linux-kernel - Re: [PATCH v4 5/9] clk: thead: Mark essential bus clocks as CLK_IGNORE

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Message-ID: <175036302986.4372.15656426260486761164@lazor>
Date: Thu, 19 Jun 2025 12:57:09 -0700
From: Stephen Boyd <sboyd@...nel.org>
To: Albert Ou <aou@...s.berkeley.edu>, Alex Gaynor <alex.gaynor@...il.com>, Alexandre Ghiti <alex@...ti.fr>, Alice Ryhl <aliceryhl@...gle.com>, Andreas Hindborg <a.hindborg@...nel.org>, Benno Lossin <lossin@...nel.org>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Boqun Feng <boqun.feng@...il.com>, Conor Dooley <conor+dt@...nel.org>, Danilo Krummrich <dakr@...nel.org>, Drew Fustini <drew@...7.com>, Fu Wei <wefu@...hat.com>, Gary Guo <gary@...yguo.net>, Guo Ren <guoren@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Marek Szyprowski <m.szyprowski@...sung.com>, Michael Turquette <mturquette@...libre.com>, Michal Wilczynski <m.wilczynski@...sung.com>, Miguel Ojeda <ojeda@...nel.org>, Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley <paul.walmsley@...ive.com>, Rob Herring <robh@...nel.org>, Trevor Gross <tmgross@...ch.edu>, Uwe Kleine-König <ukleinek@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org, linux-clk@...r.kernel.org
Subject: Re: [PATCH v4 5/9] clk: thead: Mark essential bus clocks as CLK_IGNORE_UNUSED

Quoting Michal Wilczynski (2025-06-18 05:27:38)
> Probing peripherals in the AON and PERI domains, such as the PVT thermal
> sensor and the PWM controller, can lead to boot hangs or unresponsive
> devices on the LPi4A board. The root cause is that their parent bus
> clocks ('CLK_CPU2AON_X2H' and the 'CLK_PERISYS_APB' clocks) are
> automatically gated by the kernel's power-saving mechanisms when the bus
> is perceived as idle.
> 
> Alternative solutions were investigated, including modeling the parent
> bus in the Device Tree with 'simple-pm-bus' or refactoring the clock
> driver's parentage. The 'simple-pm-bus' approach is not viable due to
> the lack of defined bus address ranges in the hardware manual and its
> creation of improper dependencies on the 'pm_runtime' API for consumer
> drivers.
> 
> Therefore, applying the'`CLK_IGNORE_UNUSED' flag directly to the
> essential bus clocks is the most direct and targeted fix. This prevents
> the kernel from auto-gating these buses and ensures peripherals remain
> accessible.
> 
> This change fixes the boot hang associated with the PVT sensor and
> resolves the functional issues with the PWM controller.
> 
> Link: https://lore.kernel.org/all/9e8a12db-236d-474c-b110-b3be96edf057@samsung.com/ [1]
> 
> Reviewed-by: Drew Fustini <drew@...7.com>
> Signed-off-by: Michal Wilczynski <m.wilczynski@...sung.com>
> ---

Acked-by: Stephen Boyd <sboyd@...nel.org>