linux-kernel - Re: [PATCH 2/2] ARM: imx: cpuidle-imx6q: configure CCM to RUN mode when CPU is active

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Date:   Wed, 24 Jan 2018 12:16:25 +0100
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Peng Fan <peng.fan@....com>, shawnguo@...nel.org,
        kernel@...gutronix.de, fabio.estevam@....com
Cc:     aisheng.dong@....com, van.freenix@...il.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] ARM: imx: cpuidle-imx6q: configure CCM to RUN mode
 when CPU is active

Hi Peng,

Am Samstag, den 30.12.2017, 21:53 +0800 schrieb Peng Fan:
> There are two states in i.MX6Q cpuidle driver.
> state[1]: ARM WFI mode
> state[2]: i.MX6Q WAIT mode
> 
> Take i.MX6DL as example, think out such a case:
> 1. CPU0/1 both run at normal mode
> 2. On CPU0, `sleep 1` is executed. And there are no workload on CPU1.
> 3. CPU0 first runs into state[2] and 'wfi' instruction. Switched to
> use
>    GPT broadcast.
> 4. CPU1 runs into state[2] and configure CCM to WAIT MODE,
>    then 'wfi' instruction. Now arm_clk and local timer clock are
>    shutdown. Switched to use GPT broadcast
> 5. GPT broadcast timer interrupt comes to GPC/GIC, then CPU0 wakes
> up.
>    CPU0 switched to use arm local timer. CPU1 is still sleeping.
> 6. No workload on CPU0, CPU0 runs into state[1]. But CCM register
>    is still not restored to Normal RUN mode. 'wfi' + CCM WAIT will
>    cause arm_clk and arm core clk.
>    Now CPU0 stops, which is not correct.
> 
> So, need to make sure CCM configured to RUN mode when any cpu exit
> state[2].
> 
> In this patch,
> When CPU exits state[2], it configures CCM to RUN mode.
> When all CPUs enters state[2], the last CPU needs to check
> whether it's ok to configure CCM to WAIT mode or not.

To me this patch seems like we are adding duct tape to fix the issue. 

It seems the whole master_lock thing doesn't properly work when the CPU
that is woken up by the timer broadcast isn't the last CPU going to
sleep. We should probably try to come up with a way to simplify this
whole master dance, instead of adding yet more complexity.

Regards,
Lucas

> Signed-off-by: Peng Fan <peng.fan@....com>
> ---
> 
> V1:
>  This is to upstream patch:
>  http://git.freescale.com/git/cgit.cgi/imx/linux-imx.git/commit/?h=im
> x_4.9.11_1.0.0_ga&id=0d980646ee068b92db71fd5e4e4efcbc33749cbd
> 
>  arch/arm/mach-imx/cpuidle-imx6q.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/arm/mach-imx/cpuidle-imx6q.c b/arch/arm/mach-
> imx/cpuidle-imx6q.c
> index bfeb25aaf9a2..4d342e2fdfe6 100644
> --- a/arch/arm/mach-imx/cpuidle-imx6q.c
> +++ b/arch/arm/mach-imx/cpuidle-imx6q.c
> @@ -30,6 +30,8 @@ static int imx6q_enter_wait(struct cpuidle_device
> *dev,
>  		if (!spin_trylock(&master_lock))
>  			goto idle;
>  		imx6_set_lpm(WAIT_UNCLOCKED);
> +		if (atomic_read(&master) != num_online_cpus())
> +			imx6_set_lpm(WAIT_CLOCKED);
>  		cpu_do_idle();
>  		imx6_set_lpm(WAIT_CLOCKED);
>  		spin_unlock(&master_lock);
> @@ -41,6 +43,7 @@ static int imx6q_enter_wait(struct cpuidle_device
> *dev,
>  done:
>  	atomic_dec(&master);
>  
> +	imx6_set_lpm(WAIT_CLOCKED);
>  	return index;
>  }
>