from userland? Maybe some kind of ioctl on the gadget serial tty?

Also, I was getting a WARNING backtrace (pasted below) as soon as I sent data into the linux box until I found this in the archives:

I commented out the "low_latency = 1" in gadget/u_serial.c and the problem seems to have gone away.

WARNING: at kernel/mutex.c:207 __mutex_lock_slowpath+0x78/0x290()
Modules linked in: g_serial musb_hdrc
[<c002922c>] (dump_backtrace+0x0/0x114) from [<c0255b5c>] (dump_stack+0x18/0x1c)
 r7:00000000 r6:c0256edc r5:c02cd8ac r4:000000cf
[<c0255b44>] (dump_stack+0x0/0x1c) from [<c0039d50>] (warn_slowpath_common+0x50/0x68)
[<c0039d00>] (warn_slowpath_common+0x0/0x68) from [<c0039d80>] (warn_slowpath_null+0x18/0x1c)
 r7:c7f50320 r6:c7e36000 r5:c7e363e0 r4:c7f5c000
[<c0039d68>] (warn_slowpath_null+0x0/0x1c) from [<c0256edc>] (__mutex_lock_slowpath+0x78/0x290)
[<c0256e64>] (__mutex_lock_slowpath+0x0/0x290) from [<c0257108>] (mutex_lock+0x14/0x28)
 r8:00000010 r7:00000000 r6:c7e36000 r5:c7e363e0 r4:c7e363e0
[<c02570f4>] (mutex_lock+0x0/0x28) from [<c0152a58>] (echo_set_canon_col+0x1c/0x40)
 r5:c7e363e0 r4:c7e36000
[<c0152a3c>] (echo_set_canon_col+0x0/0x40) from [<c0154ebc>] (n_tty_receive_buf+0xc4c/0xefc)
 r5:00000077 r4:00000012
[<c0154270>] (n_tty_receive_buf+0x0/0xefc) from [<c015767c>] (flush_to_ldisc+0x130/0x1e8)
[<c015754c>] (flush_to_ldisc+0x0/0x1e8) from [<c01577a4>] (tty_flip_buffer_push+0x70/0x80)
[<c0157734>] (tty_flip_buffer_push+0x0/0x80) from [<bf03d0b4>] (gs_rx_push+0x168/0x258 [g_serial])
 r5:c7e56980 r4:00000010
[<bf03cf4c>] (gs_rx_push+0x0/0x258 [g_serial]) from [<c003f17c>] (tasklet_action+0x84/0xf0)
[<c003f0f8>] (tasklet_action+0x0/0xf0) from [<c003f854>] (__do_softirq+0x98/0x12c)
 r5:00000018 r4:00000104
[<c003f7bc>] (__do_softirq+0x0/0x12c) from [<c003f938>] (irq_exit+0x50/0xa8)
[<c003f8e8>] (irq_exit+0x0/0xa8) from [<c0025078>] (asm_do_IRQ+0x78/0x94)
[<c0025000>] (asm_do_IRQ+0x0/0x94) from [<c0025b2c>] (__irq_svc+0x4c/0x90)
Exception stack(0xc7f5dd38 to 0xc7f5dd80)
dd20:                                                       00000000 00000002
dd40: c7f5dd48 40000013 00000000 c7e56980 00000000 c7f5c000 c7e36000 00000000
dd60: bf03e83c c7f5dda4 c7f5dd48 c7f5dd80 bf03c07c bf03cf08 40000013 ffffffff
 r5:fec48000 r4:ffffffff
[<bf03cd0c>] (gs_open+0x0/0x240 [g_serial]) from [<c0152500>] (tty_open+0x354/0x538)
[<c01521ac>] (tty_open+0x0/0x538) from [<c009b1e0>] (chrdev_open+0x1c0/0x1e0)
[<c009b020>] (chrdev_open+0x0/0x1e0) from [<c00961c0>] (__dentry_open+0x174/0x290)
 r8:c7cc8680 r7:c009b020 r6:c7839e38 r5:c7e51d80 r4:00000000
[<c009604c>] (__dentry_open+0x0/0x290) from [<c00963b8>] (nameidata_to_filp+0x4c/0x64)
[<c009636c>] (nameidata_to_filp+0x0/0x64) from [<c00a28b8>] (do_last+0x488/0x5e4)
 r5:00000000 r4:00000000
[<c00a2430>] (do_last+0x0/0x5e4) from [<c00a44d0>] (do_filp_open+0x174/0x4f8)
[<c00a435c>] (do_filp_open+0x0/0x4f8) from [<c0095f38>] (do_sys_open+0x64/0x11c)
[<c0095ed4>] (do_sys_open+0x0/0x11c) from [<c0096028>] (sys_open+0x24/0x28)
[<c0096004>] (sys_open+0x0/0x28) from [<c0025f20>] (ret_fast_syscall+0x0/0x2c)


Jon Povey

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