Re: [PATCH v2 56/65] clk: ingenic: cgu: Switch to determine_rate

[Maxime Ripard <maxime@cerno.tech>]

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Hi Paul,

On Fri, Nov 04, 2022 at 02:31:20PM +0000, Paul Cercueil wrote:
> Le ven. 4 nov. 2022 à 14:18:13 +0100, Maxime Ripard <maxime@cerno.tech> a
> écrit :
> > The Ingenic CGU clocks implements a mux with a set_parent hook, but
> > doesn't provide a determine_rate implementation.
> > 
> > This is a bit odd, since set_parent() is there to, as its name implies,
> > change the parent of a clock. However, the most likely candidate to
> > trigger that parent change is a call to clk_set_rate(), with
> > determine_rate() figuring out which parent is the best suited for a
> > given rate.
> > 
> > The other trigger would be a call to clk_set_parent(), but it's far less
> > used, and it doesn't look like there's any obvious user for that clock.
> > 
> > So, the set_parent hook is effectively unused, possibly because of an
> > oversight. However, it could also be an explicit decision by the
> > original author to avoid any reparenting but through an explicit call to
> > clk_set_parent().
> > 
> > The driver does implement round_rate() though, which means that we can
> > change the rate of the clock, but we will never get to change the
> > parent.
> > 
> > However, It's hard to tell whether it's been done on purpose or not.
> > 
> > Since we'll start mandating a determine_rate() implementation, let's
> > convert the round_rate() implementation to a determine_rate(), which
> > will also make the current behavior explicit. And if it was an
> > oversight, the clock behaviour can be adjusted later on.
> 
> So it's partly on purpose, partly because I didn't know about
> .determine_rate.
> 
> There's nothing odd about having a lonely .set_parent callback; in my case
> the clocks are parented from the device tree.
> 
> Having the clocks driver trigger a parent change when requesting a rate
> change sounds very dangerous, IMHO. My MMC controller can be parented to the
> external 48 MHz oscillator, and if the card requests 50 MHz, it could switch
> to one of the PLLs. That works as long as the PLLs don't change rate, but if
> one is configured as driving the CPU clock, it becomes messy.
> The thing is, the clocks driver has no way to know whether or not it is
> "safe" to use a designated parent.
> 
> For that reason, in practice, I never actually want to have a clock
> re-parented - it's almost always a bad idea vs. sticking to the parent clock
> configured in the DTS.

Yeah, and this is totally fine. But we need to be explicit about it. The
determine_rate implementation I did in all the patches is an exact
equivalent to the round_rate one if there was one. We will never ask to
change the parent.

Given what you just said, I would suggest to set the
CLK_SET_RATE_NO_REPARENT flag as well.

> 
> > Signed-off-by: Maxime Ripard <maxime@cerno.tech>
> > ---
> >  drivers/clk/ingenic/cgu.c | 15 ++++++++-------
> >  1 file changed, 8 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/clk/ingenic/cgu.c b/drivers/clk/ingenic/cgu.c
> > index 1f7ba30f5a1b..0c9c8344ad11 100644
> > --- a/drivers/clk/ingenic/cgu.c
> > +++ b/drivers/clk/ingenic/cgu.c
> > @@ -491,22 +491,23 @@ ingenic_clk_calc_div(struct clk_hw *hw,
> >  	return div;
> >  }
> > 
> > -static long
> > -ingenic_clk_round_rate(struct clk_hw *hw, unsigned long req_rate,
> > -		       unsigned long *parent_rate)
> > +static int ingenic_clk_determine_rate(struct clk_hw *hw,
> > +				      struct clk_rate_request *req)
> >  {
> >  	struct ingenic_clk *ingenic_clk = to_ingenic_clk(hw);
> >  	const struct ingenic_cgu_clk_info *clk_info =
> > to_clk_info(ingenic_clk);
> >  	unsigned int div = 1;
> > 
> >  	if (clk_info->type & CGU_CLK_DIV)
> > -		div = ingenic_clk_calc_div(hw, clk_info, *parent_rate, req_rate);
> > +		div = ingenic_clk_calc_div(hw, clk_info, req->best_parent_rate,
> > +					   req->rate);
> 
> Sorry but I'm not sure that this works.
> 
> You replace the "parent_rate" with the "best_parent_rate", and that means
> you only check the requested rate vs. the parent with the highest frequency,
> and not vs. the actual parent that will be used.

best_parent_rate is initialized to the current parent rate, not the
parent with the highest frequency:
https://elixir.bootlin.com/linux/v6.1-rc3/source/drivers/clk/clk.c#L1471

Maxime

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