[PATCH 0/3] cdc-acm: fix incorrect runtime wakeup in acm_tty_write

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

There is a scenario about cdc-acm utilization.Application opens

n_gsm tty and cdc-acm tty. cdc-acm tty connects to xhci device.

The application configures cdc-adm tty to n_gsm tty as ldisc tty.

     n_gsm=>cdc-acm=>xhci driver

acm_tty_write can be called from n_gsm driver by ldisc connection,

and from application when application opens cdc-acm tty directly.

acm_tty_write wakes up the device by calling usb_autopm_get_interface_async,

which calls pm_runtime_get. However, pm_runtime_get can't wake up

the device before returning as it's an async wake up. Then, acm_tty_write

might access the device when it is off.

The patchset fixes it by:

1) add a new function usb_autopm_get_interface_upgrade to deal with

above 2 requirements;

2) wake up device in n_gsm driver if n_gsm drivers calls cdc-acm driver;