clocksource/drivers/arm_arch_timer: Use event stream scaling when available

With FEAT_ECV and the 1GHz counter, it is pretty likely that the
event stream divider doesn't fit in the field that holds the
divider value (we only have 4 bits to describe counter bits [15:0]

Thankfully, FEAT_ECV also provides a scaling mechanism to switch
the field to cover counter bits [23:8] instead.

Enable this on arm64 when ECV is available (32bit doesn't have
any detection infrastructure and is unlikely to be run on an
ARMv8.6 system anyway).

Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20220203170502.2694422-1-maz@kernel.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/arm_arch_timer.c

@@ -880,10 +880,19 @@ static void __arch_timer_setup(unsigned type,
 	clockevents_config_and_register(clk, arch_timer_rate, 0xf, max_delta);
 }
 
-static void arch_timer_evtstrm_enable(int divider)
+static void arch_timer_evtstrm_enable(unsigned int divider)
 {
 	u32 cntkctl = arch_timer_get_cntkctl();
 
+#ifdef CONFIG_ARM64
+	/* ECV is likely to require a large divider. Use the EVNTIS flag. */
+	if (cpus_have_const_cap(ARM64_HAS_ECV) && divider > 15) {
+		cntkctl |= ARCH_TIMER_EVT_INTERVAL_SCALE;
+		divider -= 8;
+	}
+#endif
+
+	divider = min(divider, 15U);
 	cntkctl &= ~ARCH_TIMER_EVT_TRIGGER_MASK;
 	/* Set the divider and enable virtual event stream */
 	cntkctl |= (divider << ARCH_TIMER_EVT_TRIGGER_SHIFT)
@@ -912,7 +921,7 @@ static void arch_timer_configure_evtstream(void)
 		lsb++;
 
 	/* enable event stream */
-	arch_timer_evtstrm_enable(max(0, min(lsb, 15)));
+	arch_timer_evtstrm_enable(max(0, lsb));
 }
 
 static void arch_counter_set_user_access(void)

include/clocksource/arm_arch_timer.h

@@ -56,6 +56,7 @@ enum arch_timer_spi_nr {
 #define ARCH_TIMER_EVT_TRIGGER_MASK	(0xF << ARCH_TIMER_EVT_TRIGGER_SHIFT)
 #define ARCH_TIMER_USR_VT_ACCESS_EN	(1 << 8) /* virtual timer registers */
 #define ARCH_TIMER_USR_PT_ACCESS_EN	(1 << 9) /* physical timer registers */
+#define ARCH_TIMER_EVT_INTERVAL_SCALE	(1 << 17) /* EVNTIS in the ARMv8 ARM */
 
 #define ARCH_TIMER_EVT_STREAM_PERIOD_US	100
 #define ARCH_TIMER_EVT_STREAM_FREQ				\