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Subject[PATCH 28/79] 8250: Now honours baud rate lower bounds
From: Anton Vorontsov <>

A platform clock drives 8250 ports in most SOC systems, the clock
might run at high frequencies, and so it's not always possible to
downscale uart clock to a desired value.

Currently the 8250 uart driver accepts not supported baud rates, and
what is worse, it is doing this silently, and then passes not accepted
values to a new termios, so userspace has no chance to catch this kind
of errors (userspace verifies that settings were accepted by reading
back and comparing the settings).

This patch fixes the issue by passing minimum baud rate to the
uart_get_baud_rate() call, the call should take care of all bounds,
so userspace should now report:

# stty -F /dev/ttyS0 speed 300
stty: /dev/ttyS0: unable to perform all requested operations

p.s. uart_get_baud_rate() falls back to 9600, which still might be too
low for some 10 GHz platforms, but that's a separate issue, and
we can wait with fixing this till we find such a platform.

Signed-off-by: Anton Vorontsov <>
Signed-off-by: Alan Cox <>
Signed-off-by: Greg Kroah-Hartman <>
drivers/serial/8250.c | 4 +++-
1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index 83168a6..1fd4894 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -2272,7 +2272,9 @@ serial8250_set_termios(struct uart_port *port, struct ktermios *termios,
* Ask the core to calculate the divisor for us.
- baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk/16);
+ baud = uart_get_baud_rate(port, termios, old,
+ port->uartclk / 16 / 0xffff,
+ port->uartclk / 16);
quot = serial8250_get_divisor(port, baud);


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