linux-kernel - Re: [PATCH] clocksource/drivers/arm_global_timer: Add auto-detection for initial prescaler values

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Message-ID: <9315875c-3358-4f56-b99e-a530baa8ca0e@ti.com>
Date: Wed, 20 Aug 2025 11:47:41 -0500
From: Judith Mendez <jm@...com>
To: Markus Schneider-Pargmann <msp@...libre.com>,
        Daniel Lezcano
	<daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Patrice
 Chotard" <patrice.chotard@...s.st.com>
CC: <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
        Kevin Hilman <khilman@...libre.com>
Subject: Re: [PATCH] clocksource/drivers/arm_global_timer: Add auto-detection
 for initial prescaler values

Hi Markus,

On 8/8/25 5:35 AM, 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.
> 
> Signed-off-by: Markus Schneider-Pargmann <msp@...libre.com>

Tested-by: Judith Mendez <jm@...com>


Thanks for you patch, it also cleared the noise on my end
on am437x board.

~ Judith