linux-kernel - Re: [PATCH v7 3/3] clk: basic clock hardware types

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Date:	Fri, 16 Mar 2012 09:51:19 -0700
From:	"Turquette, Mike" <mturquette@...com>
To:	Richard Zhao <richard.zhao@...aro.org>
Cc:	Russell King <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linaro-dev@...ts.linaro.org, patches@...aro.org,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnd Bergman <arnd.bergmann@...aro.org>,
	Paul Walmsley <paul@...an.com>,
	Shawn Guo <shawn.guo@...escale.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Jamie Iles <jamie@...ieiles.com>,
	Saravana Kannan <skannan@...eaurora.org>,
	Magnus Damm <magnus.damm@...il.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Linus Walleij <linus.walleij@...ricsson.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Deepak Saxena <dsaxena@...aro.org>,
	Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [PATCH v7 3/3] clk: basic clock hardware types

On Fri, Mar 16, 2012 at 5:25 AM, Richard Zhao <richard.zhao@...aro.org> wrote:
> [...]
>> +static int clk_divider_bestdiv(struct clk_hw *hw, unsigned long rate,
>> +             unsigned long *best_parent_rate)
>> +{
>> +     struct clk_divider *divider = to_clk_divider(hw);
>> +     int i, bestdiv = 0;
>> +     unsigned long parent_rate, best = 0, now, maxdiv;
>> +
>> +     if (!rate)
>> +             rate = 1;
>> +
>> +     maxdiv = (1 << divider->width);
>> +
>> +     if (divider->flags & CLK_DIVIDER_ONE_BASED)
>> +             maxdiv--;
>> +
>> +     if (!best_parent_rate) {
>> +             parent_rate = __clk_get_rate(__clk_get_parent(hw->clk));
>> +             bestdiv = DIV_ROUND_UP(parent_rate, rate);
>> +             bestdiv = bestdiv == 0 ? 1 : bestdiv;
>> +             bestdiv = bestdiv > maxdiv ? maxdiv : bestdiv;
>> +             return bestdiv;
>> +     }
>> +
>> +     /*
>> +      * The maximum divider we can use without overflowing
>> +      * unsigned long in rate * i below
>> +      */
>> +     maxdiv = min(ULONG_MAX / rate, maxdiv);
>> +
>> +     for (i = 1; i <= maxdiv; i++) {
>> +             parent_rate = __clk_round_rate(__clk_get_parent(hw->clk),
>> +                             MULT_ROUND_UP(rate, i));
>> +             now = parent_rate / i;
>> +             if (now <= rate && now > best) {
>> +                     bestdiv = i;
>> +                     best = now;
>> +                     *best_parent_rate = parent_rate;
> Better add              if (now == rate)
>                                break;
> There may be more than one hit for (now == rate). We'd better select
> smallest div, thus smallest parent_rate.

Hmm I forgot to take this in.  Notice that the conditional above has changed:

if (now <= rate && now > best) {

So the smallest divider _will_ be selected, but adding in your check
for now == rate is nice since it will prevent unnecessary looping if
we've managed to hit the exact rate that we want.  I'll ninja this one
into the merge request.

Thanks,
Mike

>
> It's the same comment for v6, but not show stopper.
>
> Thanks
> Richard
>> +             }
>> +     }
>> +
>> +     if (!bestdiv) {
>> +             bestdiv = (1 << divider->width);
>> +             if (divider->flags & CLK_DIVIDER_ONE_BASED)
>> +                     bestdiv--;
>> +             *best_parent_rate = __clk_round_rate(__clk_get_parent(hw->clk), 1);
>> +     }
>> +
>> +     return bestdiv;
>> +}
>
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