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Date: Mon, 27 Apr 2020 09:15:32 +0000
From: Peng Fan <peng.fan@....com>
To: Leonard Crestez <leonard.crestez@....com>,
Anson Huang <anson.huang@....com>
CC: "shawnguo@...nel.org" <shawnguo@...nel.org>,
"s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
"sboyd@...nel.org" <sboyd@...nel.org>,
Abel Vesa <abel.vesa@....com>,
"kernel@...gutronix.de" <kernel@...gutronix.de>,
"festevam@...il.com" <festevam@...il.com>,
dl-linux-imx <linux-imx@....com>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Daniel Baluta <daniel.baluta@....com>,
"aford173@...il.com" <aford173@...il.com>,
Jacky Bai <ping.bai@....com>, Jun Li <jun.li@....com>,
"l.stach@...gutronix.de" <l.stach@...gutronix.de>,
"andrew.smirnov@...il.com" <andrew.smirnov@...il.com>,
"agx@...xcpu.org" <agx@...xcpu.org>,
"angus@...ea.ca" <angus@...ea.ca>,
"heiko@...ech.de" <heiko@...ech.de>,
Andy Duan <fugang.duan@....com>,
"linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>
Subject: RE: [PATCH V2 07/10] clk: imx: add mux ops for i.MX8M composite clk
> Subject: Re: [PATCH V2 07/10] clk: imx: add mux ops for i.MX8M composite
> clk
>
> On 2020-03-12 12:27 PM, Peng Fan wrote:
> > From: Peng Fan <peng.fan@....com>
> >
> > The CORE/BUS root slice has following design, simplied graph:
> > The difference is core not have pre_div block.
> > A composite core/bus clk has 8 inputs for mux to select, saying clk[0-7].
> >
> > SEL_A GA
> > +--+ +-+
> > | +->+ +------+
> > CLK[0-7]--->+ | +-+ |
> > | | | +----v---+ +----+
> > | +--+ |pre_diva+----> | +---------+
> > | +--------+ |mux +--+post_div |
> > | +--+ |pre_divb+--->+ | +---------+
> > | | | +----^---+ +----+
> > +--->+ | +-+ |
> > | +->+ +------+
> > +--+ +-+
> > SEL_B GB
> >
> > There will be system hang, when doing the following steps:
> > 1. switch mux from clk0 to clk1
> > 2. gate off clk0
> > 3. swtich from clk1 to clk2, or gate off clk1
> >
> > Step 3 triggers system hang.
> >
> > If we skip step2, keep clk0 on, step 3 will not trigger system hang.
> > However we have CLK_OPS_PARENT_ENABLE flag, which will unprepare
> > disable the clk0 which will not be used.
>
> As far as I understand when switching from clk1 to clk2 this is done by
> temporarily switching the rightmost SELECT mux to whatever was in the
> spare SEL, which is essentially arbitrary from linux POV.
No. The fixes in this patches has been confirmed by IC design owner
>
> This is quite unexpected but in theory it might be desirable to use a third
> parent as a fallback.
No. this will make things complicated. To CCM SEL_A and SEL_B,
it is controlled by a hardware counter. Saying you write n times to
the target interface.
The mux will use n % 2 to choose SEL_A or SEL_B. write twice
to make sure SEL_A and SEL_B has the same value.
Thanks,
Peng.
>
> >
> > To address this issue, we could use following simplied software flow:
> > After the first target register set
> > wait the target register set finished
> > set the target register set again
> > wait the target register set finished
> >
> > The upper flow will make sure SEL_A and SEL_B both set the new mux,
> > but with only one path gate on.
> > And there will be no system hang anymore with step3.
>
> Your fix tries to work around this scenario by always setting the mux value in
> SEL_A and SEL_B to the same value after each set_parent operation.
>
> But what if SEL_A and SEL_B are different at linux boot time and the first
> reparenting is done *after* disabling unused clocks? This doesn't happen for
> A53 because it's reparented during clock provider probe but maybe this
> scenario could be contrived if bootloader touches one of the other bus slices.
>
> It might be extra safe to assign the parent of the spare mux at the start of
> each set_parent call. This could even be done on probe and this way wouldn't
> have to duplicate mux_ops just to do a double write.
>
> > Signed-off-by: Peng Fan <peng.fan@....com>
> > ---
> >
> > V2:
> > Drop wait after write, add one line comment for write twice.
