serial: mvebu-uart: correctly calculate minimal possible baudrate

stable inclusion
from stable-5.10.50
commit 347af865b6a20d2f498644f612c0e09d2eeba199
bugzilla: 174522 https://gitee.com/openeuler/kernel/issues/I4DNFY

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=347af865b6a20d2f498644f612c0e09d2eeba199

--------------------------------

[ Upstream commit deeaf963 ]

For default (x16) scheme which is currently used by mvebu-uart.c driver,
maximal divisor of UART base clock is 1023*16. Therefore there is limit for
minimal supported baudrate. This change calculate it correctly and prevents
setting invalid divisor 0 into hardware registers.
Signed-off-by: NPali Rohár <pali@kernel.org>
Fixes: 68a0db1d ("serial: mvebu-uart: add function to change baudrate")
Link: https://lore.kernel.org/r/20210624224909.6350-4-pali@kernel.orgSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Acked-by: NWeilong Chen <chenweilong@huawei.com>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

drivers/tty/serial/mvebu-uart.c

@@ -481,7 +481,7 @@ static void mvebu_uart_set_termios(struct uart_port *port,
 				   struct ktermios *old)
 {
 	unsigned long flags;
-	unsigned int baud;
+	unsigned int baud, min_baud, max_baud;
 
 	spin_lock_irqsave(&port->lock, flags);
 
@@ -500,16 +500,21 @@ static void mvebu_uart_set_termios(struct uart_port *port,
 		port->ignore_status_mask |= STAT_RX_RDY(port) | STAT_BRK_ERR;
 
 	/*
+	 * Maximal divisor is 1023 * 16 when using default (x16) scheme.
 	 * Maximum achievable frequency with simple baudrate divisor is 230400.
 	 * Since the error per bit frame would be of more than 15%, achieving
 	 * higher frequencies would require to implement the fractional divisor
 	 * feature.
 	 */
-	baud = uart_get_baud_rate(port, termios, old, 0, 230400);
+	min_baud = DIV_ROUND_UP(port->uartclk, 1023 * 16);
+	max_baud = 230400;
+
+	baud = uart_get_baud_rate(port, termios, old, min_baud, max_baud);
 	if (mvebu_uart_baud_rate_set(port, baud)) {
 		/* No clock available, baudrate cannot be changed */
 		if (old)
-			baud = uart_get_baud_rate(port, old, NULL, 0, 230400);
+			baud = uart_get_baud_rate(port, old, NULL,
+						  min_baud, max_baud);
 	} else {
 		tty_termios_encode_baud_rate(termios, baud, baud);
 		uart_update_timeout(port, termios->c_cflag, baud);