Re: [PATCH V2 01/10] clk: clk-divider: add CLK_DIVIDER_ZERO_GATE clk support

From: Dong Aisheng
Date: Tue Jan 16 2018 - 22:01:04 EST

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On Thu, Dec 21, 2017 at 05:24:01PM -0800, Stephen Boyd wrote:
> On 12/20, Dong Aisheng wrote:
> > On Thu, Nov 02, 2017 at 12:50:39AM -0700, Stephen Boyd wrote:
> > > On 07/13, Dong Aisheng wrote:
> > > > diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
> > > > index 9bb472c..55f8c41 100644
> > > > --- a/drivers/clk/clk-divider.c
> > > > +++ b/drivers/clk/clk-divider.c
> > > > @@ -123,6 +123,9 @@ unsigned long divider_recalc_rate(struct clk_hw *hw, unsigned long parent_rate,
> > > > struct clk_divider *divider = to_clk_divider(hw);
> > > > unsigned int div;
> > > >
> > > > + if (flags & CLK_DIVIDER_ZERO_GATE && !val)
> > > > + return 0;
> > > > +
> > > > div = _get_div(table, val, flags, divider->width);
> > > > if (!div) {
> > > > WARN(!(flags & CLK_DIVIDER_ALLOW_ZERO),
> > > > @@ -141,8 +144,13 @@ static unsigned long clk_divider_recalc_rate(struct clk_hw *hw,
> > > > struct clk_divider *divider = to_clk_divider(hw);
> > > > unsigned int val;
> > > >
> > > > - val = clk_readl(divider->reg) >> divider->shift;
> > > > - val &= div_mask(divider->width);
> > > > + if ((divider->flags & CLK_DIVIDER_ZERO_GATE) &&
> > > > + !clk_hw_is_enabled(hw)) {
> > >
> > > This seems racy. Are we holding the register lock here?
> > >
> >
> > Would you please clarify what type of racy you mean?
>
> I mean a race between clk_enable() and clk_set_rate(). A
> clk_enable() can happen while a rate change is going on, and then
> the clk_hw_is_enabled() check here would be racing, unless this
> driver specifically tries to prevent that from happening by
> holding a spinlock somewhere.
>

I understand your point now.
Thanks for the careful and professional review.

Will take care of it in the next version.

