linux-kernel - Re: [PATCH v2] clocksource/drivers/arm_global_timer: Add auto-detection for initial prescaler values

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Message-ID: <db9c2cc8-d73b-4614-be9b-741510428070@linaro.org>
Date: Wed, 17 Sep 2025 18:00:42 +0200
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: Markus Schneider-Pargmann <msp@...libre.com>,
 Thomas Gleixner <tglx@...utronix.de>,
 Patrice Chotard <patrice.chotard@...s.st.com>
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 19/08/2025 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
> ---

Applied, thanks


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