linux-kernel - Re: [PATCH v2] clk: mediatek: Allow changing PLL rate when it is off

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Message-ID: <20160114022637.1942.4564@quark.deferred.io>
Date:	Wed, 13 Jan 2016 18:26:37 -0800
From:	Michael Turquette <mturquette@...libre.com>
To:	James Liao <jamesjj.liao@...iatek.com>,
	"Matthias Brugger" <matthias.bgg@...il.com>,
	"Stephen Boyd" <sboyd@...eaurora.org>
Cc:	"Sascha Hauer" <kernel@...gutronix.de>,
	"Daniel Kurtz" <djkurtz@...omium.org>, srv_heupstream@...iatek.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-mediatek@...ts.infradead.org, linux-clk@...r.kernel.org,
	"James Liao" <jamesjj.liao@...iatek.com>
Subject: Re: [PATCH v2] clk: mediatek: Allow changing PLL rate when it is off

Hi James,

Quoting James Liao (2016-01-10 19:28:56)
> 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.
> 
> On MT8173 for example, ARMPLL's enable bit can be controlled by
> other HW. That means ARMPLL may be turned on even if we (CPU / SW)
> set ARMPLL's enable bit as 0. In this case, SW may want and can
> still change ARMPLL's rate by changing its pcw and postdiv settings.
> But without this patch, new pcw setting will not be applied because
> its enable bit is 0.

Must the clock signal be enabled to change the PLL rate? If so, does
ARMPLL set the CLK_SET_RATE_GATE flag?

> 
> 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 unconditionally enables the PLL whenever clk_set_rate is called,
changing the previous behavior. The clk framework still tracks the
intent of ARMPLLs users with the enable_count. How about something like
the following:

	bool pll_en = clk_hw_is_enabled(&pll->hw);

	if (pll_en)
		con1 |= CON0_PCW_CHG;

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

	if (pll_en)
		udelay(20);

This does not rely on the hardware to tell us the intent of the user,
but instead on our framework usecounting.

Regards,
Mike

>         writel(con1, pll->base_addr + REG_CON1);
>         if (pll->tuner_addr)
>                 writel(con1 + 1, pll->tuner_addr);
>  
> -       if (pll_en)
> -               udelay(20);
> +       udelay(20);
>  }
>  
>  /*
> -- 
> 1.9.1
>