Subject: Re: linux-next: build failure after merge of the tip tree

From: "Rafael J. Wysocki" <[email-masked]>
Date: Tue, 14 Jan 2014 23:18:16 +0100

On Tuesday, January 14, 2014 04:52:20 PM Mikulas Patocka wrote:
> 
> On Tue, 14 Jan 2014, Rafael J. Wysocki wrote:
> 
> > On Tuesday, January 14, 2014 04:43:43 PM Mikulas Patocka wrote:
> > > 
> > > On Tue, 14 Jan 2014, Peter Zijlstra wrote:
> > > 
> > > > On Tue, Jan 14, 2014 at 02:06:57PM -0500, Mikulas Patocka wrote:
> > > > > On Tue, 14 Jan 2014, Peter Zijlstra wrote:
> > > > > > > Caused by commit 62b94a08da1b ("sched/preempt: Take away
> > > > > > > preempt_enable_no_resched() from modules")
> > > > 
> > > > Read these two lines, then note that:
> > > > 
> > > > > Try adding #include <linux/preempt.h> to speedstep-lib.c. Does it help?
> > > > 
> > > > this obviously will not work as preempt_check_resched() and
> > > > preempt_enable_no_resched() are no longer available to modules.
> > > 
> > > I see, you added commit 62b94a08da1bae9d187d49dfcd6665af393750f8 to 
> > > linux-next, that broke my patch.
> > > 
> > > > > > I think that pm commit is a very good example of why the sched/preempt
> > > > > > patch is a very good idea.
> > > > > > 
> > > > > > Also that Changelog fails to explain why enabling interrupts helps. What
> > > > > > interrupt is required for progress, and how does it make the progress
> > > > > > happen.
> > > > > 
> > > > > There is no explanation. It's hardware issue and I have no documentation 
> > > > > for the hardware.
> > > > 
> > > > Rafael works for Intel, he ought to be able to figure out wtf the
> > > > hardware does/needs.
> > > > 
> > > > > The general problem is that if there are bus-master transfers running (or 
> > > > > possibly for other hardware reasons), the CPU refuses to change frequency. 
> > > > > You can wait a little bit and retry and maybe you succeed changing the 
> > > > > frequency next time.
> > > > > 
> > > > > If you enable interrupts, wait, disable interrupts and retry, you may 
> > > > > succeed. If you keep interrupts disabled and retry, you never succeed, no 
> > > > > matter how long do you wait. I found it experimentally, I don't know 
> > > > > reason for that.
> > > > 
> > > > Sounds like magic goo..
> > > > 
> > > > In any case, try the below, it does the same but is far less horrid.
> > > > 
> > > > ---
> > > >  drivers/cpufreq/speedstep-smi.c | 4 ++++
> > > >  1 file changed, 4 insertions(+)
> > > > 
> > > > diff --git a/drivers/cpufreq/speedstep-smi.c b/drivers/cpufreq/speedstep-smi.c
> > > > index 0f5326d6f79f..57d31538c248 100644
> > > > --- a/drivers/cpufreq/speedstep-smi.c
> > > > +++ b/drivers/cpufreq/speedstep-smi.c
> > > > @@ -188,6 +188,7 @@ static void speedstep_set_state(unsigned int state)
> > > >  		return;
> > > >  
> > > >  	/* Disable IRQs */
> > > > +	preempt_disable();
> > > >  	local_irq_save(flags);
> > > >  
> > > >  	command = (smi_sig & 0xffffff00) | (smi_cmd & 0xff);
> > > > @@ -200,7 +201,9 @@ static void speedstep_set_state(unsigned int state)
> > > >  		if (retry) {
> > > >  			pr_debug("retry %u, previous result %u, waiting...\n",
> > > >  					retry, result);
> > > > +			local_irq_restore(flags);
> > > 
> > > ^^^ this is wrong, because the function speedstep_set_state may already be 
> > > called with interrupts disabled from speedstep_get_freqs. So, you need to 
> > > enable interrupts unconditionally, even if they were disabled at the 
> > > beginning of the function speedstep_set_state.
> > > 
> > > I know it's dirty to enable interrupts in a function that was called with 
> > > disabled interrupts, but here it must be so (you could rewrite 
> > > speedstep_get_freqs to not disable interrupts if you want to avoid this 
> > > dirtiness).
> > > 
> > > >  			mdelay(retry * 50);
> > > > +			local_irq_save(flags);
> > > >  		}
> > > >  		retry++;
> > > >  		__asm__ __volatile__(
> > > > @@ -217,6 +220,7 @@ static void speedstep_set_state(unsigned int state)
> > > >  
> > > >  	/* enable IRQs */
> > > >  	local_irq_restore(flags);
> > > > +	preempt_enable();
> > > >  
> > > >  	if (new_state == state)
> > > >  		pr_debug("change to %u MHz succeeded after %u tries "
> > > 
> > > You need also preempt_disable/enable in speedstep_get_freqs.
> > > 
> > > 
> > > Here I'm resending the patch, to account for 
> > > 62b94a08da1bae9d187d49dfcd6665af393750f8.
> > 
> > Do I think correctly that this should work regardless of whether or not
> > 62b94a08da1bae9d187d49dfcd6665af393750f8 is applied?
> 
> Yes.

OK

I'll replace your original patch with this version, then.

Thanks!

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.