linux-kernel - Re: [PATCH] clk: mediatek: Allow changing PLL rate when it is off

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Message-ID: <20160108091510.GC13058@pengutronix.de>
Date:	Fri, 8 Jan 2016 10:15:10 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	James Liao <jamesjj.liao@...iatek.com>
Cc:	Matthias Brugger <matthias.bgg@...il.com>,
	Mike Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	srv_heupstream@...iatek.com, linux-kernel@...r.kernel.org,
	Daniel Kurtz <djkurtz@...omium.org>,
	linux-mediatek@...ts.infradead.org,
	Sascha Hauer <kernel@...gutronix.de>,
	linux-clk@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] clk: mediatek: Allow changing PLL rate when it is off

Hi James,

On Fri, Jan 08, 2016 at 04:16:37PM +0800, James Liao wrote:
> Some modules may need to change its clock rate before turn on it.
> So changing PLL's rate when it is off should be allowed.
> This patch removes PLL enabled check before set rate, so that
> PLLs can set new frequency even if they are off.

This sounds like the software refused to change the rate on disabled
PLLs, but this is not the case.

> 
> Signed-off-by: James Liao <jamesjj.liao@...iatek.com>
> ---
>  drivers/clk/mediatek/clk-pll.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/clk/mediatek/clk-pll.c b/drivers/clk/mediatek/clk-pll.c
> index 966cab1..8e31fae 100644
> --- a/drivers/clk/mediatek/clk-pll.c
> +++ b/drivers/clk/mediatek/clk-pll.c
> @@ -91,9 +91,6 @@ static void mtk_pll_set_rate_regs(struct mtk_clk_pll *pll, u32 pcw,
>  		int postdiv)
>  {
>  	u32 con1, val;
> -	int pll_en;
> -
> -	pll_en = readl(pll->base_addr + REG_CON0) & CON0_BASE_EN;
>  
>  	/* set postdiv */
>  	val = readl(pll->pd_addr);
> @@ -114,15 +111,13 @@ static void mtk_pll_set_rate_regs(struct mtk_clk_pll *pll, u32 pcw,
>  
>  	con1 = readl(pll->base_addr + REG_CON1);
>  
> -	if (pll_en)
> -		con1 |= CON0_PCW_CHG;
> +	con1 |= CON0_PCW_CHG;

This bit is described as "Feedback divide ratio update". To me this
sounds like we have to inform the hardware that the PLL registers have
been updated. The current code only sets this bit when the PLL is
enabled which sounds sane to me.

>  
>  	writel(con1, pll->base_addr + REG_CON1);
>  	if (pll->tuner_addr)
>  		writel(con1 + 1, pll->tuner_addr);
>  
> -	if (pll_en)
> -		udelay(20);
> +	udelay(20);

We seem to have to wait here until the PLL is really running at the new
frequency. Normally we don't have to do this when the PLL is disabled.

I'm sure this patch solves a real problem, from looking at it it's just
not clear to me what the problem is. Could you clarify this a bit?

Sascha


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