Warning about not setting tty->port: what to set it to?

[Grant Edwards <grant.b.edwards@gmail.com>]

I maintain a serial card driver which, starting with kernel 3.7,
produces this warning when a port is opened:

 tty_init_dev: ttyXYZ driver does not set tty->port. This will crash the kernel later. Fix the driver!

The driver still seems to work fine, but apparently I shouldn't expect
it to continue to do so.  

What am I supposed to set tty->port _to_ ?

Obviously, it's supposed to point to an instance of the type (struct tty_port).

Is there somewhere in particular I'm supposed to get that structure
instance from?

Am I supposed to allocate/clear one when the port is opened and free
it when the port is closed?  Or does the contents the structure
pointed to by tty->port need to be preserved?

Am I expected to do anything with the contents of that structure?

Or am I just providing some storage space for use by the tty layer
code?

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