Message-ID: <56A64F86.9080605@gmail.com>
Date:	Mon, 25 Jan 2016 17:38:30 +0100
From:	Mario Kleiner <mario.kleiner.de@...il.com>
To:	Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:	Michel Dänzer <michel@...nzer.net>,
	Alex Deucher <alexander.deucher@....com>,
	Vlastimil Babka <vbabka@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>,
	dri-devel@...ts.freedesktop.org,
	Christian König <christian.koenig@....com>,
	Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: linux-4.4 bisected: kwin5 stuck on kde5 loading screen with
 radeon

Readding Daniel, which somehow got dropped from the cc.

On 01/25/2016 03:53 PM, Ville Syrjälä wrote:
> On Mon, Jan 25, 2016 at 02:44:53PM +0100, Mario Kleiner wrote:
>>
>>
>> On 01/25/2016 02:23 PM, Ville Syrjälä wrote:
>>> On Mon, Jan 25, 2016 at 02:16:45PM +0100, Mario Kleiner wrote:
>>>>
>>>>
>>>> On 01/25/2016 05:15 AM, Michel Dänzer wrote:
>>>>> On 23.01.2016 00:18, Ville Syrjälä wrote:
>>>>>> On Fri, Jan 22, 2016 at 12:06:00PM +0900, Michel Dänzer wrote:
>>>>>>>
>>>>>>> [ Trimming KDE folks from Cc ]
>>>>>>>
>>>>>>> On 21.01.2016 19:09, Daniel Vetter wrote:
>>>>>>>> On Thu, Jan 21, 2016 at 05:36:46PM +0900, Michel Dänzer wrote:
>>>>>>>>> On 21.01.2016 16:58, Daniel Vetter wrote:
>>>>>>>>>>
>>>>>>>>>> Can you please point me at the vblank on/off jump bug please?
>>>>>>>>>
>>>>>>>>> AFAIR I originally reported it in response to
>>>>>>>>> http://lists.freedesktop.org/archives/dri-devel/2015-August/087841.html
>>>>>>>>> , but I can't find that in the archives, so maybe that was just on IRC.
>>>>>>>>> See
>>>>>>>>> http://lists.freedesktop.org/archives/dri-devel/2016-January/099122.html
>>>>>>>>> . Basically, I ran into the bug fixed by your patch because the counter
>>>>>>>>> jumped forward on every DPMS off, so it hit the 32-bit boundary after
>>>>>>>>> just a few days.
>>>>>>>>
>>>>>>>> Ok, so just uncovered the overflow bug.
>>>>>>>
>>>>>>> Not sure what you mean by "just", but to be clear: The drm_vblank_on/off
>>>>>>> counter jumping bug (similar to the bug this thread is about), which
>>>>>>> exposed the overflow bug, is still alive and kicking in 4.5. It seems
>>>>>>> to happen when turning off the CRTC:
>>>>>>>
>>>>>>> [drm:drm_update_vblank_count] updating vblank count on crtc 0: current=218104694, diff=0, hw=916 hw_last=916
>>>>>>> [drm:radeon_get_vblank_counter_kms] crtc 0: dist from vblank start 3
>>>>>>> [drm:drm_calc_vbltimestamp_from_scanoutpos] crtc 0 : v 0x7 p(2199,-45)@ 7304.307354 -> 7304.308006 [e 0 us, 0 rep]
>>>>>>> [drm:radeon_get_vblank_counter_kms] crtc 0: dist from vblank start 3
>>>>>>> [drm:drm_update_vblank_count] updating vblank count on crtc 0: current=218104694, diff=16776301, hw=1 hw_last=916
>>>>>>
>>>>>> Not sure what bug we're talking about here, but here the hw counter
>>>>>> clearly jumps backwards.
>>>>>>
>>>>>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>>>>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>>>>>> [drm:drm_update_vblank_count] updating vblank count on crtc 1: current=0, diff=0, hw=0 hw_last=0
>>>>>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>>>>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>>>>>> [drm:drm_update_vblank_count] updating vblank count on crtc 2: current=0, diff=0, hw=0 hw_last=0
>>>>>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>>>>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 3
>>>>>>> [drm:drm_update_vblank_count] updating vblank count on crtc 3: current=0, diff=0, hw=0 hw_last=0
>>>>>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 1
>>>>>>> [drm:drm_calc_vbltimestamp_from_scanoutpos] crtc 0 : v 0x1 p(0,0)@ 7304.317140 -> 7304.317140 [e 0 us, 0 rep]
>>>>>>> [drm:radeon_get_vblank_counter_kms] Query failed! stat 1
>>>>>>> [drm:drm_update_vblank_count] updating vblank count on crtc 0: current=234880995, diff=16777215, hw=0 hw_last=1
>>>>>>
>>>>>> Same here.
>>>>>
>>>>> At least one of the jumps is expected, because this is around turning
>>>>> off the CRTC for DPMS off. Don't know yet why there are two jumps back
>>>>> though.
>>>>>
>>>>>
>>>>>> These things just don't happen on i915 because drm_vblank_off() and
>>>>>> drm_vblank_on() are always called around the times when the hw counter
>>>>>> might get reset. Or at least that's how it should be.
>>>>>
>>>>> Which is of course the idea of Daniel's patch (which is what I'm getting
>>>>> the above with) or Mario's patch as well, but clearly something's still
>>>>> wrong. It's certainly possible that it's something in the driver, but
>>>>> since calling drm_vblank_pre/post_modeset from the same places seems to
>>>>> work fine (ignoring the regression discussed in this thread)... Do
>>>>> drm_vblank_on/off require something else to handle this correctly?
>>>>>
>>>>>
>>>>
>>>> I suspect it is because vblank_disable_and_save calls
>>>> drm_update_vblank_count() unconditionally, even if vblank irqs are
>>>> already off.
>>>>
>>>> So on a manual display disable -> reenable you get something like
>>>>
>>>> At disable:
>>>>
>>>> Call to dpms-off --> atombios_crtc_dpms(DPMS_OFF) --> drm_vblank_off ->
>>>> vblank_disable_and_save -> irqs off, drm_update_vblank_count() computes
>>>> final count.
>>>>
>>>> Then the crtc is shut down and its hw counter resets to zero.
>>>>
>>>> At reenable:
>>>>
>>>> Modesetting -> drm_crtc_helper_set_mode -> crtc_funcs->prepare(crtc) ->
>>>> atombios_crtc_prepare() -> atombios_crtc_dpms(DPMS_OFF) ->
>>>> drm_vblank_off -> vblank_disable_and_save -> A pointless
>>>> drm_update_vblank_count() while the hw counter is already reset to zero
>>>> --> Unwanted counter jump.
>>>>
>>>>
>>>> The problem doesn't happen on a pure modeset to a different video
>>>> resolution/refresh rate, as then we only have one call into
>>>> atombios_crtc_dpms(DPMS_OFF).
>>>>
>>>> I think the fix is to fix vblank_disable_and_save() to only call
>>>> drm_update_vblank_count() if vblank irqs get actually disabled, not on
>>>> no-op calls. I will try that now.
>>>
>>> It does that on purpose. Otherwise the vblank counter would appear to
>>> have stalled while the interrupt was off.
>>>
>>
>> Ok, that's what the comments there say, although i don't see atm. why
>> that perceived stall would be a big problem. I checked all callers of
>> vblank_disable_and_save(). They are all careful to not call that
>> function if vblanks are already disabled. The only exception is
>> drm_vblank_off(). If drm_vblank_off/on is supposed to protect kms
>> drivers which have resetting hw counters or other problematic behaviour
>> during modesets etc. then this will break. E.g., calling the vblank
>> timestamping stuff is also not safe/well-defined during modesets when
>> the timestamping constants are not (yet) updated to reflect the new mode
>> timing of the modeset in progress.
>
> The idea is to maintain the appearance that the counter ticks all the
> time as long as the crtc is active. While that may not be really
> required in case if no one is currently interested in the vblank
> counter, I think it's a nice thing to have just to make the behaviour
> of the counter consistent.
>
> As far as calling drm_vblank_off() after the hw counter got reset, well,
> that not correct. It should be called before the reset.

