Subject: [PATCH 1/8] clk: divider: fix rate calculation for fractional rates

From: [email-masked] (Tomi Valkeinen)
Date: Mon, 17 Mar 2014 14:53:03 +0200

On 27/02/14 04:25, Mike Turquette wrote:
> Quoting Tero Kristo (2014-02-14 05:45:22)
>> On 02/13/2014 12:03 PM, Tomi Valkeinen wrote:
>>> clk-divider.c does not calculate the rates consistently at the moment.
>>>
>>> As an example, on OMAP3 we have a clock divider with a source clock of
>>> 864000000 Hz. With dividers 6, 7 and 8 the theoretical rates are:
>>>
>>> 6: 144000000
>>> 7: 123428571.428571...
>>> 8: 108000000
>>>
>>> Calling clk_round_rate() with the rate in the first column will give the
>>> rate in the second column:
>>>
>>> 144000000 -> 144000000
>>> 143999999 -> 123428571
>>> 123428572 -> 123428571
>>> 123428571 -> 108000000
>>>
>>> Note how clk_round_rate() returns 123428571 for rates from 123428572 to
>>> 143999999, which is mathematically correct, but when clk_round_rate() is
>>> called with 123428571, the returned value is surprisingly 108000000.
>>>
>>> This means that the following code works a bit oddly:
>>>
>>> rate = clk_round_rate(clk, 123428572);
>>> clk_set_rate(clk, rate);
>>>
>>> As clk_set_rate() also does clock rate rounding, the result is that the
>>> clock is set to the rate of 108000000, not 123428571 returned by the
>>> clk_round_rate.
>>>
>>> This patch changes the clk-divider.c to use DIV_ROUND_UP when
>>> calculating the rate. This gives the following behavior which fixes the
>>> inconsistency:
>>>
>>> 144000000 -> 144000000
>>> 143999999 -> 123428572
>>> 123428572 -> 123428572
>>> 123428571 -> 108000000
>>>
>>> Signed-off-by: Tomi Valkeinen <[email-masked]>
>>> Cc: Mike Turquette <[email-masked]>
>>> ---
>>>   drivers/clk/clk-divider.c | 10 +++++-----
>>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
>>> index 5543b7df8e16..ec22112e569f 100644
>>> --- a/drivers/clk/clk-divider.c
>>> +++ b/drivers/clk/clk-divider.c
>>> @@ -24,7 +24,7 @@
>>>    * Traits of this clock:
>>>    * prepare - clk_prepare only ensures that parents are prepared
>>>    * enable - clk_enable only ensures that parents are enabled
>>> - * rate - rate is adjustable.  clk->rate = parent->rate / divisor
>>> + * rate - rate is adjustable.  clk->rate = DIV_ROUND_UP(parent->rate / divisor)
>>>    * parent - fixed parent.  No clk_set_parent support
>>>    */
>>>
>>> @@ -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);
>>>   }
>>>
>>>   /*
>>> @@ -185,7 +185,7 @@ static int clk_divider_bestdiv(struct clk_hw *hw, unsigned long rate,
>>>               }
>>>               parent_rate = __clk_round_rate(__clk_get_parent(hw->clk),
>>>                               MULT_ROUND_UP(rate, i));
>>> -             now = parent_rate / i;
>>> +             now = DIV_ROUND_UP(parent_rate, i);
>>>               if (now <= rate && now > best) {
>>>                       bestdiv = i;
>>>                       best = now;
>>> @@ -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))
>>>
>>
>> Basically the patch looks good to me, but it might be good to have a 
>> testing round of sort with this. It can potentially cause regressions on 
>> multiple boards if the drivers happen to rely on the "broken" clock 
>> rates. Same for patch #2 which is a copy paste of this one, but only 
>> impacts TI boards.
> 
> Agreed. I've taken patches #1 & #2 into clk-next. Let's let them stew in
> -next for a while and see if anyone's board catches on fire.

Are these on the way to 3.15?

 Tomi

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