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 09:23:41 +0800
From:	Shawn Guo <shawnguo@...nel.org>
To:	Dong Aisheng <dongas86@...il.com>
Cc:	Stefan Agner <stefan@...er.ch>, kernel@...gutronix.de,
	mturquette@...libre.com, sboyd@...eaurora.org,
	linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
	linux-kernel@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH 1/2] clk: imx: do not sleep if IRQ's are still disabled

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 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?

> Furthermore, shouldn't it be udelay(500)?

500 is for the worst case of sleep, and 50 is good enough for delay.

Shawn