Thanks

Regards
Dong Aisheng

> >
> > Currently it only protects register write between set_rate and enable/disable,
> > and other register read are not protected.
> > e.g. in recalc_rate and is_enabled.
>
> If you're holding some lock that is used to protect the register
> writes and also the clk from getting enabled/disabled during a
> rate change then it's fine.
>
> >
> > And i did see similar users, e.g.
> > drivers/clk/sunxi-ng/ccu_mult.c
>
> Sure. Those could also be broken. I'm not sure.
>
> >
> > Should we still need protect them here?
> >
> > > > + val = divider->cached_val;
> > > > + } else {
> > > > + val = clk_readl(divider->reg) >> divider->shift;
> > > > + val &= div_mask(divider->width);
> > > > + }
> > > >
> > > > return divider_recalc_rate(hw, parent_rate, val, divider->table,
> > > > divider->flags);
> > > > @@ -392,6 +400,12 @@ static int clk_divider_set_rate(struct clk_hw *hw, unsigned long rate,
> > > > value = divider_get_val(rate, parent_rate, divider->table,
> > > > divider->width, divider->flags);
> > > >
> > > > + if ((divider->flags & CLK_DIVIDER_ZERO_GATE) &&
> > > > + !clk_hw_is_enabled(hw)) {
> > >
> > > Same racy comment here.
> > >
> > > > + divider->cached_val = value;
> > > > + return 0;
> > > > + }
> > > > +
> > > > if (divider->lock)
> > > > spin_lock_irqsave(divider->lock, flags);
> > > > else
> > > > @@ -414,10 +428,85 @@ static int clk_divider_set_rate(struct clk_hw *hw, unsigned long rate,
> > > > return 0;
> > > > }
> > > >
> > > > +static int clk_divider_enable(struct clk_hw *hw)
> > > > +{
> > > > + struct clk_divider *divider = to_clk_divider(hw);
> > > > + unsigned long flags = 0;
> > > > + u32 val;
> > > > +
> > > > + if (!(divider->flags & CLK_DIVIDER_ZERO_GATE))
> > > > + return 0;
> > >
> > > This is not good. We will always jump to these functions on
> > > enable/disable for a divider although 99.9% of all dividers that
> > > exist won't need to run this code at all.
> > >
> >
> > I absolutely understand this concern.
> >
> > > Can you please move this logic into your own divider
> > > implementation? The flag can be added to the generic layer if
> > > necessary but I'd prefer to see this logic kept in the driver
> > > that uses it. If we get more than one driver doing the cached
> > > divider thing then we can think about moving it to the more
> > > generic place like here, but for now we should be able to keep
> > > this contained away from the basic types and handled by the
> > > quirky driver that needs it.
> > >
> >
> > If only for above issue, how about invent a clk_divider_gate_ops
> > to separate the users of normal divider and zero gate divider:
> >
> > diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
> > index 4ed516c..b51f3f9 100644
> > --- a/drivers/clk/clk-divider.c
> > +++ b/drivers/clk/clk-divider.c
> > @@ -125,6 +125,9 @@ unsigned long divider_recalc_rate(struct clk_hw *hw, unsigned long parent_rate,
> >
> > div = _get_div(table, val, flags, divider->width);
> > if (!div) {
> > + if (flags & CLK_DIVIDER_ZERO_GATE)
> > + return 0;
> > +
> > WARN(!(flags & CLK_DIVIDER_ALLOW_ZERO),
> > "%s: Zero divisor and CLK_DIVIDER_ALLOW_ZERO not set\n",
> > clk_hw_get_name(hw));
> > @@ -148,6 +151,23 @@ static unsigned long clk_divider_recalc_rate(struct clk_hw *hw,
> > divider->flags);
> > }
> >
> > +static unsigned long clk_divider_gate_recalc_rate(struct clk_hw *hw,
> > + unsigned long parent_rate)
> > +{
> > + struct clk_divider *divider = to_clk_divider(hw);
> > + unsigned int val;
> > +
> > + if (!clk_hw_is_enabled(hw)) {
> > + val = divider->cached_val;
> > + } else {
> > + val = clk_readl(divider->reg) >> divider->shift;
> > + val &= div_mask(divider->width);
> > + }
> > +
> > + return divider_recalc_rate(hw, parent_rate, val, divider->table,
> > + divider->flags);
> > +}
> > +
> > static bool _is_valid_table_div(const struct clk_div_table *table,
> > unsigned int div)
> > {
> > @@ -416,6 +436,89 @@ static int clk_divider_set_rate(struct clk_hw *hw, unsigned long rate,
> > return 0;
> > }
> >
> > +static int clk_divider_gate_set_rate(struct clk_hw *hw, unsigned long rate,
> > + unsigned long parent_rate)
> > +{
> > + struct clk_divider *divider = to_clk_divider(hw);
> > + int value;
> > +
> > + if (!clk_hw_is_enabled(hw)) {
> > + value = divider_get_val(rate, parent_rate, divider->table,
> > + divider->width, divider->flags);
> > + if (value < 0)
> > + return value;
> > +
> > + divider->cached_val = value;
> > +
> > + return 0;
> > + }
> > +
> > + return clk_divider_set_rate(hw, rate, parent_rate);
> > +}
> > +
> > +static int clk_divider_enable(struct clk_hw *hw)
