[PATCH] serial: lpuart: Avoid division by zero when requested baudrate is

[Andrey Smirnov <andrew.smirnov@gmail.com>]

With serdev device support added there's now a corner case where:

 1. There is a DT node for a serdev device on one of the UARTs
 2. There is no driver that binds against serdev device's compatibility
    string

with 1 and 2 being true it is possible to end up in a situation where
a particualr UART has not been initalized to any baudrate when
clock_notifier_call_chain() gets called. This effectively translates
to

	set_baudrate(uart, 0);

which for LPUART driver result in a division by zero.

To avoid this problem, convert lpuart_serial_setbaudrate() to treat
zero baudrate as a request to disable the UART. While we are at it add
a BUG_ON() to lpuart_setbrg() to simplify finding any future bugs.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
---
 drivers/serial/serial_lpuart.c | 13 ++++++++-----
 include/serial/lpuart.h        |  2 ++
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/drivers/serial/serial_lpuart.c b/drivers/serial/serial_lpuart.c
index f28035a32..246fc3d3a 100644
--- a/drivers/serial/serial_lpuart.c
+++ b/drivers/serial/serial_lpuart.c
@@ -68,11 +68,14 @@ static int lpuart_serial_setbaudrate(struct console_device *cdev,
 
 	lpuart_enable(lpuart, false);
 
-	lpuart_setbrg(lpuart->base,
-		      clk_get_rate(lpuart->clk),
-		      baudrate);
-
-	lpuart_enable(lpuart, true);
+	/*
+	 * We treat baudrate of 0 as a request to disable UART
+	 */
+	if (baudrate) {
+		lpuart_setbrg(lpuart->base, clk_get_rate(lpuart->clk),
+			      baudrate);
+		lpuart_enable(lpuart, true);
+	}
 
 	lpuart->baudrate = baudrate;
 
diff --git a/include/serial/lpuart.h b/include/serial/lpuart.h
index a920291de..9c6e271eb 100644
--- a/include/serial/lpuart.h
+++ b/include/serial/lpuart.h
@@ -228,6 +228,8 @@ static inline void lpuart_setbrg(void __iomem *base,
 	unsigned int bfra;
 	u16 sbr;
 
+	BUG_ON(!baudrate);
+
 	sbr = (u16) (refclock / (16 * baudrate));
 
 	writeb(sbr >> 8,   base + UARTBDH);
-- 
2.17.0


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