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

From: James Liao
Date: Thu Jan 14 2016 - 00:56:21 EST


Hi Mike,

On Wed, 2016-01-13 at 18:26 -0800, Michael Turquette wrote:
> 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?

No. The controlling of PLL's rate and enable are independent. So we can
change the PLL's rate when it's enabled or disabled.

> >
> > Signed-off-by: James Liao <jamesjj.liao@xxxxxxxxxxxx>
> > ---
> > 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:

Not really. This patch will not enable or disable PLL when we change its
rate. The PLL enable bit is bit[0], we didn't change bit[0] in
mtk_pll_set_rate_regs(), so its state will not be changed.

This patch set CON0_PCW_CHG (bit[31]) no matter the PLL is enabled or
not. So we don't need to check the enable bit[0] nor PLL state in the
framework before set bit[31].

> 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.


Best regards,

James