clocksource/drivers/arm_global_timer: Add auto-detection for initial prescaler values

am43xx has a clock tree where the global timer clock is an indirect child
of the CPU clock used for frequency scaling:

  dpll_mpu_ck -- CPU/cpufreq
        |
        v
  dpll_mpu_m2_ck -- divider
        |
        v
  mpu_periphclk -- fixed divider by 2 used for global timer

When CPU frequency changes, the global timer's clock notifier rejects
the change because the hardcoded prescaler (1 or 2) cannot accommodate
the frequency range across all CPU OPPs (300, 600, 720, 800, 1000 MHz).

Add platform-specific prescaler auto-detection to solve this issue:

- am43xx: prescaler = 50 (calculated as initial_freq/GCD of all OPP
  freqs) This allows the timer to work across all CPU frequencies after
  the fixed divider by 2. Tested on am4372-idk-evm.

- zynq-7000: prescaler = 2 (preserves previous Kconfig default)

- Other platforms: prescaler = 1 (previous default)

The Kconfig option now defaults to 0 (auto-detection) but can still
override the auto-detected value when set to a non-zero value,
preserving existing customization workflows.

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com>
Tested-by: Judith Mendez <jm@ti.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20250819-topic-am43-arm-global-timer-v6-16-v2-1-6d082e2a5161@baylibre.com

Author: scosu

drivers/clocksource/Kconfig

@@ -395,8 +395,7 @@ config ARM_GLOBAL_TIMER
 
 config ARM_GT_INITIAL_PRESCALER_VAL
 	int "ARM global timer initial prescaler value"
-	default 2 if ARCH_ZYNQ
-	default 1
+	default 0
 	depends on ARM_GLOBAL_TIMER
 	help
 	  When the ARM global timer initializes, its current rate is declared
@@ -406,6 +405,7 @@ config ARM_GT_INITIAL_PRESCALER_VAL
 	  bounds about how much the parent clock is allowed to decrease or
 	  increase wrt the initial clock value.
 	  This affects CPU_FREQ max delta from the initial frequency.
+	  Use 0 to use auto-detection in the driver.
 
 config ARM_TIMER_SP804
 	bool "Support for Dual Timer SP804 module"

drivers/clocksource/arm_global_timer.c

@@ -263,14 +263,13 @@ static void __init gt_delay_timer_init(void)
 	register_current_timer_delay(&gt_delay_timer);
 }
 
-static int __init gt_clocksource_init(void)
+static int __init gt_clocksource_init(unsigned int psv)
 {
 	writel(0, gt_base + GT_CONTROL);
 	writel(0, gt_base + GT_COUNTER0);
 	writel(0, gt_base + GT_COUNTER1);
 	/* set prescaler and enable timer on all the cores */
-	writel(FIELD_PREP(GT_CONTROL_PRESCALER_MASK,
-			  CONFIG_ARM_GT_INITIAL_PRESCALER_VAL - 1) |
+	writel(FIELD_PREP(GT_CONTROL_PRESCALER_MASK, psv - 1) |
 	       GT_CONTROL_TIMER_ENABLE, gt_base + GT_CONTROL);
 
 #ifdef CONFIG_CLKSRC_ARM_GLOBAL_TIMER_SCHED_CLOCK
@@ -338,11 +337,45 @@ static int gt_clk_rate_change_cb(struct notifier_block *nb,
 	return NOTIFY_DONE;
 }
 
+struct gt_prescaler_config {
+	const char *compatible;
+	unsigned long prescaler;
+};
+
+static const struct gt_prescaler_config gt_prescaler_configs[] = {
+	/*
+	 * On am43 the global timer clock is a child of the clock used for CPU
+	 * OPPs, so the initial prescaler has to be compatible with all OPPs
+	 * which are 300, 600, 720, 800 and 1000 with a fixed divider of 2, this
+	 * gives us a GCD of 10. Initial frequency is 1000, so the prescaler is
+	 * 50.
+	 */
+	{ .compatible = "ti,am43", .prescaler = 50 },
+	{ .compatible = "xlnx,zynq-7000", .prescaler = 2 },
+	{ .compatible = NULL }
+};
+
+static unsigned long gt_get_initial_prescaler_value(struct device_node *np)
+{
+	const struct gt_prescaler_config *config;
+
+	if (CONFIG_ARM_GT_INITIAL_PRESCALER_VAL != 0)
+		return CONFIG_ARM_GT_INITIAL_PRESCALER_VAL;
+
+	for (config = gt_prescaler_configs; config->compatible; config++) {
+		if (of_machine_is_compatible(config->compatible))
+			return config->prescaler;
+	}
+
+	return 1;
+}
+
 static int __init global_timer_of_register(struct device_node *np)
 {
 	struct clk *gt_clk;
 	static unsigned long gt_clk_rate;
 	int err;
+	unsigned long psv;
 
 	/*
 	 * In A9 r2p0 the comparators for each processor with the global timer
@@ -378,8 +411,9 @@ static int __init global_timer_of_register(struct device_node *np)
 		goto out_unmap;
 	}
 
+	psv = gt_get_initial_prescaler_value(np);
 	gt_clk_rate = clk_get_rate(gt_clk);
-	gt_target_rate = gt_clk_rate / CONFIG_ARM_GT_INITIAL_PRESCALER_VAL;
+	gt_target_rate = gt_clk_rate / psv;
 	gt_clk_rate_change_nb.notifier_call =
 		gt_clk_rate_change_cb;
 	err = clk_notifier_register(gt_clk, &gt_clk_rate_change_nb);
@@ -404,7 +438,7 @@ static int __init global_timer_of_register(struct device_node *np)
 	}
 
 	/* Register and immediately configure the timer on the boot CPU */
-	err = gt_clocksource_init();
+	err = gt_clocksource_init(psv);
 	if (err)
 		goto out_irq;