On Fri, 21 Feb 2014, loody wrote:

quoted

quoted

after tracing the kernel src, I found the usb resume call back is
called in drivers/base/power/main.c -> dpm_resume.

is that the only way usb resume functions could be called?

Yes.

I found all usb devices will be register as dev->power.async_suspend
=1 in usb_new_device.

if all usb devices' resume will be finished in
drivers/base/power/main.c -> dpm_resume, what is the difference to
register these devices as async_suspend =1?
Doesn't async_schedule in dpm_resume will let those async_suspend =1
devices resume after calling dpm_resume?

Sorry, I should have been more careful in my reply.

The resume routines are always called from device_resume.  If
async_suspend is set to 0 then dpm_resume calls device_resume directly.  
If async_suspend is set to 1 then dpm_resume starts up an async thread,
which calls device_resume indirectly (by way of async_resume).

Alan Stern