linux-kernel - Re: [PATCH 2/3] clk: WARN_ON about to disable a critical clock

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Date:   Mon, 3 Jul 2017 12:53:24 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Dirk Behme <dirk.behme@...bosch.com>
Cc:     Michael Turquette <mturquette@...libre.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel@...inux.com, s.hauer@...gutronix.de, sboyd@...eaurora.org,
        geert@...ux-m68k.org, maxime.ripard@...e-electrons.com,
        maxime.coquelin@...com
Subject: Re: [PATCH 2/3] clk: WARN_ON about to disable a critical clock

On Tue, 27 Jun 2017, Dirk Behme wrote:

> On 11.02.2016 01:43, Michael Turquette wrote:
> > Quoting Lee Jones (2016-01-18 06:28:50)
> > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > 
> > Looks good to me.
> > 
> > Regards,
> > Mike
> > 
> > > ---
> > >   drivers/clk/clk.c | 6 ++++++
> > >   1 file changed, 6 insertions(+)
> > > 
> > > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> > > index 835cb85..178b364 100644
> > > --- a/drivers/clk/clk.c
> > > +++ b/drivers/clk/clk.c
> > > @@ -575,6 +575,9 @@ static void clk_core_unprepare(struct clk_core *core)
> > >          if (WARN_ON(core->prepare_count == 0))
> > >                  return;
> > > +       if (WARN_ON(core->prepare_count == 1 && core->flags & CLK_IS_CRITICAL))
> > > +               return;
> > > +
> > >          if (--core->prepare_count > 0)
> > >                  return;
> > > @@ -680,6 +683,9 @@ static void clk_core_disable(struct clk_core *core)
> > >          if (WARN_ON(core->enable_count == 0))
> > >                  return;
> > > +       if (WARN_ON(core->enable_count == 1 && core->flags & CLK_IS_CRITICAL))
> > > +               return;
> > > +
> > >          if (--core->enable_count > 0)
> > >                  return;
> 
> 
> I have a question regarding this patch, which is mainline meanwhile [1]:
> 
> Having the following clock configuration:
> 
>                                         |--> child clk '1' (crit)
> clk source --> parent clk 'A' (crit) -->|
>                                         |--> child clk '2'
> 
> 
> Clock '2' might be used, or not. It might be disabled or not. It doesn't
> matter. Clock '1' is not allowed to be disabled. Therefore its marked as
> critical.
> 
> Parent clock 'A' is marked as critical because its not allowed to be
> disabled, even if the enable_count of all child clocks is 0. To avoid that
> by disabling parent clock 'A' the child clock '1' is disabled, too, whats
> not allowed as its marked as critical.
> 
> 
> Now, child clock '2' is used and enabled & disabled continuously by a (SPI)
> driver. What is ok. But:
> 
> Disabling child clock '2' results in the attempt to disable parent clock
> 'A', too, which has correct enable_count 1 (from enabling the child '2').
> What results
> 
> a) in the WARN_ON output
> 
> and
> 
> b) enable_count of 'A' never decreases to 0. Being off by one after the
> WARN_ON
> 
> 
> It sounds like both is wrong for a configuration like above.

Clock A still has one user, Clock 1.

Why is that wrong?

> Opinions or proposal how to fix/change this?
> 
> 
> Best regards
> 
> Dirk
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/clk/clk.c?id=2e20fbf592621b2c2aeddd82e0fa3dad053cce03

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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