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SubjectRe: [PATCH v2 1/3] clk: bcm2835: Always round up clock divisor
Hi,

On Wed, Nov 18, 2015 at 10:25:45AM -0800, Eric Anholt wrote:
> Remi Pommarel <repk@triplefau.lt> writes:
>
> > Make bcm2835_clock_choose_div always round up the chosen MASH divisor so that
> > the resulting average rate will not be higher than the requested one.
> >
> > Signed-off-by: Remi Pommarel <repk@triplefau.lt>
> > ---
> > drivers/clk/bcm/clk-bcm2835.c | 15 ++++++++-------
> > 1 file changed, 8 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
> > index 39bf582..1237716 100644
> > --- a/drivers/clk/bcm/clk-bcm2835.c
> > +++ b/drivers/clk/bcm/clk-bcm2835.c
> > @@ -1152,18 +1152,19 @@ static u32 bcm2835_clock_choose_div(struct clk_hw *hw,
> > {
> > struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
> > const struct bcm2835_clock_data *data = clock->data;
> > - u32 unused_frac_mask = GENMASK(CM_DIV_FRAC_BITS - data->frac_bits, 0);
> > + u32 unused_frac_mask =
> > + GENMASK(CM_DIV_FRAC_BITS - data->frac_bits, 0) >> 1;
> > u64 temp = (u64)parent_rate << CM_DIV_FRAC_BITS;
> > + u64 rem;
> > u32 div;
> >
> > - do_div(temp, rate);
> > + rem = do_div(temp, rate);
> > div = temp;
> >
> > - /* Round and mask off the unused bits */
> > - if (unused_frac_mask != 0) {
> > - div += unused_frac_mask >> 1;
> > - div &= ~unused_frac_mask;
> > - }
> > + /* Round up and mask off the unused bits */
> > + if ((div & unused_frac_mask) != 0 || rem != 0)
> > + div += unused_frac_mask + 1;
> > + div &= ~unused_frac_mask;
>
> Suppose we've got 8 of our 12 frac bits populated. You've added a ">>
> 1" to the unused_frac_mask, so it's only 0x7 instead of 0xf. When you
> say "round up", you add 0x8 (the high bit of the unused mask") then and
> with ~0x7. If you started with 0x1 in the low bits of div, you'd end up
> with 0x8, so you've set an unused bit instead of actually rounding up.
>
> Did my logic work, here? I think you just want to drop the ">>1" in
> unused_frac_mask.

I don't think so.

If we have 8 of our 12 frac bits populated GENMASK(12 - 8, 0) will be 0x1f
because GENMASK(4, 0) generates a mask from bit at position 0 to bit at
position 4 inclusively (which is the fifth bit). So GENMASK(4, 0) >> 1 will
be 0xf which is what we want here.

Thanks,

--
Remi Pommarel


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