[PATCH 2/3] drm: Warn if vblank state has become inconsistent.

[Mario Kleiner]

On Apr 28, 2011, at 9:47 AM, Christopher James Halse Rogers wrote:

> On Wed, 2011-04-27 at 17:50 +0200, Mario Kleiner wrote:
>> On Apr 27, 2011, at 10:58 AM, dri-devel-request at lists.freedesktop.org
>> wrote:
>>> Message: 5
>>> Date: Wed, 27 Apr 2011 10:38:14 +0200
>>> From: Michel D?nzer <michel at daenzer.net>
>>> Subject: Re: [PATCH 2/3] drm: Warn if vblank state has become
>>> 	inconsistent.
>>> To: christopher.halse.rogers at canonical.com
>>> Cc: dri-devel at lists.freedesktop.org
>>> Message-ID: <1303893494.5633.129.camel at thor.local>
>>> Content-Type: text/plain; charset="UTF-8"
>>>
>>> On Mit, 2011-04-27 at 16:10 +1000,
>>> christopher.halse.rogers at canonical.com wrote:
>>>> From: Christopher James Halse Rogers
>>>> <christopher.halse.rogers at canonical.com>
>>>>
>>>> After emitting all the waiting vblank events no-one should hold
>>>> a vblank reference.  Emit a warning if this is not the case.
>>>>
>>>> Signed-off-by: Christopher James Halse Rogers
>>>> <christopher.halse.rogers at canonical.com>
>>>> ---
>>>>  drivers/gpu/drm/drm_irq.c |    1 +
>>>>  1 files changed, 1 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
>>>> index a1f12cb..72407fa 100644
>>>> --- a/drivers/gpu/drm/drm_irq.c
>>>> +++ b/drivers/gpu/drm/drm_irq.c
>>>> @@ -960,6 +960,7 @@ void drm_vblank_off(struct drm_device *dev,
>>>> int crtc)
>>>>  						 e->event.sequence);
>>>>  	}
>>>>
>>>> +	WARN_ON(atomic_read(&dev->vblank_refcount[crtc]) != 0);
>>>>  	spin_unlock_irqrestore(&dev->vbl_lock, irqflags);
>>>>  }
>>>>  EXPORT_SYMBOL(drm_vblank_off);
>>>
>>> Reviewed-by: Michel D?nzer <michel at daenzer.net>
>>>
>>
>>
>> Any pending kms pageflip will also hold a reference on the vblank of
>> a crtc, so having the refcount non-zero there is not really a sign of
>> inconsistency, so i'm not sure if a warning is appropriate there.
>
> But at this point the vblank interrupts have been disabled, or at  
> least
> the driver's disable function has been called.  Will that not mean  
> that
> any pending pageflips will wait indefinitely for a vblank interrupt
> that's not going to come - ie: exactly the state we're warning  
> against?
>


Ok, if that is the state we're warning against, then the warning  
makes sense. But it would make more sense to somehow handle this more  
gracefully. The pageflip completion path does depend on vblank irq's  
to unpin buffers, release fences and notify user apps of swap  
completion. Hanging there due to disabled vblank irq sounds bad.

Otoh the only caller of drm_vblank_off() i can find in 2.6.38 is the  
crtc disable code in intel_display.c and those routines protect  
against pending pageflips - they wait for all pageflips to finish  
before calling drm_vblank_off().

If this warning is just a precaution against possible future bugs  
then i happily rest my case.
-mario