Remi Pommarel writes: > Hi, > > On Wed, Nov 18, 2015 at 10:25:45AM -0800, Eric Anholt wrote: >> Remi Pommarel 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 >> > --- >> > 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. Oh, that was an existing bug you were fixing! You're right. I even got the GENMASK call right at the end of the function, I'd just botched this one. Reviewed-by: Eric Anholt