Re: [PATCH v4 5/9] clk: thead: Mark essential bus clocks as CLK_IGNORE_UNUSED

[Stephen Boyd]

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@xxxxxxxxxxx/ [1]
> 
> Reviewed-by: Drew Fustini <drew@xxxxxxxx>
> Signed-off-by: Michal Wilczynski <m.wilczynski@xxxxxxxxxxx>
> ---

Acked-by: Stephen Boyd <sboyd@xxxxxxxxxx>