serial: 8250: Add 8250 port clock update method

Some platforms can be designed in a way so the UART port reference clock
might be asynchronously changed at some point. In Baikal-T1 SoC this may
happen due to the reference clock being shared between two UART ports, on
the Allwinner SoC the reference clock is derived from the CPU clock, so
any CPU frequency change should get to be known/reflected by/in the UART
controller as well. But it's not enough to just update the
uart_port->uartclk field of the corresponding UART port, the 8250
controller reference clock divisor should be altered so to preserve
current baud rate setting. All of these things is done in a coherent
way by calling the serial8250_update_uartclk() method provided in this
patch. Though note that it isn't supposed to be called from within the
UART port callbacks because the locks using to the protect the UART port
data are already taken in there.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20200723003357.26897-2-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index d64ca77d9cfa9d4c342e1a48dbb296d2bc624caf..09475695effd472dd027922b5642d189a23921ec 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2632,6 +2632,46 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
                                  (port->uartclk + tolerance) / 16);
 }
 
+/*
+ * Note in order to avoid the tty port mutex deadlock don't use the next method
+ * within the uart port callbacks. Primarily it's supposed to be utilized to
+ * handle a sudden reference clock rate change.
+ */
+void serial8250_update_uartclk(struct uart_port *port, unsigned int uartclk)
+{
+       struct uart_8250_port *up = up_to_u8250p(port);
+       unsigned int baud, quot, frac = 0;
+       struct ktermios *termios;
+       unsigned long flags;
+
+       mutex_lock(&port->state->port.mutex);
+
+       if (port->uartclk == uartclk)
+               goto out_lock;
+
+       port->uartclk = uartclk;
+       termios = &port->state->port.tty->termios;
+
+       baud = serial8250_get_baud_rate(port, termios, NULL);
+       quot = serial8250_get_divisor(port, baud, &frac);
+
+       serial8250_rpm_get(up);
+       spin_lock_irqsave(&port->lock, flags);
+
+       uart_update_timeout(port, termios->c_cflag, baud);
+
+       serial8250_set_divisor(port, baud, quot, frac);
+       serial_port_out(port, UART_LCR, up->lcr);
+       serial8250_out_MCR(up, UART_MCR_DTR | UART_MCR_RTS);
+
+       spin_unlock_irqrestore(&port->lock, flags);
+       serial8250_rpm_put(up);
+
+out_lock:
+       mutex_unlock(&port->state->port.mutex);
+}
+EXPORT_SYMBOL_GPL(serial8250_update_uartclk);
+
 void
 serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,
                          struct ktermios *old)

diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index 6545f8cfc8fa23709c026c523089b09a4dae5efe..2b70f736b091dde3bf1898d96a16e8ca6d90f752 100644 (file)
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -155,6 +155,8 @@ extern int early_serial_setup(struct uart_port *port);
 
 extern int early_serial8250_setup(struct earlycon_device *device,
                                         const char *options);
+extern void serial8250_update_uartclk(struct uart_port *port,
+                                     unsigned int uartclk);
 extern void serial8250_do_set_termios(struct uart_port *port,
                struct ktermios *termios, struct ktermios *old);
 extern void serial8250_do_set_ldisc(struct uart_port *port,