What radeon does is calling drm_vblank_off at beginning of DPMS_OFF. The 
first call to DMPS_OFF will call drm_vblank_off() and really disable 
vblank-irqs if they were running, updating the counts/ts a last time. 
But then the dpms off will reset the hw counter to zero. When one 
reenables the display, a second call to DPMS_OFF will now call 
drm_vblank_off again when it apparently shouldn't.

I just tested this patch, which fixes the counter jumps on radeon-kms 
with my or Daniel's drm_vblank_off patches to radeon:

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 607f493..d739d93 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -1313,7 +1313,10 @@ void drm_vblank_off(struct drm_device *dev, 
unsigned int pipe)
         spin_lock_irqsave(&dev->event_lock, irqflags);

         spin_lock(&dev->vbl_lock);
-       vblank_disable_and_save(dev, pipe);
+       DRM_DEBUG_VBL("crtc %d, vblank enabled %d\n", pipe, 
vblank->enabled);
+
+       if (vblank->enabled)
+               vblank_disable_and_save(dev, pipe);
         wake_up(&vblank->queue);

         /*
@@ -1415,6 +1418,8 @@ void drm_vblank_on(struct drm_device *dev, 
unsigned int pipe)
                 return;

         spin_lock_irqsave(&dev->vbl_lock, irqflags);
+       DRM_DEBUG_VBL("crtc %d, vblank enabled %d\n", pipe, 
vblank->enabled);
+
         /* Drop our private "prevent drm_vblank_get" refcount */
         if (vblank->inmodeset) {
                 atomic_dec(&vblank->refcount);



Another, maybe better, approach might be to no-op redundant calls to 
drm_vblank_off() iff vblank->inmodeset and no-op redundant calls to 
drm_vblank_on() iff !vblank->inmodeset.

-mario


>
>>
>> -mario
>>
>>
>>>>
>>>> Otherwise kms drivers would have to be careful to never call
>>>> drm_vblank_off multiple times before calling drm_vblank_on, but the help
>>>> text to drm_vblank_on() claims that unbalanced calls to these functions
>>>> are perfectly fine.
>>>>
>>>> -mario
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>

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