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 13:51:13 +0800
From:	Dong Aisheng <dongas86@...il.com>
To:	Shawn Guo <shawnguo@...nel.org>
Cc:	Stefan Agner <stefan@...er.ch>,
	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, linux-clk@...r.kernel.org,
	"linux-kernel@...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

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?

>> Furthermore, shouldn't it be udelay(500)?
>
> 500 is for the worst case of sleep, and 50 is good enough for delay.
>

Yes, you''re right.
We have a loop, so 50ns one time should be good.

> Shawn

Regards
Dong Aisheng