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

[Thomas Gleixner]

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@xxxxxx>

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>