> > +{
> > + struct clk_divider *divider = to_clk_divider(hw);
> > + unsigned long uninitialized_var(flags);
> > + u32 val;
> > +
> > + if (!divider->cached_val) {
> > + pr_err("%s: no valid preset rate\n", clk_hw_get_name(hw));
> > + return -EINVAL;
> > + }
> > +
> > + if (divider->lock)
> > + spin_lock_irqsave(divider->lock, flags);
> > + else
> > + __acquire(divider->lock);
> > +
> > + /* restore div val */
> > + val = clk_readl(divider->reg);
> > + val |= divider->cached_val << divider->shift;
> > + clk_writel(val, divider->reg);
> > +
> > + if (divider->lock)
> > + spin_unlock_irqrestore(divider->lock, flags);
> > + else
> > + __release(divider->lock);
> > +
> > + return 0;
> > +}
> > +
> > +static void clk_divider_disable(struct clk_hw *hw)
> > +{
> > + struct clk_divider *divider = to_clk_divider(hw);
> > + unsigned long uninitialized_var(flags);
> > + u32 val;
> > +
> > + if (divider->lock)
> > + spin_lock_irqsave(divider->lock, flags);
> > + else
> > + __acquire(divider->lock);
> > +
> > + /* store the current div val */
> > + val = clk_readl(divider->reg) >> divider->shift;
> > + val &= div_mask(divider->width);
> > + divider->cached_val = val;
> > + clk_writel(0, divider->reg);
> > +
> > + if (divider->lock)
> > + spin_unlock_irqrestore(divider->lock, flags);
> > + else
> > + __release(divider->lock);
> > +}
> > +
> > +static int clk_divider_is_enabled(struct clk_hw *hw)
> > +{
> > + struct clk_divider *divider = to_clk_divider(hw);
> > + u32 val;
> > +
> > + val = clk_readl(divider->reg) >> divider->shift;
> > + val &= div_mask(divider->width);
> > +
> > + return val ? 1 : 0;
> > +}
> > +
> > const struct clk_ops clk_divider_ops = {
> > .recalc_rate = clk_divider_recalc_rate,
> > .round_rate = clk_divider_round_rate,
> > @@ -423,6 +526,16 @@ const struct clk_ops clk_divider_ops = {
> > };
> > EXPORT_SYMBOL_GPL(clk_divider_ops);
> >
> > +const struct clk_ops clk_divider_gate_ops = {
> > + .recalc_rate = clk_divider_gate_recalc_rate,
> > + .round_rate = clk_divider_round_rate,
> > + .set_rate = clk_divider_gate_set_rate,
> > + .enable = clk_divider_enable,
> > + .disable = clk_divider_disable,
> > + .is_enabled = clk_divider_is_enabled,
> > +};
> > +EXPORT_SYMBOL_GPL(clk_divider_gate_ops);
> > +
> > const struct clk_ops clk_divider_ro_ops = {
> > .recalc_rate = clk_divider_recalc_rate,
> > .round_rate = clk_divider_round_rate,
> > @@ -438,6 +551,7 @@ static struct clk_hw *_register_divider(struct device *dev, const char *name,
> > struct clk_divider *div;
> > struct clk_hw *hw;
> > struct clk_init_data init;
> > + u32 val;
> > int ret;
> >
> > if (clk_divider_flags & CLK_DIVIDER_HIWORD_MASK) {
> > @@ -455,6 +569,8 @@ static struct clk_hw *_register_divider(struct device *dev, const char *name,
> > init.name = name;
> > if (clk_divider_flags & CLK_DIVIDER_READ_ONLY)
> > init.ops = &clk_divider_ro_ops;
> > + else if (clk_divider_flags & CLK_DIVIDER_ZERO_GATE)
> > + init.ops = &clk_divider_gate_ops;
> > else
> > init.ops = &clk_divider_ops;
> > init.flags = flags | CLK_IS_BASIC;
> > @@ -470,6 +586,12 @@ static struct clk_hw *_register_divider(struct device *dev, const char *name,
> > div->hw.init = &init;
> > div->table = table;
> >
> > + if (div->flags & CLK_DIVIDER_ZERO_GATE) {
> > + val = clk_readl(reg) >> shift;
> > + val &= div_mask(width);
> > + div->cached_val = val;
> > + }
> > +
> > /* register the clock */
> > hw = &div->hw;
> > ret = clk_hw_register(dev, hw);
> > diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h
> > index 7c925e6..5f33b73 100644
> > --- a/include/linux/clk-provider.h
> > +++ b/include/linux/clk-provider.h
> > @@ -358,6 +358,7 @@ struct clk_div_table {
> > * @shift: shift to the divider bit field
> > * @width: width of the divider bit field
> > * @table: array of value/divider pairs, last entry should have div = 0
> > + * @cached_val: cached div hw value used for CLK_DIVIDER_ZERO_GATE
> > * @lock: register lock
> > *
> > * Clock with an adjustable divider affecting its output frequency. Implements
> > @@ -386,6 +387,12 @@ struct clk_div_table {
> > * CLK_DIVIDER_MAX_AT_ZERO - For dividers which are like CLK_DIVIDER_ONE_BASED
> > * except when the value read from the register is zero, the divisor is
> > * 2^width of the field.
> > + * CLK_DIVIDER_ZERO_GATE - For dividers which are like CLK_DIVIDER_ONE_BASED
> > + * when the value read from the register is zero, it means the divisor
> > + * is gated. For this case, the cached_val will be used to store the
> > + * intermediate div for the normal rate operation, like set_rate/get_rate/
> > + * recalc_rate. When the divider is ungated, the driver will actually
