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From:   Benjamin Gaignard <benjamin.gaignard@linaro.org>
Date:   Tue, 29 Oct 2019 13:43:14 +0100
Message-ID: <CA+M3ks51SNOfM9YJFv8wkLDar0qvbwGQVzVwxEVP7T=bGeTcKw@mail.gmail.com>
Subject: Re: [PATCH] arm: kernel: initialize broadcast hrtimer based clock
 event device
To:     Benjamin Gaignard <benjamin.gaignard@st.com>
Cc:     Russell King - ARM Linux <linux@armlinux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-stm32@st-md-mailman.stormreply.com, maz@kernel.org
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Le mar. 15 oct. 2019 =C3=A0 10:42, Benjamin Gaignard
<benjamin.gaignard@st.com> a =C3=A9crit :
>
> On platforms implementing CPU power management, the CPUidle subsystem
> can allow CPUs to enter idle states where local timers logic is lost on p=
ower
> 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 alway=
s-on
> HW timer or implement it in a broken way, this patch adds code to initial=
ize
> the kernel hrtimer based clock event device upon boot (which can be chose=
n as
> tick broadcast device by the kernel).
> It relies on a dynamically chosen CPU to be always powered-up. This CPU t=
hen
> relays the timer interrupt to CPUs in deep-idle states through its HW loc=
al
> 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.

Gentle ping,

Thanks,
Benjamin
>
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
> ---
> Note:
> - The same reasons lead to same patch than for arm64 so I have copy the
>   commit message from: 9358d755bd5c ("arm64: kernel: initialize broadcast
>   hrtimer based clock event device")
>  arch/arm/kernel/time.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/arm/kernel/time.c b/arch/arm/kernel/time.c
> index b996b2cf0703..dddc7ebf4db4 100644
> --- a/arch/arm/kernel/time.c
> +++ b/arch/arm/kernel/time.c
> @@ -9,6 +9,7 @@
>   *  reading the RTC at bootup, etc...
>   */
>  #include <linux/clk-provider.h>
> +#include <linux/clockchips.h>
>  #include <linux/clocksource.h>
>  #include <linux/errno.h>
>  #include <linux/export.h>
> @@ -107,5 +108,6 @@ void __init time_init(void)
>                 of_clk_init(NULL);
>  #endif
>                 timer_probe();
> +               tick_setup_hrtimer_broadcast();
>         }
>  }
> --
> 2.15.0
>
>
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