clocksource/drivers/mediatek: Use GPT as sched clock source

When cpu is in deep idle, arch timer will stop counting. Setup GPT as
sched clock source so it can keep counting in idle.
Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>

drivers/clocksource/mtk_timer.c

@@ -24,6 +24,7 @@
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/of_irq.h>
+#include <linux/sched_clock.h>
 #include <linux/slab.h>
 
 #define GPT_IRQ_EN_REG		0x00
@@ -59,6 +60,13 @@ struct mtk_clock_event_device {
 	struct clock_event_device dev;
 };
 
+static void __iomem *gpt_sched_reg __read_mostly;
+
+static u64 notrace mtk_read_sched_clock(void)
+{
+	return readl_relaxed(gpt_sched_reg);
+}
+
 static inline struct mtk_clock_event_device *to_mtk_clk(
 				struct clock_event_device *c)
 {
@@ -230,6 +238,8 @@ static void __init mtk_timer_init(struct device_node *node)
 	mtk_timer_setup(evt, GPT_CLK_SRC, TIMER_CTRL_OP_FREERUN);
 	clocksource_mmio_init(evt->gpt_base + TIMER_CNT_REG(GPT_CLK_SRC),
 			node->name, rate, 300, 32, clocksource_mmio_readl_up);
+	gpt_sched_reg = evt->gpt_base + TIMER_CNT_REG(GPT_CLK_SRC);
+	sched_clock_register(mtk_read_sched_clock, 32, rate);
 
 	/* Configure clock event */
 	mtk_timer_setup(evt, GPT_CLK_EVT, TIMER_CTRL_OP_REPEAT);