> > + * program the hardware to have the requested divider value.
> > */
> > struct clk_divider {
> > struct clk_hw hw;
> > @@ -394,6 +401,7 @@ struct clk_divider {
> > u8 width;
> > u8 flags;
> > const struct clk_div_table *table;
> > + u32 cached_val;
> > spinlock_t *lock;
> > };
> >
> > @@ -406,6 +414,7 @@ struct clk_divider {
> > #define CLK_DIVIDER_ROUND_CLOSEST BIT(4)
> > #define CLK_DIVIDER_READ_ONLY BIT(5)
> > #define CLK_DIVIDER_MAX_AT_ZERO BIT(6)
> > +#define CLK_DIVIDER_ZERO_GATE BIT(7)
> >
> > extern const struct clk_ops clk_divider_ops;
> > extern const struct clk_ops clk_divider_ro_ops;
> >
> > Anyway, if you still think it's not proper, i can put it in platform
> > driver as you wish, just in the cost of a few duplicated codes.
>
> Ok. Keeping it in the basic types but split into different ops
> path looks good.
>
> >
> > > > +
> > > > + if (!divider->cached_val) {
> > > > + pr_err("%s: no valid preset rate\n", clk_hw_get_name(hw));
> > > > + return -EINVAL;
> > > > + }
> > > > +
> > > > + if (divider->lock)
> > > > + spin_lock_irqsave(divider->lock, flags);
> > > > + else
> > > > + __acquire(divider->lock);
> > > > +
> > > > + /* restore div val */
> > > > + val = clk_readl(divider->reg);
> > > > + val |= divider->cached_val << divider->shift;
> > > > + clk_writel(val, divider->reg);
> > > > +
> > > > + if (divider->lock)
> > > > + spin_unlock_irqrestore(divider->lock, flags);
> > > > + else
> > > > + __release(divider->lock);
> > > > +
> > > > + return 0;
> > > > +}
> > > > +
> > > > +static void clk_divider_disable(struct clk_hw *hw)
> > > > +{
> > > > + struct clk_divider *divider = to_clk_divider(hw);
> > > > + unsigned long flags = 0;
> > > > + u32 val;
> > > > +
> > > > + if (!(divider->flags & CLK_DIVIDER_ZERO_GATE))
> > > > + return;
> > > > +
> > > > + if (divider->lock)
> > > > + spin_lock_irqsave(divider->lock, flags);
> > > > + else
> > > > + __acquire(divider->lock);
> > > > +
> > > > + /* store the current div val */
> > > > + val = clk_readl(divider->reg) >> divider->shift;
> > > > + val &= div_mask(divider->width);
> > > > + divider->cached_val = val;
> > > > + clk_writel(0, divider->reg);
> > > > +
> > > > + if (divider->lock)
> > > > + spin_unlock_irqrestore(divider->lock, flags);
> > > > + else
> > > > + __release(divider->lock);
> > > > +}
> > > > +
> > > > +static int clk_divider_is_enabled(struct clk_hw *hw)
> > > > +{
> > > > + struct clk_divider *divider = to_clk_divider(hw);
> > > > + u32 val;
> > > > +
> > > > + if (!(divider->flags & CLK_DIVIDER_ZERO_GATE))
> > > > + return __clk_get_enable_count(hw->clk);
> > >
> > > The plan was to delete this API once OMAP stopped using it.
> > > clk_hw_is_enabled() doesn't work?
> >
> > No, it did not work before because clk_hw_is_enabled will result
> > in the dead loop by calling .is_enabled() callback again.
> >
> > That's why __clk_get_enable_count is used instead.
> >
> > However, with above new patch method, this issue was gone.
>
> Great!
>
> >
> > >
> > > > +
> > > > + val = clk_readl(divider->reg) >> divider->shift;
> > > > + val &= div_mask(divider->width);
> > > > +
> > > > + return val ? 1 : 0;
> > > > +}
> > > > +
> > > > const struct clk_ops clk_divider_ops = {
> > > > .recalc_rate = clk_divider_recalc_rate,
> > > > .round_rate = clk_divider_round_rate,
> > > > .set_rate = clk_divider_set_rate,
> > > > + .enable = clk_divider_enable,
> > > > + .disable = clk_divider_disable,
> > > > + .is_enabled = clk_divider_is_enabled,
> > > > };
> > > > EXPORT_SYMBOL_GPL(clk_divider_ops);
> > > >
> > > > @@ -436,6 +525,7 @@ static struct clk_hw *_register_divider(struct device *dev, const char *name,
> > > > struct clk_divider *div;
> > > > struct clk_hw *hw;
> > > > struct clk_init_data init;
> > > > + u32 val;
> > > > int ret;
> > > >
> > > > if (clk_divider_flags & CLK_DIVIDER_HIWORD_MASK) {
> > > > @@ -468,6 +558,12 @@ static struct clk_hw *_register_divider(struct device *dev, const char *name,
> > > > div->hw.init = &init;
> > > > div->table = table;
> > > >
> > > > + if (div->flags & CLK_DIVIDER_ZERO_GATE) {
> > > > + val = clk_readl(reg) >> shift;
> > > > + val &= div_mask(width);
> > > > + div->cached_val = val;
> > > > + }
> > >
> > > What if it isn't on? Setting cached_val to 0 is ok?
> > >
> >
> > If it isn't on, then the cache_val should be 0.
> > And recalc_rate will catch this case and return 0 as there's
> > no proper pre-set rate.
> >
>
> Ok.
>
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> a Linux Foundation Collaborative Project

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