linux-kernel - Re: [PATCH 3/3] clk: meson: clk-pll: always enable a critical PLL when setting the rate

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Message-Id: <20190919170610.541D620644@mail.kernel.org>
Date:   Thu, 19 Sep 2019 10:06:09 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Jerome Brunet <jbrunet@...libre.com>,
        Neil Armstrong <narmstrong@...libre.com>
Cc:     Neil Armstrong <narmstrong@...libre.com>,
        linux-clk@...r.kernel.org, linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] clk: meson: clk-pll: always enable a critical PLL when setting the rate

Quoting Jerome Brunet (2019-09-19 06:01:28)
> On Thu 19 Sep 2019 at 11:38, Neil Armstrong <narmstrong@...libre.com> wrote:
> 
> > Make sure we always enable a PLL on a set_rate() when the PLL is
> > flagged as critical.
> >
> > This fixes the case when the Amlogic G12A SYS_PLL gets disabled by the
> > PSCI firmware when resuming from suspend-to-memory, in the case
> > where the CPU was not clocked by the SYS_PLL, but by the fixed PLL
> > fixed divisors.
> > In this particular case, when changing the PLL rate, CCF doesn't handle
> > the fact the PLL could have been disabled in the meantime and set_rate()
> > only changes the rate and never enables it again.
> >
> > Fixes: d6e81845b7d9 ("clk: meson: clk-pll: check if the clock is already enabled')
> > Signed-off-by: Neil Armstrong <narmstrong@...libre.com>
> > ---
> >  drivers/clk/meson/clk-pll.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/clk/meson/clk-pll.c b/drivers/clk/meson/clk-pll.c
> > index ddb1e5634739..8c5adccb7959 100644
> > --- a/drivers/clk/meson/clk-pll.c
> > +++ b/drivers/clk/meson/clk-pll.c
> > @@ -379,7 +379,7 @@ static int meson_clk_pll_set_rate(struct clk_hw *hw, unsigned long rate,
> >       }
> >  
> >       /* If the pll is stopped, bail out now */
> > -     if (!enabled)
> > +     if (!(hw->init->flags & CLK_IS_CRITICAL) && !enabled)
> 
> This is surely a work around to the issue at hand but:
> 
> * Enabling the clock, critical or not, should not be done but the
> set_rate() callback. This is not the purpose of this callback.
> 
> * Enabling the clock in such way does not walk the tree. So, if there is
> ever another PSCI Fw which disable we would get into the same issue
> again. IOW, This is not specific to the PLL driver so it should not have
> to deal with this.

Exactly.

> 
> Since this clock can change out of CCF maybe it should be marked with
> CLK_GET_RATE_NOCACHE ?

Yes, or figure out a way to make the clk state match what PSCI leaves it
in on resume from suspend.


> 
> When CCF hits a clock with CLK_GET_RATE_NOCACHE while walking the tree,
> in addition to to calling get_rate(), CCF could also call is_enabled()
> if the clock has CLK_IS_CRITICAL and possibly .enable() ?

This logic should go under a new flag. The CLK_GET_RATE_NOCACHE flag
specifically means get rate shouldn't be a cached operation. It doesn't
relate to the enable state. I hope that you can implement some sort of
resume hook that synchronizes the state though so that you don't need to
rely on clk_set_rate() or clk_get_rate() to trigger a sync.