linux-kernel - Re: [RFC PATCH V1 2/2] clk: add lock for clk_core_is

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Date:   Tue, 26 Dec 2017 17:29:18 -0800
From:   Stephen Boyd <sboyd@...eaurora.org>
To:     Dong Aisheng <aisheng.dong@....com>
Cc:     linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, mturquette@...libre.com
Subject: Re: [RFC PATCH V1 2/2] clk: add lock for clk_core_is_enabled

On 12/22, Dong Aisheng wrote:
> According to design doc, .is_enabled should be protected by enable lock.
> Then users don't have to protect it against enable/disable operation
> in clock drivers.
> 
> See: Documentation/clk.txt
> "The enable lock is a spinlock and is held across calls to the .enable,
> .disable and .is_enabled operations."
> 
> Cc: Stephen Boyd <sboyd@...eaurora.org>
> Cc: Michael Turquette <mturquette@...libre.com>
> Signed-off-by: Dong Aisheng <aisheng.dong@....com>
> ---
>  drivers/clk/clk.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index e24968f..d6e2d5c 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -198,14 +198,19 @@ static bool clk_core_is_prepared(struct clk_core *core)
>  
>  static bool clk_core_is_enabled(struct clk_core *core)
>  {
> +	unsigned long flags;
>  	bool ret = false;
>  
> +	flags = clk_enable_lock();
> +
>  	/*
>  	 * .is_enabled is only mandatory for clocks that gate
>  	 * fall back to software usage counter if .is_enabled is missing
>  	 */
> -	if (!core->ops->is_enabled)
> +	if (!core->ops->is_enabled) {
> +		clk_enable_unlock(flags);
>  		return core->enable_count;
> +	}
>  
>  	/*
>  	 * Check if clock controller's device is runtime active before
> @@ -230,6 +235,8 @@ static bool clk_core_is_enabled(struct clk_core *core)
>  	if (core->dev)
>  		pm_runtime_put(core->dev);
>  
> +	clk_enable_unlock(flags);
> +
>  	return ret;
>  }

It doesn't really make any sense to hold the enable lock inside
the clk_core_is_enabled() function, unless you want to do
something else with the information of the enable state with that
lock held. Otherwise, seeing if a clk is enabled is a one-shot
read of the enabled state, which could just as easily change
after the function returns because the lock is released.

We should update the documentation.

-- 
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