linux-kernel - atmel_serial might lose modem status change

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Message-Id: <20081113.021601.30439144.anemo@mba.ocn.ne.jp>
Date:	Thu, 13 Nov 2008 02:16:01 +0900 (JST)
From:	Atsushi Nemoto <anemo@....ocn.ne.jp>
To:	hskinnemoen@...el.com
Cc:	linux@...mer.net, marc.pignat@...s.ch, linux-kernel@...r.kernel.org
Subject: atmel_serial might lose modem status change

I found a problem of handling of modem status of atmel_serial driver.

With the commit 1ecc26 ("atmel_serial: split the interrupt handler"),
handling of modem status signal was splitted into two parts.  The
atmel_tasklet_func() compares new status with irq_status_prev, but
irq_status_prev is not correct if signal status was changed while the
port is closed.

Here is a sequence to cause problem:

1. Remote side sets CTS (and DSR).
2. Local side close the port.
3. Local side clears RTS and DTR.
4. Remote side clears CTS and DSR.
5. Local side reopen the port.  hw_stopped becomes 1.
6. Local side sets RTS and DTR.
7. Remote side sets CTS and DSR.

Then CTS change interrupt can be received, but since CTS bit in
irq_status_prev and new status is same, uart_handle_cts_change() will
not be called (so hw_stopped will not be cleared, i.e. cannot send any
data).

I suppose irq_status_prev should be initialized at somewhere in open
sequence.  Is this fix correct?

diff --git a/drivers/serial/atmel_serial.c b/drivers/serial/atmel_serial.c
index 4c535f7..9f3b697 100644
--- a/drivers/serial/atmel_serial.c
+++ b/drivers/serial/atmel_serial.c
@@ -877,6 +877,9 @@ static int atmel_startup(struct uart_port *port)
 		}
 	}

+	/* Save current CSR for comparison in atmel_tasklet_func() */
+	atmel_port->irq_status_prev = UART_GET_CSR(port);
+
 	/*
 	 * Finally, enable the serial port
 	 */

---
Atsushi Nemoto
--
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