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

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Date:	Thu, 03 Dec 2015 16:31:25 -0800
From:	Eric Anholt <eric@...olt.net>
To:	Remi Pommarel <repk@...plefau.lt>
Cc:	Stephen Warren <swarren@...dotorg.org>, Lee Jones <lee@...nel.org>,
	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-rpi-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] clk: bcm2835: Always round up clock divisor

Remi Pommarel <repk@...plefau.lt> writes:

> Hi,
>
> On Wed, Nov 18, 2015 at 10:25:45AM -0800, Eric Anholt wrote:
>> Remi Pommarel <repk@...plefau.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@...plefau.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@...olt.net>

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