linux-kernel - Re: [PATCH 1/5] clk: sun6i: Protect CPU clock

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Date:	Mon, 24 Feb 2014 16:30:35 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Emilio Lopez <emilio@...pez.com.ar>,
	Dan Williams <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>, devicetree@...r.kernel.org,
	Mike Turquette <mturquette@...aro.org>,
	linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com,
	dmaengine@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/5] clk: sun6i: Protect CPU clock

On Mon, Feb 24, 2014 at 05:22:43PM +0100, Maxime Ripard wrote:
> Right now, AHB is an indirect child clock of the CPU clock. If that happens to
> change, since the CPU clock has no other consumers declared in Linux, it would
> be shut down, which is not really a good idea.
> 
> Prevent this by forcing it enabled.
> 
> Signed-off-by: Maxime Ripard <maxime.ripard@...e-electrons.com>
> ---
>  drivers/clk/sunxi/clk-sunxi.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/clk/sunxi/clk-sunxi.c b/drivers/clk/sunxi/clk-sunxi.c
> index 23baad9..cedaf4b 100644
> --- a/drivers/clk/sunxi/clk-sunxi.c
> +++ b/drivers/clk/sunxi/clk-sunxi.c
> @@ -1301,6 +1301,14 @@ static void __init sunxi_clock_protect(void)
>  		clk_prepare_enable(clk);
>  		clk_put(clk);
>  	}
> +
> +	/* CPU clocks - sun6i */
> +	clk = clk_get(NULL, "cpu");
> +	if (!IS_ERR(clk)) {
> +		clk_prepare_enable(clk);
> +		clk_put(clk);
> +	}

This is broken.  I'm not sure what's difficult to grasp about the concept
of "while a clock is in use, you should keep a reference to that clock".

That implies that if you get a clock, and then enable it, you don't
put the clock until you've disabled it.

The only reason the core doesn't check for this kind of thing is that
a clock may be shared, so it's entirely possible for a correctly written
driver to have a clock which is still enabled at put time - but enabled
by an entirely different driver.

However, that's no excuse for this kind of sloppiness.

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