cdc-acm: fix incorrect runtime wakeup in acm_tty_write

Resend as V1/V2 have email format issue. Sorry for bothering.

I use Thunderbird. It has no a button to enable LKML email simply. :)

V3: Change email config to resend.
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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;

Greg,

We have to abandon this patchset. Zhuang Jin Can, a USB expert, reviewed the patches. He says acm_tty_write already considers it carefully.

acm_tty_write puts the urb to a delayed queue when acm->susp_count is not 0. acm_suspend adds 1 to acm->susp_count and acm_resume decreases 1 from it .

Sorry for the bothering. Also thank Jin Can for the comments.

Yanmin