serial: 8250: Fix max baud limit in generic 8250 port

Standard 8250 UART ports are designed in a way so they can communicate
with baud rates up to 1/16 of a reference frequency. It's expected from
most of the currently supported UART controllers. That's why the former
version of serial8250_get_baud_rate() method called uart_get_baud_rate()
with min and max baud rates passed as (port->uartclk / 16 / UART_DIV_MAX)
and ((port->uartclk + tolerance) / 16) respectively. Doing otherwise, like
it was suggested in commit ("serial: 8250_mtk: support big baud rate."),
caused acceptance of bauds, which was higher than the normal UART
controllers actually supported. As a result if some user-space program
requested to set a baud greater than (uartclk / 16) it would have been
permitted without truncation, but then serial8250_get_divisor(baud)
(which calls uart_get_divisor() to get the reference clock divisor) would
have returned a zero divisor. Setting zero divisor will cause an
unpredictable effect varying from chip to chip. In case of DW APB UART the
communications just stop.

Lets fix this problem by getting back the limitation of (uartclk +
tolerance) / 16 maximum baud supported by the generic 8250 port. Mediatek
8250 UART ports driver developer shouldn't have touched it in the first
place  notably seeing he already provided a custom version of set_termios()
callback in that glue-driver which took into account the extended baud
rate values and accordingly updated the standard and vendor-specific
divisor latch registers anyway.

Fixes: 81bb549fdf14 ("serial: 8250_mtk: support big baud rate.")
Signed-off-by: Serge Semin <[email protected]>
Cc: Alexey Malahov <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: Paul Burton <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Long Cheng <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Russell King <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index f77bf820b7a30aeea8133b15b02b55784f2c9c3c..4d83c85a7389bd948391ef8ed2e5083d31fe4563 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2615,6 +2615,8 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
                                             struct ktermios *termios,
                                             struct ktermios *old)
 {
+       unsigned int tolerance = port->uartclk / 100;
+
        /*
         * Ask the core to calculate the divisor for us.
         * Allow 1% tolerance at the upper limit so uart clks marginally
@@ -2623,7 +2625,7 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
         */
        return uart_get_baud_rate(port, termios, old,
                                  port->uartclk / 16 / UART_DIV_MAX,
-                                 port->uartclk);
+                                 (port->uartclk + tolerance) / 16);
 }
 
 void