Re: [PATCH] irqchip/exynos-combiner: switch to raw_spinlock

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-05-21 00:04:22 [+0200], Marek Szyprowski wrote:
> The exynos-combiner driver uses a regular spinlock to protect access to
> the combiner interrupt status register in combiner_handle_cascade_irq(),
> which is invoked in hard IRQ context as a chained interrupt handler.
> 
> When PREEMPT_RT is enabled on ARM, regular spinlock is converted to a
> sleeping lock (mutex-based), which must not be used in atomic context
> such as hard interrupt handlers. Switch the irq_controller_lock to
> raw_spinlock, which remains a true non-sleeping spinlock even under
> PREEMPT_RT.
> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Fixes: a900e5d99718 ("ARM: exynos: move exynos4210-combiner to drivers/irqchip")

For the change based on the reasoning
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

but why do you need a lock around a single read of a register? 
As far as I can tell, it was introduced in commit
   84bbc16c1f621 ("ARM: S5PV310: Add IRQ support")

and then dragged through the kernel, merged, renamed, unmerged until it
got where it is today.

This
	https://lore.kernel.org/all/1277476037-8806-5-git-send-email-kgene.kim@samsung.com/

is v1, as you see the lock is used in multiple places. Then someone
asked "why locking"
	https://lore.kernel.org/all/20100628144743.GB3287@debian/

and in v2
	https://lore.kernel.org/all/1279270714-15146-5-git-send-email-kgene.kim@samsung.com/

it went down to a single user. 

It looks like a leftover from initial development. So it would make
sense to einer remove it or add a comment why it is still there after
all these years since it is not obvious.

Sebastian