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

From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: gregkh@linuxfoundation.org,
	linux-stm32@st-md-mailman.stormreply.com,
	Benjamin Gaignard <benjamin.gaignard@st.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arm: kernel: initialize broadcast hrtimer based clock event device
Date: Tue, 12 Nov 2019 14:55:41 +0000	[thread overview]
Message-ID: <20191112145541.GK25745@shell.armlinux.org.uk> (raw)
In-Reply-To: <alpine.DEB.2.21.1911121547490.1833@nanos.tec.linutronix.de>

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@st.com>
> 
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

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

-- 
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