tty: add bits to manage multidrop mode

Multidrop mode differentiates the data characters and the address
characters. Data is transmitted with the parity bit to 0 and addresses
are transmitted with the parity bit to 1. However this usually slow
down communication by adding a delay between the first byte and the
others.

This patch defines two non-stadard bits PARMD (that enables multidrop)
and SENDA (that marks the next transmitted byte as address) that can
be used to completely remove the delay during transmission by
correctly managing the parity bit generation in hardware.

Signed-off-by: Rodolfo Giometti <giometti@linux.it>
Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
[change SENDA/PARMD to use bits 23 and 24]

Author: Rodolfo Giometti

include/linux/tty.h

@@ -167,6 +167,8 @@ struct tty_bufhead {
 #define C_CIBAUD(tty)	_C_FLAG((tty), CIBAUD)
 #define C_CRTSCTS(tty)	_C_FLAG((tty), CRTSCTS)
 #define C_CMSPAR(tty)	_C_FLAG((tty), CMSPAR)
+#define C_PARMD(tty)	_C_FLAG((tty), PARMD)
+#define C_SENDA(tty)	_C_FLAG((tty), SENDA)
 
 #define L_ISIG(tty)	_L_FLAG((tty), ISIG)
 #define L_ICANON(tty)	_L_FLAG((tty), ICANON)

include/uapi/asm-generic/termbits.h

@@ -141,6 +141,8 @@ struct ktermios {
 #define HUPCL	0002000
 #define CLOCAL	0004000
 #define CBAUDEX 0010000
+#define PARMD   040000000
+#define SENDA   0100000000
 #define    BOTHER 0010000
 #define    B57600 0010001
 #define   B115200 0010002