linux-kernel - Re: [PATCH 7/9] clk: sunxi-ng: mux: Add clk notifier functions

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Date:	Wed, 27 Jul 2016 09:30:05 +0200
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Chen-Yu Tsai <wens@...e.org>
Cc:	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Rob Herring <robh+dt@...nel.org>,
	Mark Rutland <mark.rutland@....com>, dev@...ux-sunxi.org,
	linux-clk@...r.kernel.org, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/9] clk: sunxi-ng: mux: Add clk notifier functions

On Tue, Jul 26, 2016 at 03:04:29PM +0800, Chen-Yu Tsai wrote:
> On sunxi we support cpufreq by changing the clock rate of PLL-CPU.
> It's possible the clock output of the PLL goes out of the CPU's
> operational limits when the PLL's multipliers / dividers are changed
> and it hasn't stabilized yet. This would result in the CPU hanging.
> 
> To circumvent this, we temporarily switch the CPU mux clock to another
> stable clock before the rate change, and switch it back after the PLL
> stabilizes. This is done with clk notifiers registered on the PLL.
> 
> This patch adds common functions for notifiers to reparent mux clocks.
> 
> Signed-off-by: Chen-Yu Tsai <wens@...e.org>
> ---
>  drivers/clk/sunxi-ng/ccu_mux.c | 36 ++++++++++++++++++++++++++++++++++++
>  drivers/clk/sunxi-ng/ccu_mux.h | 14 ++++++++++++++
>  2 files changed, 50 insertions(+)
> 
> diff --git a/drivers/clk/sunxi-ng/ccu_mux.c b/drivers/clk/sunxi-ng/ccu_mux.c
> index f96eabb5d1f3..8a6e9065cb85 100644
> --- a/drivers/clk/sunxi-ng/ccu_mux.c
> +++ b/drivers/clk/sunxi-ng/ccu_mux.c
> @@ -8,7 +8,9 @@
>   * the License, or (at your option) any later version.
>   */
>  
> +#include <linux/clk.h>
>  #include <linux/clk-provider.h>
> +#include <linux/delay.h>
>  
>  #include "ccu_gate.h"
>  #include "ccu_mux.h"
> @@ -199,3 +201,37 @@ const struct clk_ops ccu_mux_ops = {
>  	.determine_rate	= __clk_mux_determine_rate,
>  	.recalc_rate	= ccu_mux_recalc_rate,
>  };
> +
> +/*
> + * This clock notifier is called when the frequency of the of the parent
> + * PLL clock is to be changed. The idea is to switch the parent to a
> + * stable clock, such as the main oscillator, while the PLL frequency
> + * stabilizes.
> + */
> +static int ccu_mux_notifier_cb(struct notifier_block *nb,
> +			       unsigned long event, void *data)
> +{
> +	struct ccu_mux_nb *mux = to_ccu_mux_nb(nb);
> +	int ret = 0;
> +
> +	if (event == PRE_RATE_CHANGE) {
> +		mux->original_index = ccu_mux_helper_get_parent(mux->common,
> +								mux->cm);
> +		ret = ccu_mux_helper_set_parent(mux->common, mux->cm,
> +						mux->bypass_index);
> +	} else if (event == POST_RATE_CHANGE) {
> +		ret = ccu_mux_helper_set_parent(mux->common, mux->cm,
> +						mux->original_index);
> +	}
> +
> +	udelay(mux->delay_us);

Are you sure we need that delay?

set_rate will end and notify you once the PLL rate is stable, so it
looks redundant.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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