[PATCH v4 11/13] serial: asc: Adopt readl_/writel_relaxed()

[Daniel Thompson]

The architectures supported by this driver 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 absense of spin locks during console I/O) if it uses the
relaxed variants.

This driver is cross compilable for testing purposes and remains
compilable on all architectures by falling back to writel() when
writel_relaxed() does not exist. We also include explicit compiler
barriers. There are redundant on ARM and SH but important on
x86 because it defines "relaxed" differently.

Signed-off-by: Daniel Thompson <daniel.thompson at linaro.org>
Cc: Srinivas Kandagatla <srinivas.kandagatla at gmail.com>
Cc: Maxime Coquelin <maxime.coquelin at st.com>
Cc: Patrice Chotard <patrice.chotard at st.com>
Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Cc: Jiri Slaby <jslaby at suse.cz>
Cc: kernel at stlinux.com
Cc: linux-serial at vger.kernel.org
---
 drivers/tty/serial/st-asc.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/st-asc.c b/drivers/tty/serial/st-asc.c
index 4f376d8..58aa1c6 100644
--- a/drivers/tty/serial/st-asc.c
+++ b/drivers/tty/serial/st-asc.c
@@ -152,12 +152,21 @@ 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);
+	u32 r;
+
+	r = readl_relaxed(port->membase + offset);
+	barrier();
+	return r;
 }
 
 static inline void asc_out(struct uart_port *port, u32 offset, u32 value)
 {
+#ifdef writel_relaxed
+	writel_relaxed(value, port->membase + offset);
+	barrier();
+#else
 	writel(value, port->membase + offset);
+#endif
 }
 
 /*
-- 
1.9.3