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Date:	Fri, 8 Aug 2014 11:37:01 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Chris Wilson <chris@...is-wilson.co.uk>,
	Daniel Vetter <daniel@...ll.ch>, oscar.mateo@...el.com,
	intel-gfx@...ts.freedesktop.org,
	Greg KH <gregkh@...uxfoundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Intel-gfx] [PATCH 45/53] drm/i915/bdw: Do not call
 intel_runtime_pm_get() in an interrupt

On Fri, Aug 08, 2014 at 10:20:40AM +0100, Chris Wilson wrote:
> On Tue, Jul 29, 2014 at 12:26:36PM +0200, Daniel Vetter wrote:
> > On Tue, Jul 29, 2014 at 08:37:48AM +0100, Chris Wilson wrote:
> > > On Mon, Jul 28, 2014 at 10:54:06AM +0200, Daniel Vetter wrote:
> > > > On Sat, Jul 26, 2014 at 11:27:38AM +0100, Chris Wilson wrote:
> > > > > On Wed, Jun 18, 2014 at 10:54:13PM +0200, Daniel Vetter wrote:
> > > > > > On Fri, Jun 13, 2014 at 04:38:03PM +0100, oscar.mateo@...el.com wrote:
> > > > > > > From: Oscar Mateo <oscar.mateo@...el.com>
> > > > > > > 
> > > > > > > Or with a spinlock grabbed, because it might sleep, which is not
> > > > > > > a nice thing to do. Instead, do the runtime_pm get/put together
> > > > > > > with the create/destroy request, and handle the forcewake get/put
> > > > > > > directly.
> > > > > > > 
> > > > > > > Signed-off-by: Oscar Mateo <oscar.mateo@...el.com>
> > > > > > 
> > > > > > Looks like a fixup that should be squashed into relevant earlier patches.
> > > > > 
> > > > > The whole gen6_gt_force_wake_get() calling intel_runtime_pm_get() is
> > > > > broken due to this - we must be able to read registers in atomic
> > > > > context!
> > > > > 
> > > > > Please revert c8c8fb33b37766acf6474784b0d5245dab9a1690
> > > > 
> > > > force_wake_get can't call runtime_pm_get becuase pm_get can sleep. So if
> > > > you want to read registers from atomic context you have to have a runtime
> > > > pm reference from someone else.
> > > 
> > > Nope. That cannot work.
> > 
> > Well it works currently. So where do you see the problem?
> 
> Sampling registers from an timer - in particular, we really do not want
> to disable runtime pm whilst trying to monitor the impact of runtime pm.

In that case you can grab a runtime pm reference iff the device is powered
on already. Which won't call anything scary, just amounts to an
atomic_add_unless or so, and then drop it again. 

Unfortunately there doesn't seem to be such a thing around already, so
need to add it first. Greg, how much would you freak out if we add
something like

/**
 * pm_runtime_get_unless_suspended - grab a rpm ref if the device is on
 * 
 * Returns true if an rpm ref has been acquire, false otherwise. Can be
 * called from atomic context to e.g. sample perfomance counters (where we
 * obviously don't want to disturb system state if everything is off atm).
 */
static inline bool pm_runtime_get_unless_suspended(struct device *dev)
{
	return atomic_add_unless(&dev->power.usage_count, 1, 0);
}

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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