linux-kernel - Re: [PATCH 1/2] clk: imx: do not sleep if IRQ's are still disabled

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Date:	Tue, 26 Apr 2016 11:31:12 +0200
From:	Lucas Stach <l.stach@...gutronix.de>
To:	Dong Aisheng <dongas86@...il.com>
Cc:	Shawn Guo <shawnguo@...nel.org>,
	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stefan Agner <stefan@...er.ch>, mingo@...hat.com,
	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	tglx@...utronix.de, linux-clk@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/2] clk: imx: do not sleep if IRQ's are still disabled

Am Dienstag, den 26.04.2016, 13:51 +0800 schrieb Dong Aisheng:
> Hi Shawn,
> 
> On Tue, Apr 26, 2016 at 9:23 AM, Shawn Guo <shawnguo@...nel.org> wrote:
> > On Thu, Apr 21, 2016 at 11:45:20AM +0800, Dong Aisheng wrote:
> >> On Fri, Jan 29, 2016 at 02:49:23PM -0800, Stefan Agner wrote:
> >> > If a clock gets enabled early during boot time, it can lead to a PLL
> >> > startup. The wait_lock function makes sure that the PLL is really
> >> > stareted up before it gets used. However, the function sleeps which
> >> > leads to scheduling and an error:
> >> > bad: scheduling from the idle thread!
> >> > ...
> >> >
> >> > Use udelay in case IRQ's are still disabled.
> >> >
> >> > Signed-off-by: Stefan Agner <stefan@...er.ch>
> >> > ---
> >> >  drivers/clk/imx/clk-pllv3.c | 5 ++++-
> >> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >> >
> >> > diff --git a/drivers/clk/imx/clk-pllv3.c b/drivers/clk/imx/clk-pllv3.c
> >> > index c05c43d..b5ff561 100644
> >> > --- a/drivers/clk/imx/clk-pllv3.c
> >> > +++ b/drivers/clk/imx/clk-pllv3.c
> >> > @@ -63,7 +63,10 @@ static int clk_pllv3_wait_lock(struct clk_pllv3 *pll)
> >> >                     break;
> >> >             if (time_after(jiffies, timeout))
> >> >                     break;
> >> > -           usleep_range(50, 500);
> >> > +           if (unlikely(irqs_disabled()))
> >>
> >> This causes a bit confusion that clk_pllv3_prepare is sleepable.
> >> But this line indicates it's possible to be called in irq context.
> >> Although it's only happened during kernel boot phase where irq is
> >> still not enabled.
> >> It seems schedule_debug() somehow did not catch it during early boot
> >> phase. Maybe schedule guys can help explain.
> >>
> >> My question is if it's really worthy to introduce this confusion
> >> to fix the issue since the delay is minor?
> >
> > I do not understand why it's confusing.  The code already obviously
> > indicates this is a special handling for cases where irq is still not
> > enabled, rather than for irq context.
> >
> 
> The code itself has nothing telling it's a special handling for the
> case where irq is
> still not enabled.
> Even it tells, it may still cause confusing by adding complexity in
> clk_pllv3_prepare()
> which actually should be called in non-atomic context as it could sleep.
> 
> > The patch is to fix the "bad: scheduling from the idle thread!" warning
> > rather than minimize the delay.  Do you have an opinion on how to fix
> > the warning?
> >
> 
> I just wonder maybe we could simply just using udelay(50) instead of
> usleep_range(50, 500) to eliminate the confusing since it's minor cast.
> What do you think of it?

Why should we needless burn CPU time in the likely case of
clk_pllv3_prepare() being called in sleepable context?

Using udelay() in a sleepable context is even more confusing than having
the special case for clk_prepare() being called in atomic context IMHO.

Regards,
Lucas