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

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Date:	Wed, 18 Nov 2015 20:11:41 +0100
From:	Remi Pommarel <repk@...plefau.lt>
To:	Eric Anholt <eric@...olt.net>
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

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.

Thanks,

-- 
Remi Pommarel
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