[PATCH] serial: 8250: Add flag so drivers can avoid probe

[Joel Stanley]

The probe assumes the other end will be reading bytes out of the buffer
in order to probe the port at driver init. In some cases the other
end cannot be relied upon to read these bytes, so provide a flag for
them to skip this step.

Bit 16 was chosen as the flags are a int and the top bits are taken.

Signed-off-by: Joel Stanley <joel at jms.id.au>
Acked-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>
---
 drivers/tty/serial/8250/8250_port.c | 2 +-
 drivers/tty/serial/aspeed-vuart.c   | 2 +-
 include/linux/serial_core.h         | 1 +
 3 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 427dd789c0e5..90994e0389df 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2167,7 +2167,7 @@ int serial8250_do_startup(struct uart_port *port)
 		serial_port_out(port, UART_LCR, 0);
 	}
 
-	if (port->irq) {
+	if (port->irq && !(up->port.flags & UPF_NO_THRE_TEST)) {
 		unsigned char iir1;
 		/*
 		 * Test for UARTs that do not reassert THRE when the
diff --git a/drivers/tty/serial/aspeed-vuart.c b/drivers/tty/serial/aspeed-vuart.c
index 42ccb380af41..b9559025d710 100644
--- a/drivers/tty/serial/aspeed-vuart.c
+++ b/drivers/tty/serial/aspeed-vuart.c
@@ -260,7 +260,7 @@ static int ast_vuart_probe(struct platform_device *pdev)
 	port.port.type = PORT_16550A;
 	port.port.uartclk = clk;
 	port.port.flags = UPF_SHARE_IRQ | UPF_BOOT_AUTOCONF
-		| UPF_FIXED_PORT | UPF_FIXED_TYPE;
+		| UPF_FIXED_PORT | UPF_FIXED_TYPE | UPF_NO_THRE_TEST;
 
 	if (of_find_property(np, "no-loopback-test", NULL))
 		port.port.flags |= UPF_SKIP_TEST;
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index a3d7c0d4a03e..2d1fb7423999 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -193,6 +193,7 @@ struct uart_port {
 #define UPF_NO_TXEN_TEST	((__force upf_t) (1 << 15))
 #define UPF_MAGIC_MULTIPLIER	((__force upf_t) ASYNC_MAGIC_MULTIPLIER /* 16 */ )
 
+#define UPF_NO_THRE_TEST	((__force upf_t) (1 << 19))
 /* Port has hardware-assisted h/w flow control */
 #define UPF_AUTO_CTS		((__force upf_t) (1 << 20))
 #define UPF_AUTO_RTS		((__force upf_t) (1 << 21))
-- 
2.11.0