Re: [PATCH v2 1/3] clk: bcm2835: Always round up clock divisor

Eric Anholt
2015-12-04T00:31:25Z

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Remi Pommarel <repk@triplefau.lt> writes:

> 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.

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 <eric@anholt.net>

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