linux-kernel - Re: [PATCH RESEND] clk: set initial best mux parent to current parent when determining rate

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Date: Wed, 28 Feb 2024 18:25:18 -0800
From: Stephen Boyd <sboyd@...nel.org>
To: Michael Turquette <mturquette@...libre.com>, Yang Xiwen <forbidden405@...look.com>, Yang Xiwen via B4 Relay <devnull+forbidden405.outlook.com@...nel.org>
Cc: linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND] clk: set initial best mux parent to current parent when determining rate

Quoting Yang Xiwen (2024-02-28 18:13:04)
> On 2/29/2024 9:58 AM, Stephen Boyd wrote:
> > Quoting Yang Xiwen via B4 Relay (2024-02-23 09:18:52)
> >> From: Yang Xiwen <forbidden405@...look.com>
> >>
> >> Originally, the initial clock rate is hardcoded to 0, this can lead to
> >> some problem when setting a very small rate with CLK_MUX_ROUND_NEAREST.
> > 
> > Did you mean CLK_MUX_ROUND_CLOSEST?
> 
> You are right :).
> 
> > 
> >>
> >> For example, if the lowest possible rate privided by the mux is 1000Hz,
> > 
> > s/privided/provided/
> > 
> >> setting a rate below 500Hz will fail, because no clock can provide a
> >> better rate than the non-existant 0. But it should succeed with 1000Hz
> >> being set.
> >>
> >> Setting the initial best parent to current parent could solve this bug
> >> very well.
> >>
> >> Signed-off-by: Yang Xiwen <forbidden405@...look.com>
> >> ---
> >> This is actually a v2 of [1], but seems too simple to have a unittest.
> >> It's tested in a mmc host driver.
> > 
> > It's not too simple for a unittest.
> > 
> >>
> >> [1]: https://lore.kernel.org/linux-clk/20230421-clk-v3-1-9ff79e7e7fed@outlook.com/
> > 
> > In that thread I asked you to please Cc Maxime. Please do that.
> > 
> >> ---
> >>  drivers/clk/clk.c | 4 ++++
> >>  1 file changed, 4 insertions(+)
> >>
> >> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> >> index 2253c154a824..d98cebd7ff03 100644
> >> --- a/drivers/clk/clk.c
> >> +++ b/drivers/clk/clk.c
> >> @@ -649,6 +649,10 @@ int clk_mux_determine_rate_flags(struct clk_hw *hw,
> >>  
> >>         /* find the parent that can provide the fastest rate <= rate */
> >>         num_parents = core->num_parents;
> >> +       if (core->parent) {
> >> +               best_parent = core->parent;
> >> +               best = clk_core_get_rate_nolock(best_parent);
> >> +       }
> > 
> > Is the problem that we're not using abs_diff()?
> 
> 
> No, i think. It has nothing to do with the code here. It's because of
> the initial best_parent/best_parent_rate.

Alright.

> 
> > 
> > ----8<----
> > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> > index a3bc7fb90d0f..91023345595f 100644
> > --- a/drivers/clk/clk.c
> > +++ b/drivers/clk/clk.c
> > @@ -542,7 +542,7 @@ static bool mux_is_better_rate(unsigned long rate, unsigned long now,
> >                          unsigned long best, unsigned long flags)
> >  {
> >       if (flags & CLK_MUX_ROUND_CLOSEST)
> > -             return abs(now - rate) < abs(best - rate);
> > +             return abs_diff(now, rate) < abs_diff(best, rate);
> 
> Without this patch, the initial `best` rate would be always 0. This is
> wrong for most cases, 0Hz might (usually) be unavailable. We should use
> a valid rate(i.e. current rate) initially.

Ok. But you set best to the parent rate. So why not use 'core->rate'
directly as 'best'?