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

[Mike Turquette <mturquette@linaro.org>]

Quoting Tomi Valkeinen (2014-03-17 05:53:03)
> 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 <tomi.valkeinen@ti.com>
> >>> Cc: Mike Turquette <mturquette@linaro.org>
> >>> ---
> >>>   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?

Yes, they've been in -next for a couple of weeks.

Regards,
Mike

> 
>  Tomi
> 
>