linux-kernel - Re: [PATCH] clk: Re-evaluate clock rate on min/max update

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Date:   Thu, 13 Apr 2017 10:48:19 +0300
From:   Peter De Schrijver <pdeschrijver@...dia.com>
To:     Stephen Boyd <sboyd@...eaurora.org>
CC:     Michael Turquette <mturquette@...libre.com>,
        <linux-clk@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] clk: Re-evaluate clock rate on min/max update

On Wed, Apr 12, 2017 at 09:46:05AM -0700, Stephen Boyd wrote:
> On 03/21, Peter De Schrijver wrote:
> > Whenever a user change its min or max rate limit of a clock, we need to
> > re-evaluate the current clock rate and possibly change it if the new limits
> > require so. To do this clk_set_rate_range() already calls
> > clk_core_set_rate_nolock, however this won't have the intended effect
> > because the core clock rate hasn't changed. To fix this, move the test to
> > avoid setting the same core clock rate again, to clk_set_rate() so
> > clk_core_set_rate_nolock() can change the clock rate when min or max have
> > been updated, even when the core clock rate has not changed.
> 
> I'd expect some sort of Fixes: tag here? Or it never worked!?

I don't think this ever worked.

> 
> > 
> > Signed-off-by: Peter De Schrijver <pdeschrijver@...dia.com>
> 
> I seem to recall some problems here around rate aggregation that
> we fixed after the patches merged. Sorry, but I have to go back
> and look at those conversations to refresh my memory and make
> sure this is all fine.
> 
> Are you relying on the rate setting op to be called with the new
> min/max requirements if the aggregated rate is the same? I don't
> understand why clk drivers care.
> 

No. But I do rely on the rate setting op to be called when a new min or max
rate would cause the rate to be changed even when there is no new rate request.

Eg:

min = 100MHz, max = 500MHz, current rate request is 400MHz, then max changes to
300MHz. Today the rate setting op will not be called, while I think it should
be called to lower the rate to 300MHz.

Peter.



> > ---
> >  drivers/clk/clk.c | 13 +++++++------
> >  1 file changed, 7 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> > index 2fa2fb8..0b815d1 100644
> > --- a/drivers/clk/clk.c
> > +++ b/drivers/clk/clk.c
> > @@ -1569,10 +1569,6 @@ static int clk_core_set_rate_nolock(struct clk_core *core,
> >  	if (!core)
> >  		return 0;
> >  
> > -	/* bail early if nothing to do */
> > -	if (rate == clk_core_get_rate_nolock(core))
> > -		return 0;
> > -
> >  	if ((core->flags & CLK_SET_RATE_GATE) && core->prepare_count)
> >  		return -EBUSY;
> >  
> > @@ -1621,16 +1617,21 @@ static int clk_core_set_rate_nolock(struct clk_core *core,
> >   */
> >  int clk_set_rate(struct clk *clk, unsigned long rate)
> >  {
> > -	int ret;
> > +	int ret = 0;
> >  
> >  	if (!clk)
> > -		return 0;
> > +		return ret;
> 
> Why? Noise?
> 
> >  
> >  	/* prevent racing with updates to the clock topology */
> >  	clk_prepare_lock();
> >  
> > +	/* bail early if nothing to do */
> > +	if (rate == clk_core_get_rate_nolock(clk->core))
> > +		goto out;
> > +
> >  	ret = clk_core_set_rate_nolock(clk->core, rate);
> >  
> > +out:
> >  	clk_prepare_unlock();
> >  
> 
> -- 
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> a Linux Foundation Collaborative Project