The current code tries to allocate memory with GFP_KERNEL at
interrupt context, it would show below warning during the enumeration
when I test it with chipidea hardware, change GFP flag as GFP_ATOMIC
can fix this issue.

[   40.438237] zero gadget: high-speed config #2: loopback
[   40.444924] ------------[ cut here ]------------
[   40.449609] WARNING: CPU: 0 PID: 0 at kernel/locking/lockdep.c:2755 lockdep_trace_alloc+0x108/0x128()
[   40.461715] DEBUG_LOCKS_WARN_ON(irqs_disabled_flags(flags))
[   40.467130] Modules linked in:
[   40.470216]  usb_f_ss_lb g_zero libcomposite evbug
[   40.473822] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.3.0-rc5-00168-gb730aaf #604
[   40.481496] Hardware name: Freescale i.MX6 SoloX (Device Tree)
[   40.487345] Backtrace:
[   40.489857] [<80014e94>] (dump_backtrace) from [<80015088>] (show_stack+0x18/0x1c)
[   40.497445]  r6:80b67a80 r5:00000000 r4:00000000 r3:00000000
[   40.503234] [<80015070>] (show_stack) from [<802e27b4>] (dump_stack+0x8c/0xa4)
[   40.510503] [<802e2728>] (dump_stack) from [<8002cfe8>] (warn_slowpath_common+0x80/0xbc)
[   40.518612]  r6:8007510c r5:00000009 r4:80b49c88 r3:00000001
[   40.524396] [<8002cf68>] (warn_slowpath_common) from [<8002d05c>] (warn_slowpath_fmt+0x38/0x40)
[   40.533109]  r8:bcfdef80 r7:bdb705cc r6:000080d0 r5:be001e80 r4:809cc278
[   40.539965] [<8002d028>] (warn_slowpath_fmt) from [<8007510c>] (lockdep_trace_alloc+0x108/0x128)
[   40.548766]  r3:809d0128 r2:809cc278
[   40.552401]  r4:600b0193
[   40.554990] [<80075004>] (lockdep_trace_alloc) from [<801093d4>] (kmem_cache_alloc+0x28/0x15c)
[   40.563618]  r4:000080d0 r3:80b4aa8c
[   40.567270] [<801093ac>] (kmem_cache_alloc) from [<804d95e4>] (ep_alloc_request+0x58/0x68)
[   40.575550]  r10:7f01f104 r9:00000001 r8:bcfdef80 r7:bdb705cc r6:bc178700 r5:00000000
[   40.583512]  r4:bcfdef80 r3:813c0a38
[   40.587183] [<804d958c>] (ep_alloc_request) from [<7f01f7ec>] (loopback_set_alt+0x114/0x21c [usb_f_ss_lb])
[   40.596929] [<7f01f6d8>] (loopback_set_alt [usb_f_ss_lb]) from [<7f006910>] (composite_setup+0xbd0/0x17e8 [libcomposite])
[   40.607902]  r10:bd3a2c0c r9:00000000 r8:bcfdef80 r7:bc178700 r6:bdb702d0 r5:bcfdefdc
[   40.615866]  r4:7f0199b4 r3:00000002
[   40.619542] [<7f005d40>] (composite_setup [libcomposite]) from [<804dae88>] (udc_irq+0x784/0xd1c)
[   40.628431]  r10:80bb5619 r9:c0876140 r8:00012001 r7:bdb71010 r6:bdb70568 r5:00010001
[   40.636392]  r4:bdb70014
[   40.638985] [<804da704>] (udc_irq) from [<804d64f8>] (ci_irq+0x5c/0x118)
[   40.645702]  r10:80bb5619 r9:be11e000 r8:00000117 r7:00000000 r6:bdb71010 r5:be11e060
[   40.653666]  r4:bdb70010
[   40.656261] [<804d649c>] (ci_irq) from [<8007f638>] (handle_irq_event_percpu+0x7c/0x13c)
[   40.664367]  r6:00000000 r5:be11e060 r4:bdb05cc0 r3:804d649c
[   40.670149] [<8007f5bc>] (handle_irq_event_percpu) from [<8007f740>] (handle_irq_event+0x48/0x6c)
[   40.679036]  r10:00000000 r9:be008000 r8:00000001 r7:00000000 r6:bdb05cc0 r5:be11e060
[   40.686998]  r4:be11e000
[   40.689581] [<8007f6f8>] (handle_irq_event) from [<80082850>] (handle_fasteoi_irq+0xd4/0x1b0)
[   40.698120]  r6:80b56a30 r5:be11e060 r4:be11e000 r3:00000000
[   40.703898] [<8008277c>] (handle_fasteoi_irq) from [<8007ec04>] (generic_handle_irq+0x28/0x3c)
[   40.712524]  r7:00000000 r6:80b4aaf4 r5:00000117 r4:80b445fc
[   40.718304] [<8007ebdc>] (generic_handle_irq) from [<8007ef20>] (__handle_domain_irq+0x6c/0xe8)
[   40.727033] [<8007eeb4>] (__handle_domain_irq) from [<800095d4>] (gic_handle_irq+0x48/0x94)
[   40.735402]  r9:c080f100 r8:80b4ac6c r7:c080e100 r6:80b67d40 r5:80b49f00 r4:c080e10c
[   40.743290] [<8000958c>] (gic_handle_irq) from [<80015d38>] (__irq_svc+0x58/0x78)
[   40.750791] Exception stack(0x80b49f00 to 0x80b49f48)
[   40.755873] 9f00: 00000001 00000001 00000000 80024320 80b48000 80b4a9d0 80b4a984 80b433e4
[   40.764078] 9f20: 00000001 807f4680 00000000 80b49f5c 80b49f20 80b49f50 80071ca4 800113fc
[   40.772272] 9f40: 200b0013 ffffffff
[   40.775776]  r9:807f4680 r8:00000001 r7:80b49f34 r6:ffffffff r5:200b0013 r4:800113fc
[   40.783677] [<800113d4>] (arch_cpu_idle) from [<8006c5bc>] (default_idle_call+0x28/0x38)
[   40.791798] [<8006c594>] (default_idle_call) from [<8006c6dc>] (cpu_startup_entry+0x110/0x1b0)
[   40.800445] [<8006c5cc>] (cpu_startup_entry) from [<807e95dc>] (rest_init+0x12c/0x168)
[   40.808376]  r7:80b4a8c0 r3:807f4b7c
[   40.812030] [<807e94b0>] (rest_init) from [<80ad7cc0>] (start_kernel+0x360/0x3d4)
[   40.819528]  r5:80bcb000 r4:80bcb050
[   40.823171] [<80ad7960>] (start_kernel) from [<8000807c>] (0x8000807c)

It fixes commit 91c42b0d

 ("usb: gadget: loopback: Fix looping back
logic implementation").

Cc: <stable@vger.kernel.org> # v3.18+
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Reviewed-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>