linux-kernel - Re: [PATCH] arm: kernel: initialize broadcast hrtimer based clock event device

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Message-ID: <alpine.DEB.2.21.1911121547490.1833@nanos.tec.linutronix.de>
Date:   Tue, 12 Nov 2019 15:48:10 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Benjamin Gaignard <benjamin.gaignard@...com>
cc:     linux@...linux.org.uk, gregkh@...uxfoundation.org,
        rmk+kernel@...linux.org.uk, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH] arm: kernel: initialize broadcast hrtimer based clock
 event device

On Tue, 12 Nov 2019, Benjamin Gaignard wrote:

> On platforms implementing CPU power management, the CPUidle subsystem
> can allow CPUs to enter idle states where local timers logic is lost on power
> down. To keep the software timers functional the kernel relies on an
> always-on broadcast timer to be present in the platform to relay the
> interrupt signalling the timer expiries.
> 
> For platforms implementing CPU core gating that do not implement an always-on
> HW timer or implement it in a broken way, this patch adds code to initialize
> the kernel hrtimer based clock event device upon boot (which can be chosen as
> tick broadcast device by the kernel).
> It relies on a dynamically chosen CPU to be always powered-up. This CPU then
> relays the timer interrupt to CPUs in deep-idle states through its HW local
> timer device.
> 
> Having a CPU always-on has implications on power management platform
> capabilities and makes CPUidle suboptimal, since at least a CPU is kept
> always in a shallow idle state by the kernel to relay timer interrupts,
> but at least leaves the kernel with a functional system with some working
> power management capabilities.
> 
> The hrtimer based clock event device is unconditionally registered, but
> has the lowest possible rating such that any broadcast-capable HW clock
> event device present will be chosen in preference as the tick broadcast
> device.
> 
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>