From: Sean Wang <sean.wang@mediatek.com>

Bluetooth Interface (BTIF) is designed dedicatedly for MediaTek SOC with
BT in order to be instead of the UART interface between BT module and Host
CPU, and not exported to user space to access.

As the UART design, BTIF will be an APB slave and can transmit or receive
data by MCU access, but doesn't provide termios function like baudrate and
flow control setup.

Even LCR on offset 0xC that is just a FAKELCR
a. If FAKELCR[7] is equaled to 1, RBR(0x00), THR(0x00), IER(0x04)
   will not be readable/writable.

b. If FAKELCR is equaled to 0xBF, RBR(0x00), THR(0x00), IER(0x04),
   IIR(0x08), and LSR(0x14) will not be readable/writable.

So adding a new capability 'UART_CAP_NTIO' for the unusual unsupported
case.

The bluetooth driver would use BTIF device as a serdev. So the termios
still function would be called in kernelspace from ttyport_open in
drivers/tty/serdev/serdev-ttyprt.c.

Fixes: 1c16ae65e250 ("serial: 8250: of: Add new port type for MediaTek BTIF controller on MT7622/23 SoC")
Cc: Steven Liu <redacted>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>

--
v1->v2:
no change on termios->c_cflag and refine commit message

v2->v3:
change the naming from NMOD to NTIO as TIO is a well established prefix
for termios IOCTLs.

v3->v4:
1. remove appropriate tag
2. add the explanation why the termios is required even when the connection
   isn't exported to userspace.
---
 drivers/tty/serial/8250/8250.h      | 1 +
 drivers/tty/serial/8250/8250_port.c | 5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index 52bb21205bb6..0d9d3bfe48af 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h

@@ -82,6 +82,7 @@ struct serial8250_config {
 #define UART_CAP_MINI	(1 << 17)	/* Mini UART on BCM283X family lacks:
 					 * STOP PARITY EPAR SPAR WLEN5 WLEN6
 					 */
+#define UART_CAP_NTIO	(1 << 18)	/* UART doesn't do termios */