Re: [PATCH 1/5] clk: zynqmp: fix check for fractional clock

[Stephen Boyd <sboyd@kernel.org>]

Quoting Michael Tretter (2019-03-12 10:25:46)
> On Tue, 12 Mar 2019 09:49:21 -0700, Stephen Boyd wrote:
> > Quoting Michael Tretter (2019-03-12 04:00:12)
> > > CLK_FRAC is not set in the divider->flags, but in the hw->flags.
> > > 
> > > The firmware sets CLK_FRAC for fractional clocks in the clkflag field.
> > > When registering the devider, these clkflags are copied to hw->flags.
> > > 
> > > Moreover, divider->flags field is a u8 type, but CLK_FRAG is BIT(13). So
> > > this check would never work.
> > > 
> > > Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
> > > ---
> > >  drivers/clk/zynqmp/divider.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/clk/zynqmp/divider.c b/drivers/clk/zynqmp/divider.c
> > > index a371c66e72ef..fc70950c1e24 100644
> > > --- a/drivers/clk/zynqmp/divider.c
> > > +++ b/drivers/clk/zynqmp/divider.c
> > > @@ -117,7 +117,7 @@ static long zynqmp_clk_divider_round_rate(struct clk_hw *hw,
> > >         bestdiv = zynqmp_divider_get_val(*prate, rate);
> > >  
> > >         if ((clk_hw_get_flags(hw) & CLK_SET_RATE_PARENT) &&
> > > -           (divider->flags & CLK_FRAC))
> > > +           (clk_hw_get_flags(hw) & CLK_FRAC))  
> > 
> > CLK_FRAC shouldn't be set in the struct clk_hw::core::flags field. It's
> > not a clk framework flag so it shouldn't go there. Please fix the user
> > of this flag to place the CLK_FRAC flag somewhere else. Even adding it
> > into divider::flags is not a good idea because that numberspace is for
> > dividers, and this flag seems to be zynqmp driver specific, so maybe
> > just add a bool to the zynqmp_clk_divider?
> > 
> 
> Thanks. The driver sets the clk_hw::core::flags based on a response
> from the ATF and this response includes this flag with other clk
> frameworks flags. I can test for the flag when registering the clock
> and set another flag or a bool for the zynqmp_clk_divider and will do
> so in v2.

Cool. Thanks!

> 
> However, this merely sounds like a workaround for an issue in the ATF,
> which should not define and use this flag in the first place. 
> 

What is ATF doing with these flags? Hopefully ATF and the Linux kernel
aren't using the same numberspace to describe these things. For example,
I would be concerned if ATF was looking at the CLK_SET_RATE_PARENT flag
and passing that value from firmware to the kernel, blindly assuming
that the kernel wouldn't change those numbers to be something else.
Obviously that type of kernel change would be invasive but it's not an
ABI that we've ever published so we're free to do these sorts of things.