clocksource/drivers/arm_arch_timer_mmio: Add MMIO clocksource

The MMIO driver can also double as a clocksource, something that was
missing in its previous incarnation. Add it for completeness.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20250814154622.10193-5-maz@kernel.org

Author: Marc Zyngier

drivers/clocksource/arm_arch_timer_mmio.c

@@ -45,13 +45,15 @@ enum arch_timer_access {
 
 struct arch_timer {
 	struct clock_event_device	evt;
+	struct clocksource		cs;
 	struct arch_timer_mem		*gt_block;
 	void __iomem			*base;
 	enum arch_timer_access		access;
 	u32				rate;
 };
 
 #define evt_to_arch_timer(e) container_of(e, struct arch_timer, evt)
+#define cs_to_arch_timer(c) container_of(c, struct arch_timer, cs)
 
 static void arch_timer_mmio_write(struct arch_timer *timer,
 				  enum arch_timer_reg reg, u64 val)
@@ -128,6 +130,13 @@ static noinstr u64 arch_counter_mmio_get_cnt(struct arch_timer *t)
 	return ((u64) cnt_hi << 32) | cnt_lo;
 }
 
+static u64 arch_mmio_counter_read(struct clocksource *cs)
+{
+	struct arch_timer *at = cs_to_arch_timer(cs);
+
+	return arch_counter_mmio_get_cnt(at);
+}
+
 static int arch_timer_mmio_shutdown(struct clock_event_device *clk)
 {
 	struct arch_timer *at = evt_to_arch_timer(clk);
@@ -256,6 +265,16 @@ static void arch_timer_mmio_setup(struct arch_timer *at, int irq)
 					(unsigned long)CLOCKSOURCE_MASK(56));
 
 	enable_irq(at->evt.irq);
+
+	at->cs = (struct clocksource) {
+		.name	= "arch_mmio_counter",
+		.rating	= 300,
+		.read	= arch_mmio_counter_read,
+		.mask	= CLOCKSOURCE_MASK(56),
+		.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
+	};
+
+	clocksource_register_hz(&at->cs, at->rate);
 }
 
 static int arch_timer_mmio_frame_register(struct platform_device *pdev,