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Message-ID: <000f67ea-8d1a-4b04-a5e6-60bcce4390af@foss.st.com>
Date: Tue, 26 Aug 2025 11:33:29 +0200
From: Patrice CHOTARD <patrice.chotard@...s.st.com>
To: Markus Schneider-Pargmann <msp@...libre.com>,
Daniel Lezcano
<daniel.lezcano@...aro.org>,
Thomas Gleixner <tglx@...utronix.de>
CC: <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
"Mendez, Judith" <jm@...com>, Kevin Hilman <khilman@...libre.com>
Subject: Re: [PATCH v2] clocksource/drivers/arm_global_timer: Add
auto-detection for initial prescaler values
On 8/19/25 09:52, Markus Schneider-Pargmann wrote:
> 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.
>
> Reviewed-by: Kevin Hilman <khilman@...libre.com>
> Tested-by: Kevin Hilman <khilman@...libre.com>
> Signed-off-by: Markus Schneider-Pargmann <msp@...libre.com>
> ---
> Changes in v2:
> - Rebased to v6.17-rc1
> - Link to v1: https://lore.kernel.org/r/20250808-topic-am43-arm-global-timer-v6-16-v1-1-82067d327580@baylibre.com
> ---
> drivers/clocksource/Kconfig | 4 ++--
> drivers/clocksource/arm_global_timer.c | 44 ++++++++++++++++++++++++++++++----
> 2 files changed, 41 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
> index 645f517a1ac26d0c12d4b2b46591321ddf6e20b3..92df191b25b1af339dffc791f81ff541b09a87a8 100644
> --- a/drivers/clocksource/Kconfig
> +++ b/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"
> diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
> index 2d86bbc2764a04944e453038013142b2495d26b9..5e3d6bb7e437badeba45d8bf2c1df14eb3427448 100644
> --- a/drivers/clocksource/arm_global_timer.c
> +++ b/drivers/clocksource/arm_global_timer.c
> @@ -263,14 +263,13 @@ static void __init gt_delay_timer_init(void)
> register_current_timer_delay(>_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, >_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;
>
>
> ---
> base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
> change-id: 20250808-topic-am43-arm-global-timer-v6-16-66fc0c6981ab
>
> Best regards,
Tested on STMicroelectronics STiH410-B2260 board
Tested-by: Patrice Chotard <patrice.chotard@...s.st.com>
Thanks
Patrice
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