Re: [PATCH] clk: sunxi-ng: ccu_common: Replace ktime-dependent lock wait loop with udelay

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] clk: sunxi-ng: custom udelay loop instead of readl_relaxed_poll_timeout_atomic()
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commit 303433ce9fcf08821cef4553ec1030effcdd3fb9
Author: Alastair D'Silva <alastair@d-silva.org>

    clk: sunxi-ng: ccu_common: Replace ktime-dependent lock wait loop with udelay

This commit replaces the use of readl_relaxed_poll_timeout() in the sunxi-ng
CCU driver with a custom udelay() loop. The goal is to avoid system freezes
caused by timekeeping suspensions during early boot or CPUfreq DVFS scaling
when waiting for PLLs to lock.

> diff --git a/drivers/clk/sunxi-ng/ccu_common.c b/drivers/clk/sunxi-ng/ccu_common.c
> index 43d8eca6abeed..00daedadb5c83 100644
> --- a/drivers/clk/sunxi-ng/ccu_common.c
> +++ b/drivers/clk/sunxi-ng/ccu_common.c

[ ... ]

> @@ -35,7 +36,14 @@ void ccu_helper_wait_for_lock(struct ccu_common *common, u32 lock)
>  	else
>  		addr = common->base + common->reg;
>  
> -	WARN_ON(readl_relaxed_poll_timeout(addr, reg, reg & lock, 100, 70000));
> +	for (i = 0; i < 100000; i++) {
> +		if (readl_relaxed(addr) & lock)
> +			return;
> +		udelay(1);
> +	}

[Severity: Low]
Does this custom busy-wait loop unnecessarily reinvent the kernel's standard
polling abstractions?

Since ccu_helper_wait_for_lock() executes whenever a PLL lock is waited upon
during clock configuration, such as early boot or CPUfreq transitions, avoiding
ktime dependencies is definitely necessary. However, the kernel already natively
provides readl_relaxed_poll_timeout_atomic() in <linux/iopoll.h> which is
designed specifically to perform timekeeping-independent hardware polling.

Would it be better to use readl_relaxed_poll_timeout_atomic() here instead of
circumventing the standard I/O polling macros with a hand-rolled loop?

> +
> +	if (!(readl_relaxed(addr) & lock))
> +		pr_warn("%s: clock lock timeout\n", clk_hw_get_name(&common->hw));
>  }
>  EXPORT_SYMBOL_NS_GPL(ccu_helper_wait_for_lock, "SUNXI_CCU");

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Sashiko AI review · https://sashiko.dev/#/patchset/20260916041104.239999-1-alastair@d-silva.org?part=1