Initialization of musb on board-omap2beagle fails in platform_driver_probe

[Ben Gamari <bgamari@gmail.com>]

Today I've had a very frustrating time trying to bring up a BeagleBoard
(arch/arm/mach-omap2/board-omap2beagle.c) with the 2.6.38
kernel. Unfortunately musb has been giving me great difficulties,

 # modprobe g_ether
[   25.276855] usb_gadget_probe_driver 1815: usb_gadget_probe_driver, no dev??
modprobe: Failed to load module g_ether: No such device.

After a great deal of printking, I've determined that the error is
coming from platform_driver_probe (platform.c:499). The code in question
appears to be a check ensuring that any devices being probe were first
registered. When the musb-hdrc module is probed, this check seems to
fail with code = 1 and list_empty(&drv->driver.p->klist_devices.k_list)
= 1.

What exactly is this check supposed to accomplish? Why would it fail
during musb initialization? The relevant call path is,

  kernel_init -> do_one_initcall -> musb_init -> platform_driver_probe
  
Both musb_init and platform_driver_probe return with -19. As far as I
can tell, the board's init_machine successfully calls usb_musb_init
from omap3_beagle_init, so the device should be registered well before
arriving in musb_init.

Any ideas would be greatly appreciated. Cheers,

 - Ben