serial: asc: Adopt readl_/writel_relaxed()

The architectures supported by this driver, arm and sh, have expensive
implementations of writel(), reliant on spin locks and explicit L2 cache
management. These architectures provide a cheaper writel_relaxed() which
is much better suited to peripherals that do not perform DMA. The
situation with readl()/readl_relaxed()is similar although less acute.

This driver does not use DMA and will be more power efficient and more
robust (due to absence of spin locks during console I/O) if it uses the
relaxed variants.

The driver supports COMPILE_TEST and therefore falls back to writel()
when writel_relaxed() does not exist.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@gmail.com>
Cc: Maxime Coquelin <maxime.coquelin@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: kernel@stlinux.com
Cc: linux-serial@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/st-asc.c

@@ -151,12 +151,16 @@ static inline struct asc_port *to_asc_port(struct uart_port *port)
 
 static inline u32 asc_in(struct uart_port *port, u32 offset)
 {
-	return readl(port->membase + offset);
+	return readl_relaxed(port->membase + offset);
 }
 
 static inline void asc_out(struct uart_port *port, u32 offset, u32 value)
 {
+#ifdef writel_relaxed
+	writel_relaxed(value, port->membase + offset);
+#else
 	writel(value, port->membase + offset);
+#endif
 }
 
 /*