[PATCH] clk: divider: Use DIV_ROUND_CLOSEST

[Sören Brinkmann]

Hi Mike,

On Tue, Mar 19, 2013 at 05:16:09PM -0700, Mike Turquette wrote:
> Quoting Soren Brinkmann (2013-01-29 17:25:44)
> > Use the DIV_ROUND_CLOSEST macro to calculate divider values and minimize
> > rounding errors.
> >
> > Cc: linux-arm-kernel at lists.infradead.org
> > Cc: linux-kernel at vger.kernel.org
> > Signed-off-by: Soren Brinkmann <soren.brinkmann at xilinx.com>
> > ---
> > Hi,
> >
> > I just came across this behavior:
> > I'm using the clk-divider as cpuclock which can be scaled through cpufreq.
> > During boot I create an opp table which in this case holds the frequencies [MHz]
> > 666, 333 and 222. To make sure those are actually valid frequencies I call
> > clk_round_rate().
> > I added a debug line in clk-divider.c:clk_divider_bestdiv() before the return
> > in line 163 giving me:
> >         prate:1333333320, rate:333333330, div:4
> > for adding the 333 MHz operating point.
> >
> > In the running system this gives me:
> > zynq:/sys/devices/system/cpu/cpu0/cpufreq # cat scaling_available_frequencies
> > 222222 333333 666666
> > zynq:/sys/devices/system/cpu/cpu0/cpufreq # cat scaling_cur_freq
> >  666666
> >
> >  So far, so good. But now, let's scale the frequency:
> > zynq:/sys/devices/system/cpu/cpu0/cpufreq # echo 333333 > scaling_setspeed
> > zynq:/sys/devices/system/cpu/cpu0/cpufreq # cat scaling_cur_freq
> >  266666
> >
> > And the corresponding debug line:
> >         prate:1333333320, rate:333333000, div:5
> >
> > So, with DIV_ROUND_UP an actual divider of 4.00000396 becomes 5, resulting in a
> > huge error.
> >
>
> Soren,
>
> Thanks for the patch, and apologies for it getting lost for so long.  I
> think that your issue is more about policy.  E.g. should we round the
> divider up or round the divider down?  The correct answer will vary from
> platform to platform and the clk.h api doesn't specify how
> clk_round_rate should round, other than it must specify a rate that the
> clock can actually run at.
Sure, my problem seems to be caused by different subsystems using a different resolution for clock frequencies.

>
> Unless everyone can agree that DIV_ROUND_CLOSEST gives them what they
> want I'm more inclined to make this behavior a flag specific to struct
> clk-divider.  E.g. CLK_DIVIDER_ROUND_CLOSEST.
My understanding would have been, that clk_round_rate() gives me a frequency as close to the requested one as possible. If the caller doesn't like the returned frequency he can request a different one. And he's eventually happy with the return value he calls clk_set_rate() requesting the frequency clk_round_rate() returned.
Always rounding down seems a bit odd to me.

Another issue with the current implmentation:
clk_divider_round_rate() calls clk_divider_bestdiv(), which uses the ROUND_UP macro, returning a rather low frequency.
clk_divider_set_rate() does plain integer divisions. IMHO, that should cause the actually set frequency to differ from what round_rate() returns, and in that case it is even higher than the expected.

> Care to take a crack at that approach?
If a new flag is the preferred solution, I'll sure do that.

        Sören

>
> Regards,
> Mike
>
> >         Regards,
> >         Sören
> >
> >  drivers/clk/clk-divider.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
> > index a9204c6..32e1b6a 100644
> > --- a/drivers/clk/clk-divider.c
> > +++ b/drivers/clk/clk-divider.c
> > @@ -157,7 +157,7 @@ static int clk_divider_bestdiv(struct clk_hw *hw, unsigned long rate,
> >
> >         if (!(__clk_get_flags(hw->clk) & CLK_SET_RATE_PARENT)) {
> >                 parent_rate = *best_parent_rate;
> > -               bestdiv = DIV_ROUND_UP(parent_rate, rate);
> > +               bestdiv = DIV_ROUND_CLOSEST(parent_rate, rate);
> >                 bestdiv = bestdiv == 0 ? 1 : bestdiv;
> >                 bestdiv = bestdiv > maxdiv ? maxdiv : bestdiv;
> >                 return bestdiv;
> > @@ -207,7 +207,7 @@ 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_CLOSEST(parent_rate, rate);
> >         value = _get_val(divider, div);
> >
> >         if (value > div_mask(divider))
> > --
> > 1.8.1.2
>


This email and any attachments are intended for the sole use of the named recipient(s) and contain(s) confidential information that may be proprietary, privileged or copyrighted under applicable law. If you are not the intended recipient, do not read, copy, or forward this email message or any attachments. Delete this email message and any attachments immediately.