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

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Date:	Thu, 03 Dec 2015 16:37:07 -0800
From:	Eric Anholt <eric@...olt.net>
To:	Remi Pommarel <repk@...plefau.lt>
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

Remi Pommarel <repk@...plefau.lt> writes:

> 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().

If we pass bcm2835_clock_rate_from_divisor(bcm2835_clock_choose_div()),
to bcm2835_clock_choose_div(), will it actually give a different divisor
than the first call?  (That seems like an unfortunate problem in our
implementation, if so).

I'd be willing to go along with this, but if so I'd like a comment
explaining why we aren't setting the field that we should pretty
obviously be setting.

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