linux-kernel - Re: [PATCH RFC] clk: divider: Tolerate 0 divider for one based dividers

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Message-ID: <c9f78c47-4926-4233-8cc3-51e0a4cc9c5e@AM1EHSMHS003.ehs.local>
Date:	Fri, 22 Mar 2013 11:00:15 -0700
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Mike Turquette <mturquette@...aro.org>
CC:	Shawn Guo <shawn.guo@...aro.org>, Rajendra Nayak <rnayak@...com>,
	Andrew Lunn <andrew@...n.ch>,
	James Hogan <james.hogan@...tec.com>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] clk: divider: Tolerate 0 divider for one based
 dividers

Hi Mike,

On Fri, Mar 22, 2013 at 10:11:41AM -0700, Mike Turquette wrote:
> Quoting Soren Brinkmann (2013-03-11 14:13:37)
> > Handle a zero divider value as one/bypass for dividers which have the
> > CLK_DIVIDER_ONE_BASED flag set.
> > 
> > Signed-off-by: Soren Brinkmann <soren.brinkmann@...inx.com>
> > ---
> > In Zynq we have a lot of dividers which are one based, but at the same time
> > zero is a valid value which is handled as one/bypass. Also, the reset value of
> > some of these registers is zero, resulting in warnings when the clock framework
> > encounters this.
> > 
> > So, my question here is: Are our dividers odd? Does it make sense to allow zero
> > for all one based dividers, as shown in this patch? Or does this behavior
> > qualify for another flag for the divider clocks (e.g. CLK_DIVIDER_ZERO_OKAY)?
> > 
> 
> Everyone's dividers are odd ;)
> 
> For the case where a bitfield value of zero or one translates into
> divide-by-one, then I think a flag can be introduced.
> 
> But handling bypass and reset is likely to not be as generic or common
> across implementation, so you might need your own custom clk_hw_foo for
> that.
What I mean is that 0 corresponds to not modify the input frequency.
Whether hardware realizes this through bypass or divide by 1 is an irrelevant
implementation detail. I don't want to handle reset. Just avoid the warning
about a zero divider in case the CLK_DIVIDER_ONE_BASED flag is set.

> 
> The basic clk_divider type is meant to do exactly what the name says:
> divide an incoming clock rate.  If your clock does more than that then
> you might need to cook up your own clock type.
> 
> >         Thanks,
> >         Sören
> > 
> >  drivers/clk/clk-divider.c    | 5 +++--
> >  include/linux/clk-provider.h | 4 ++--
> >  2 files changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
> > index 68b4021..6c2a431 100644
> > --- a/drivers/clk/clk-divider.c
> > +++ b/drivers/clk/clk-divider.c
> > @@ -109,8 +109,9 @@ static unsigned long clk_divider_recalc_rate(struct clk_hw *hw,
> >  
> >         div = _get_div(divider, val);
> >         if (!div) {
> > -               WARN(1, "%s: Invalid divisor for clock %s\n", __func__,
> > -                                               __clk_get_name(hw->clk));
> > +               WARN(!(divider->flags & CLK_DIVIDER_ONE_BASED),
> > +                               "%s: Invalid divisor for clock %s\n", __func__,
> > +                               __clk_get_name(hw->clk));
> 
> div in this code is not the value programmed into the divider, but the
> numerical divider applied to the parent clock rate.  Thus dividing by
> zero never makes sense here.
Right, and for dividers which have the ONE_BASED flags set, this value
is just the register value read from HW. Our HW, allows the register
to be 0 and the resulting output frequency is its input frequency - just
as if it was set to 1.
Since some of our dividers have a reset value of 0, I see this warning
during boot up. My intention here is to get rid of the warning for clocks
which deem 0 a valid divider value.
I thought, reusing the ONE_BASED flag might be okay, since all such
dividers have this redundant 0 state and might handle it similar.
Otherwise a new flag might be required.

	Sören


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