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

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Date:	Mon, 6 Jun 2016 15:20:03 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dong Aisheng <dongas86@...il.com>
cc:	Shawn Guo <shawnguo@...nel.org>,
	Lucas Stach <l.stach@...gutronix.de>,
	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>,
	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

On Thu, 2 Jun 2016, Dong Aisheng wrote:
> On Wed, Apr 27, 2016 at 12:15:00PM +0200, Thomas Gleixner wrote:
> > Calling a function which might sleep _BEFORE_ kernel_init() is wrong. Don't
> > try to work around such an issue by doing magic irq_disabled() checks and busy
> > loops. Fix the call site and be done with it.
> > 
> 
> IRQ chip and clocksource are also initialised before kernel_init()
> which may call clk APIs like clk_prepare_enable().
> We may not be able to guarantee those clocks are unsleepable.
> 
> e.g.
> For IRQ chip, the arm,gic documents the optional clocks property in
> Documentation/devicetree/bindings/interrupt-controller/arm,gic.txt.
> Although there seems no user currently, not sure there might be
> a exception in the future.
> 
> For clocksource driver, it's quite common to call clk APIs to
> enable and get clock rate which may also cause sleep.
> e.g. ARM twd clock in arch/arm/kernel/smp_twd.c.
> 
> If we have to avoid the possible sleep caused by these clocks,
> we may need to manually check it and change the related clocks
> (e.g PLLs) from sleepable to busy loops.
> But that is not a good solution and may also not stable when
> clock varies.
> 
> It looks like not easy to find a generic solution for them.
> What's your suggestion?

I think we should split the prepare callbacks up and move the handling logic
into the core.

clk_ops.prepare()	Legacy callback
clk_ops.prepare_hw()	Callback which writes to the hardware
clk_ops.prepare_done()	Callback which checks whether the prepare is completed

So now the core can do:

clk_prepare()
{
	/* Legacy code should go away */
	if (ops->prepare) {
	      ops->prepare();
	      return;
	}

	if (ops->prepare_hw)
	      ops->prepare_hw();

   	if (!ops->prepare_done())
		return;

	if (early_boot_or_suspend_resume()) {
		/*
		 * Busy loop when we can't schedule in early boot,
		 * suspend and resume.
		 */
		while (!ops->prepare_done())
		      ;
	} else {
		while (!ops->prepare_done())
		      usleep(clk->prepare_delay);
	}
}

As a side effect that allows us to remove the gazillion of

     	    while (!hw_ready)
	    	  usleep();

copies all over the place and we have a single point of control where we allow
the clocks to busy loop.

Toughts?

Thanks,

	tglx