serial: meson: acquire port->lock in startup()

The uart_ops startup() callback is called without interrupts
disabled and without port->lock locked, relatively late during the
boot process (from the call path of console_on_rootfs()). If the
device is a console, it was already previously registered and could
be actively printing messages.

Since the startup() callback is reading/writing registers used by
the console write() callback (AML_UART_CONTROL), its access must
be synchronized using the port->lock. Currently it is not.

The startup() callback is the only function that explicitly enables
interrupts. Without the synchronization, it is possible that
interrupts become accidentally permanently disabled.

CPU0                           CPU1
meson_serial_console_write     meson_uart_startup
--------------------------     ------------------
spin_lock(port->lock)
val = readl(AML_UART_CONTROL)
uart_console_write()
                               writel(INT_EN, AML_UART_CONTROL)
writel(val, AML_UART_CONTROL)
spin_unlock(port->lock)

Add port->lock synchronization to meson_uart_startup() to avoid
racing with meson_serial_console_write().

Also add detailed comments to meson_uart_reset() explaining why it
is *not* using port->lock synchronization.

Link: https://lore.kernel.org/lkml/2a82eae7-a256-f70c-fd82-4e510750906e@samsung.com
Fixes: ff7693d0 ("ARM: meson: serial: add MesonX SoC on-chip uart driver")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Link: https://lore.kernel.org/r/20220508103547.626355-1-john.ogness@linutronix.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/meson_uart.c

@@ -258,6 +258,14 @@ static const char *meson_uart_type(struct uart_port *port)
 	return (port->type == PORT_MESON) ? "meson_uart" : NULL;
 }
 
+/*
+ * This function is called only from probe() using a temporary io mapping
+ * in order to perform a reset before setting up the device. Since the
+ * temporarily mapped region was successfully requested, there can be no
+ * console on this port at this time. Hence it is not necessary for this
+ * function to acquire the port->lock. (Since there is no console on this
+ * port at this time, the port->lock is not initialized yet.)
+ */
 static void meson_uart_reset(struct uart_port *port)
 {
 	u32 val;
@@ -272,9 +280,12 @@ static void meson_uart_reset(struct uart_port *port)
 
 static int meson_uart_startup(struct uart_port *port)
 {
+	unsigned long flags;
 	u32 val;
 	int ret = 0;
 
+	spin_lock_irqsave(&port->lock, flags);
+
 	val = readl(port->membase + AML_UART_CONTROL);
 	val |= AML_UART_CLEAR_ERR;
 	writel(val, port->membase + AML_UART_CONTROL);
@@ -290,6 +301,8 @@ static int meson_uart_startup(struct uart_port *port)
 	val = (AML_UART_RECV_IRQ(1) | AML_UART_XMIT_IRQ(port->fifosize / 2));
 	writel(val, port->membase + AML_UART_MISC);
 
+	spin_unlock_irqrestore(&port->lock, flags);
+
 	ret = request_irq(port->irq, meson_uart_interrupt, 0,
 			  port->name, port);