linux-kernel - Re: [PATCH v2 2/3] clk: bcm2835: Support for clock parent selection

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Date:	Wed, 18 Nov 2015 20:24:23 +0100
From:	Remi Pommarel <repk@...plefau.lt>
To:	Eric Anholt <eric@...olt.net>
Cc:	Stephen Warren <swarren@...dotorg.org>, Lee Jones <lee@...nel.org>,
	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-rpi-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] clk: bcm2835: Support for clock parent selection

On Wed, Nov 18, 2015 at 10:30:17AM -0800, Eric Anholt wrote:

[...]

> > +static int bcm2835_clock_determine_rate(struct clk_hw *hw,
> > +		struct clk_rate_request *req)
> > +{
> > +	struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
> > +	struct clk_hw *parent, *best_parent = NULL;
> > +	struct clk_rate_request parent_req;
> > +	unsigned long rate, best_rate = 0;
> > +	unsigned long prate, best_prate = 0;
> > +	size_t i;
> > +	u32 div;
> > +
> > +	/*
> > +	 * Select parent clock that results in the closest but lower rate
> > +	 */
> > +	for (i = 0; i < clk_hw_get_num_parents(hw); ++i) {
> > +		parent = clk_hw_get_parent_by_index(hw, i);
> > +		if (!parent)
> > +			continue;
> > +		parent_req = *req;
> 
> parent_req appears dead, so it should be removed.

Yes, will do thanks.

> > +		prate = clk_hw_get_rate(parent);
> > +		div = bcm2835_clock_choose_div(hw, req->rate, prate);
> > +		rate = bcm2835_clock_rate_from_divisor(clock, prate, div);
> > +		if (rate > best_rate && rate <= req->rate) {
> > +			best_parent = parent;
> > +			best_prate = prate;
> > +			best_rate = rate;
> > +		}
> > +	}
> > +
> > +	if (!best_parent)
> > +		return -EINVAL;
> > +
> > +	req->best_parent_hw = best_parent;
> > +	req->best_parent_rate = best_prate;
> 
> I think you're supposed to req->rate = best_rate, here, too.  With these
> two fixes,

I did not set req->rate to best_rate in order to avoid rounding down
twice the actual clock rate.

Indeed with patch 1 from this patchset bcm2835_clock_choose_div()
chooses a divisor that produces a rate lower or equal to the requested
one. As we call bcm2835_clock_choose_div() twice when using
clk_set_rate() (once with ->determine_rate() and once with ->set_rate()),
if I set req->rate in bcm2835_clock_determine_rate to the rounded down
one, the final rate will likely be again rounded down in
bcm2835_clock_set_rate().

Thanks,

-- 
Rémi Pommarel
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