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

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Message-ID: <20191112145541.GK25745@shell.armlinux.org.uk>
Date:   Tue, 12 Nov 2019 14:55:41 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Benjamin Gaignard <benjamin.gaignard@...com>,
        gregkh@...uxfoundation.org, 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, Nov 12, 2019 at 03:48:10PM +0100, Thomas Gleixner wrote:
> 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>

Thanks.  Benjamin, please put it in the patch system, thanks.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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