Re: [PATCH] clk: sunxi: Accept a greater rate when setting a parent clock

[Maxime Ripard <maxime.ripard@free-electrons.com>]

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Hi,

On Mon, Mar 21, 2016 at 09:25:49AM +0100, Jean-Francois Moine wrote:
> On Mon, 21 Mar 2016 08:25:46 +0100
> Maxime Ripard <maxime.ripard@free-electrons.com> wrote:
> 
> > > -	/* find the parent that can help provide the fastest rate <= rate */
> > > +	/* find the parent that can help provide the fastest rate */
> > >  	num_parents = clk_hw_get_num_parents(hw);
> > >  	for (i = 0; i < num_parents; i++) {
> > >  		parent = clk_hw_get_parent_by_index(hw, i);
> > > @@ -100,7 +100,7 @@ static int clk_factors_determine_rate(struct clk_hw *hw,
> > >  		child_rate = clk_factors_round_rate(hw, req->rate,
> > >  						    &parent_rate);
> > >  
> > > -		if (child_rate <= req->rate && child_rate > best_child_rate) {
> > > +		if (child_rate > best_child_rate) {
> > 
> > I'm not sure this would work, since you'll end up picking the fastest
> > rate without considering whether it is the closest or not.
> > 
> > I guess what you want here is using the absolute difference between
> > the requested rate and the rate you're evaluating.
> > 
> > That being said, we had a similar discussion for SPI around a month
> > ago where we wanted a rate strictly lower than the requested one. I
> > guess it's time to add a flag to tell how you want to round.
> 
> You are right, I just removed half of the constraint, but I still wonder
> why does this sequence introduced by the commit 862b728387aef3a37
> (clk: sunxi: factors: automatic reparenting support) do
> 	"provide the fastest rate <= rate"
> instead of
> 	"provide the closest rate" ?

I guess it's a confusing wording, in this case where you try to have
the closest rate but below the requested rate, what you want is
actually the fastest rate that doesn't trip above the requested rate,
hence the comment I guess.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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