Re: Rounding issue in drivers/clk/clk-divider.c

[Tomi Valkeinen <tomi.valkeinen@ti.com>]

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Hi,

On 08/10/13 16:17, Tomi Valkeinen wrote:
> Hi,
> 
> I'm seeing the following issue on omap3 with dpll4_m4 clock. dpll4_m4's
> parent is a PLL set to 864000000 and dpll4_m4 is a divider, handled by
> clk-divider.c.
> 
> Now, if I call clk_round_rate(dpll4_m4, 143999999), I get 123428571
> which is correct. However, if I call clk_round_rate(dpll4_m4,
> 123428571), I would presume to get the same answer, 123428571, as that
> was already "verified" by the previous clk_round_rate() call. However, I
> get 108000000.
> 
> So, if I have the following code:
> 
> rate = clk_round_rate(dpll4_m4, 143999999);
> /* rate is 123428571 */
> clk_set_rate(dpll4_m4, rate);
> 
> the resulting rate is 108000000.

I continued testing with this, and with the following RFC patch I get
consistent rates:

diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
index 8d3009e..ba20314 100644
--- a/drivers/clk/clk-divider.c
+++ b/drivers/clk/clk-divider.c
@@ -115,7 +115,7 @@ static unsigned long clk_divider_recalc_rate(struct clk_hw *hw,
 		return parent_rate;
 	}
 
-	return parent_rate / div;
+	return DIV_ROUND_UP(parent_rate, div);
 }
 
 /*
@@ -207,7 +207,7 @@ static long clk_divider_round_rate(struct clk_hw *hw, unsigned long rate,
 	int div;
 	div = clk_divider_bestdiv(hw, rate, prate);
 
-	return *prate / div;
+	return DIV_ROUND_UP(*prate, div);
 }
 
 static int clk_divider_set_rate(struct clk_hw *hw, unsigned long rate,
@@ -218,7 +218,7 @@ static int clk_divider_set_rate(struct clk_hw *hw, unsigned long rate,
 	unsigned long flags = 0;
 	u32 val;
 
-	div = parent_rate / rate;
+	div = DIV_ROUND_UP(parent_rate, rate);
 	value = _get_val(divider, div);
 
 	if (value > div_mask(divider))


Now clk_round_rate for this clock returns the following:

144000000 -> 144000000
143999999 -> 123428572
123428572 -> 123428572
123428571 -> 108000000

So now multiple nested calls to clk_round_rate return consistent values, and
calling clk_set_rate with the rate returned by clk_round_rate will not modify
the rate.

I believe the patch is missing pieces, at least for clk_divider_bestdiv() for
the case when CLK_SET_RATE_PARENT is set. Also, 864000000 / 7 = 123428571.4...,
so in reality 123428571 would be a better answer than 123428572. But rounding
to 123428572 makes things work consistently.

However, even if the patch fixes the issue for me, I'm a bit confused on the
clock rate rounding. How should it happen? Is it even defined how the rate is
rounded?

In my particular use case I want to iterate the possible clock rates, so that I
can find the best one to use. I do it with this kind of code:

/* start with the max rate my IP allows */
rate = max_allowed_fck;
while (true) {
	rate = clk_round_rate(rate);
	test_rate(rate);
	/* -1, so that the next round will return the next lowest rate */
	rate -= 1;
}

The code above presumes that the clk_round_rate will round down, but I don't
see the rounding explicitly specified in any documentation. Is that kind of
code valid?

Another use case I have is to set the clock rate to something which is higher
than what I need. I.e. I know that I need at least 100MHz clock so that the IP
performs the job quickly enough. If I call clk_round_rate(100M), I'll get a
lower clock, not higher. So in this case I'd actually like the rounding to be
up. And if the rate is rounded down, I have no idea what rate should I use to
get at least 100MHz.

Am I doing something silly here? =) Should there be multiple clk_round_rate
versions, for different roundings?

 Tomi



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