linux-kernel - Re: [PATCH 1/9] ARM: get rid of arch_cpu_idle

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Date:	Mon, 27 Jan 2014 12:45:11 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org,
	linux-pm@...r.kernel.org, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH 1/9] ARM: get rid of arch_cpu_idle_prepare()

On Mon, Jan 27, 2014 at 01:08:16AM -0500, Nicolas Pitre wrote:
> ARM and ARM64 are the only two architectures implementing
> arch_cpu_idle_prepare() simply to call local_fiq_enable().
> 
> We have secondary_start_kernel() already calling local_fiq_enable() and
> this is done a second time in arch_cpu_idle_prepare() in that case. And
> enabling FIQs has nothing to do with idling the CPU to start with.
> 
> So let's introduce init_fiq_boot_cpu() to take care of FIQs on the boot
> CPU and remove arch_cpu_idle_prepare(). This is now done a bit earlier
> at late_initcall time but this shouldn't make a difference in practice
> i.e. when FIQs are actually used.
> 
> Signed-off-by: Nicolas Pitre <nico@...aro.org>
> ---
>  arch/arm/kernel/process.c | 5 -----
>  arch/arm/kernel/setup.c   | 7 +++++++
>  2 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
> index 92f7b15dd2..725b8c95e0 100644
> --- a/arch/arm/kernel/process.c
> +++ b/arch/arm/kernel/process.c
> @@ -142,11 +142,6 @@ static void default_idle(void)
>  	local_irq_enable();
>  }
>  
> -void arch_cpu_idle_prepare(void)
> -{
> -	local_fiq_enable();
> -}
> -
>  void arch_cpu_idle_enter(void)
>  {
>  	ledtrig_cpu(CPU_LED_IDLE_START);
> diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> index 987a7f5bce..d027b1a6fe 100644
> --- a/arch/arm/kernel/setup.c
> +++ b/arch/arm/kernel/setup.c
> @@ -789,6 +789,13 @@ static int __init init_machine_late(void)
>  }
>  late_initcall(init_machine_late);
>  
> +static int __init init_fiq_boot_cpu(void)
> +{
> +	local_fiq_enable();
> +	return 0;
> +}
> +late_initcall(init_fiq_boot_cpu);

arch_cpu_idle_prepare() gets called from the swapper thread, and changes
the swapper thread's CPSR. init_fiq_boot_cpu() gets called from PID1, the
init thread, and changes the init thread's CPSR, which will already have
FIQs enabled by way of how kernel threads are created.

Hence, the above code fragment has no effect what so ever, and those
platforms using FIQs will not have FIQs delivered if they're idle
(because the swapper will have FIQs masked at the CPU.)

NAK.

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