[PATCH v2] serial: 8250_dw: Honor clk_round_rate errors in dw8250_set_termios

[Heiko Stuebner]

clk_round_rate returns a signed long and may possibly return errors
in it, for example if there is no possible rate.

Till now dw8250_set_termios ignored any error, the signednes and would
just use the value as input to clk_set_rate. This of course falls apart
if there is an actual error, so check for errors and only try to set
a rate if the value is actually valid.

This turned up on some Rockchip platforms after commit
6a171b299379 ("serial: 8250_dw: Allow hardware flow control to be used")
enabled set_termios callback in all cases, not only ACPI.

Fixes: 6a171b299379 ("serial: 8250_dw: Allow hardware flow control to be used")
Signed-off-by: Heiko Stuebner <heiko@xxxxxxxxx>
Reviewed-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
---
There is also another patch floating around, fixing a separate issue
on top of this one: "serial: 8250_dw: Fix breakage when HAVE_CLK=n"

changes in v2:
- adapt commit message to make it more explicit, that this is a
  somewhat critical fix
- add Andy's Reviewed-by

 drivers/tty/serial/8250/8250_dw.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 6ee55a2d47bb..223ac234ddb2 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -257,7 +257,7 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios,
 {
 	unsigned int baud = tty_termios_baud_rate(termios);
 	struct dw8250_data *d = p->private_data;
-	unsigned int rate;
+	long rate;
 	int ret;
 
 	if (IS_ERR(d->clk) || !old)
@@ -265,7 +265,10 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios,
 
 	clk_disable_unprepare(d->clk);
 	rate = clk_round_rate(d->clk, baud * 16);
-	ret = clk_set_rate(d->clk, rate);
+	if (rate < 0)
+		ret = rate;
+	else
+		ret = clk_set_rate(d->clk, rate);
 	clk_prepare_enable(d->clk);
 
 	if (!ret)
-- 
2.11.0