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

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Message-ID: <20170703142517.tyojzmzofvyiicsn@dell>
Date:   Mon, 3 Jul 2017 15:25:17 +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 Mon, 03 Jul 2017, Dirk Behme wrote:

> On 03.07.2017 13:53, Lee Jones wrote:
> > 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?
> 
> 
> Because clock 1 is not a (Linux kernel clock framework) used clock. Its
> enable count is 0. So from Linux kernel (clock framework) point of view
> clock 1 is unused.

All critical clocks are 'used'.  That's the point of critical clocks.

> The increase/decrease of enable count of parent clock A is only driven by
> the Linux kernel usage of clock 2.
> 
> > > 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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