Is tty->receive_room no longer usable w/ SMP?

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

A couple serial drivers I maintain check the value of tty->receive_room
to decide the max number of bytes to pull out of the UART's receive
FIFO and shove into a flip buffer.

After checking tty->receive room to decide how many bytes to read, one
of the drivers uses this sequence:

 tty_prepare_flip_string_flags(...)
 <fill up char buffer and flag buffer>
 tty_flip_buffer_push(...)

The other uses

 for (i=0; i<count; ++i)
   uart_insert_char(...);
 tty_flip_buffer_push(...);   
 
But, starting with kernel 3.12.0, whenSMP is enabled, tty->receive_room
is always 0 and never changes.  With SMP disabled, it seems to work the
way it always has.

Is use of tty->receive room no longer supported for SMP kernels?

How _should_ a serial driver decide how many rx characters there are
room for?

-- 
Grant Edwards               grant.b.edwards        Yow! What I need is a
                                  at               MATURE RELATIONSHIP with a
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