linux-kernel - Re: [PATCH v2 2/5] clk: bcm281xx: implement prerequisite clocks

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Date:	Thu, 29 May 2014 09:35:52 -0700
From:	Mike Turquette <mturquette@...aro.org>
To:	Alex Elder <elder@...aro.org>, mporter@...aro.org,
	bcm@...thebug.org, devicetree@...r.kernel.org
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/5] clk: bcm281xx: implement prerequisite clocks

Quoting Alex Elder (2014-05-29 06:26:15)
> On 05/23/2014 07:53 PM, Mike Turquette wrote:
> > The above seems like a lot effort to go to. Why not skip all of this and
> > just implement the prerequisite logic in the .enable & .disable
> > callbacks? E.g. your kona clk .enable callback would look like:
> 
> I think the problem is that it means the clock consumers
> would have to know that prerequisite relationship.  And
> that is dependent on the clock tree.  The need for it in
> this case was because the boot loader didn't initialize
> all the clocks that were needed.  If we could count on
> the boot loader setting things up initially we might not
> need to do this.
> 
> > 
> > diff --git a/drivers/clk/bcm/clk-kona.c b/drivers/clk/bcm/clk-kona.c
> > index d603c4e..51f35b4 100644
> > --- a/drivers/clk/bcm/clk-kona.c
> > +++ b/drivers/clk/bcm/clk-kona.c
> > @@ -987,6 +987,12 @@ static int kona_peri_clk_enable(struct clk_hw *hw)
> >  {
> >       struct kona_clk *bcm_clk = to_kona_clk(hw);
> >       struct bcm_clk_gate *gate = &bcm_clk->u.peri->gate;
> > +     int ret;
> > +
> > +     hw->prereq_bus_clk = clk_get(hw->ccu, hw->prereq);
> > +     ret = clk_enable(prereq_bus_clk);
> > +     if (ret)
> > +             return ret;
> >  
> >       return clk_gate(bcm_clk->ccu, bcm_clk->init_data.name, gate, true);
> >  }
> > @@ -997,6 +1003,9 @@ static void kona_peri_clk_disable(struct clk_hw *hw)
> >       struct bcm_clk_gate *gate = &bcm_clk->u.peri->gate;
> >  
> >       (void)clk_gate(bcm_clk->ccu, bcm_clk->init_data.name, gate, false);
> > +
> > +     clk_disable(hw->prereq_bus_clk);
> > +     clk_put(hw->prereq_bus_clk);
> >  }
> >  
> >  static int kona_peri_clk_is_enabled(struct clk_hw *hw)
> > 
> > 
> > I guess it might take some trickery to get clk_get to work like that.
> > Let me know if I've completely lost the plot.
> 
> I don't think so, but I think there's a lot of stuff
> here to try to understand, and you're trying to extract
> it from the code without the benefit of some background
> of how and why it's done this way.
> 
> Hopefully all this verbiage is moving you closer to
> understanding...  I appreciate your patience.

Hi Alex,

Can you comment on my diff above? I basically tossed up some pseudo-code
to show how clk_enable calls can be nested inside of each other. I'd
like to know if that approach makes sense for your prereq clocks case.

Note that Linux device drivers that consume leaf clocks do NOT need to
know about the prereq clocks. All of that prereq clock knowledge is
stored in the .enable callback for the leaf clock (see above).

Regards,
Mike

> 
>                                         -Alex
> 
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