linux-kernel - [PATCH] serial: 8250: Fix UART_BUG

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Date:	Mon,  2 Feb 2015 15:47:07 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Jiri Slaby <jslaby@...e.cz>, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Peter Hurley <peter@...leysoftware.com>,
	Dick Hollenbeck <dick@...tplc.com>
Subject: [PATCH] serial: 8250: Fix UART_BUG_TXEN workaround

UARTs which do not trigger THRE interrupt if the fifo is already
empty when the interrupt is enabled need tx primed manually. These
UARTs are identified by the UART_BUG_TXEN flag to enable the
required workaround.

However, the current workaround is broken; if the fifo is already
empty but the shifter is still transmitting, then serial8250_tx_chars()
will not be called but no further THRE interrupt will occur, and
tx will stall. The appropriate check is for fifo empty (THRE), not
transmitter empty (TEMT).

Signed-off-by: Dick Hollenbeck <dick@...tplc.com>
[pjh: rewrote commit log]
Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
 drivers/tty/serial/8250/8250_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 285694b..0c61a9b 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -1365,7 +1365,7 @@ static void serial8250_start_tx(struct uart_port *port)
 			unsigned char lsr;
 			lsr = serial_in(up, UART_LSR);
 			up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
-			if (lsr & UART_LSR_TEMT)
+			if (lsr & UART_LSR_THRE)
 				serial8250_tx_chars(up);
 		}
 	}
-- 
2.2.2

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