> >
> > drivers/clk/imx/clk-composite-8m.c | 62
> +++++++++++++++++++++++++++++++++++++-
> > 1 file changed, 61 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/clk/imx/clk-composite-8m.c
> > b/drivers/clk/imx/clk-composite-8m.c
> > index 99773519b5a5..eae02c151ced 100644
> > --- a/drivers/clk/imx/clk-composite-8m.c
> > +++ b/drivers/clk/imx/clk-composite-8m.c
> > @@ -24,6 +24,12 @@
> >
> > #define PCG_CGC_SHIFT 28
> >
> > +#define PRE_REG_OFF 0x30
> > +#define PRE_MUXA_SHIFT 24
> > +#define PRE_MUXA_MASK 0x7
> > +#define PRE_MUXB_SHIFT 8
> > +#define PRE_MUXB_MASK 0x7
>
> These are unused.
>
> > +
> > static unsigned long imx8m_clk_composite_divider_recalc_rate(struct
> clk_hw *hw,
> > unsigned long parent_rate)
> > {
> > @@ -124,6 +130,57 @@ static const struct clk_ops
> imx8m_clk_composite_divider_ops = {
> > .set_rate = imx8m_clk_composite_divider_set_rate,
> > };
> >
> > +static u8 imx8m_clk_composite_mux_get_parent(struct clk_hw *hw) {
> > + struct clk_mux *mux = to_clk_mux(hw);
> > + u32 val;
> > +
> > + val = readl(mux->reg) >> mux->shift;
> > + val &= mux->mask;
> > +
> > + return clk_mux_val_to_index(hw, mux->table, mux->flags, val); }
> > +
> > +static int imx8m_clk_composite_mux_set_parent(struct clk_hw *hw, u8
> > +index) {
> > + struct clk_mux *mux = to_clk_mux(hw);
> > + u32 val = clk_mux_index_to_val(mux->table, mux->flags, index);
> > + unsigned long flags = 0;
> > + u32 reg;
> > +
> > + if (mux->lock)
> > + spin_lock_irqsave(mux->lock, flags);
> > +
> > + reg = readl(mux->reg);
> > + reg &= ~(mux->mask << mux->shift);
> > + val = val << mux->shift;
> > + reg |= val;
> > + /* write twice to make sure SEL_A/B point the same mux */
> > + writel(reg, mux->reg);
> > + writel(reg, mux->reg);
> > +
> > + if (mux->lock)
> > + spin_unlock_irqrestore(mux->lock, flags);
> > +
> > + return 0;
> > +}
> > +
> > +static int
> > +imx8m_clk_composite_mux_determine_rate(struct clk_hw *hw,
> > + struct clk_rate_request *req) {
> > + struct clk_mux *mux = to_clk_mux(hw);
> > +
> > + return clk_mux_determine_rate_flags(hw, req, mux->flags); }
> > +
> > +
> > +const struct clk_ops imx8m_clk_composite_mux_ops = {
> > + .get_parent = imx8m_clk_composite_mux_get_parent,
> > + .set_parent = imx8m_clk_composite_mux_set_parent,
> > + .determine_rate = imx8m_clk_composite_mux_determine_rate,
> > +};
> > +
> > struct clk_hw *imx8m_clk_hw_composite_flags(const char *name,
> > const char * const *parent_names,
> > int num_parents, void __iomem *reg, @@
> -136,6 +193,7 @@ struct
> > clk_hw *imx8m_clk_hw_composite_flags(const char *name,
> > struct clk_gate *gate = NULL;
> > struct clk_mux *mux = NULL;
> > const struct clk_ops *divider_ops;
> > + const struct clk_ops *mux_ops;
> >
> > mux = kzalloc(sizeof(*mux), GFP_KERNEL);
> > if (!mux)
> > @@ -157,10 +215,12 @@ struct clk_hw
> *imx8m_clk_hw_composite_flags(const char *name,
> > div->shift = PCG_DIV_SHIFT;
> > div->width = PCG_CORE_DIV_WIDTH;
> > divider_ops = &clk_divider_ops;
> > + mux_ops = &imx8m_clk_composite_mux_ops;
> > } else {
> > div->shift = PCG_PREDIV_SHIFT;
> > div->width = PCG_PREDIV_WIDTH;
> > divider_ops = &imx8m_clk_composite_divider_ops;
> > + mux_ops = &clk_mux_ops;
> > }
> >
> > div->lock = &imx_ccm_lock;
> > @@ -176,7 +236,7 @@ struct clk_hw
> *imx8m_clk_hw_composite_flags(const char *name,
> > gate->lock = &imx_ccm_lock;
> >
> > hw = clk_hw_register_composite(NULL, name, parent_names,
> num_parents,
> > - mux_hw, &clk_mux_ops, div_hw,
> > + mux_hw, mux_ops, div_hw,
> > divider_ops, gate_hw, &clk_gate_ops, flags);
> > if (IS_ERR(hw))
> > goto fail;
> >
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