linux-kernel - Re: linux-4.4 bisected: kwin5 stuck on kde5 loading screen with radeon

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Date:	Thu, 21 Jan 2016 09:28:34 +0100
From:	Mario Kleiner <mario.kleiner.de@...il.com>
To:	Michel Dänzer <michel@...nzer.net>,
	Vlastimil Babka <vbabka@...e.cz>,
	Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:	Daniel Vetter <daniel.vetter@...ll.ch>,
	LKML <linux-kernel@...r.kernel.org>,
	dri-devel@...ts.freedesktop.org, mgraesslin@....org, kwin@....org,
	Alex Deucher <alexander.deucher@....com>,
	Christian König <christian.koenig@....com>
Subject: Re: linux-4.4 bisected: kwin5 stuck on kde5 loading screen with
 radeon

On 01/21/2016 07:38 AM, Michel Dänzer wrote:
> On 21.01.2016 14:31, Mario Kleiner wrote:
>> On 01/21/2016 04:43 AM, Michel Dänzer wrote:
>>> On 21.01.2016 05:32, Mario Kleiner wrote:
>>>>
>>>> So the problem is that AMDs hardware frame counters reset to
>>>> zero during a modeset. The old DRM code dealt with drivers doing that by
>>>> keeping vblank irqs enabled during modesets and incrementing vblank
>>>> count by one during each vblank irq, i think that's what
>>>> drm_vblank_pre_modeset() and drm_vblank_post_modeset() were meant for.
>>>
>>> Right, looks like there's been a regression breaking this. I suspect the
>>> problem is that vblank->last isn't getting updated from
>>> drm_vblank_post_modeset. Not sure which change broke that though, or how
>>> to fix it. Ville?
>>>
>>
>> The whole logic has changed and the software counter updates are now
>> driven all the time by the hw counter.
>>
>>>
>>> BTW, I'm seeing a similar issue with drm_vblank_on/off as well, which
>>> exposed the bug fixed by 209e4dbc ("drm/vblank: Use u32 consistently for
>>> vblank counters"). I've been meaning to track that down since then; one
>>> of these days hopefully, but if anybody has any ideas offhand...
>>
>> I spent the last few hours reading through the drm and radeon code and i
>> think what should probably work is to replace the
>> drm_vblank_pre/post_modeset calls in radeon/amdgpu by drm_vblank_off/on
>> calls. These are apparently meant for drivers whose hw counters reset
>> during modeset, [...]
>
> ... just like drm_vblank_pre/post_modeset. That those were broken is a
> regression which needs to be fixed anyway. I don't think switching to
> drm_vblank_on/off is suitable for stable trees.
>
> Looking at Vlastimil's original post again, I'd say the most likely
> culprit is 4dfd6486 ("drm: Use vblank timestamps to guesstimate how many
> vblanks were missed").
>

Yes, i think reverting that one alone would likely fix it by reverting 
to the old vblank update logic.

>
>> Once drm_vblank_off is called, drm_vblank_get will no-op and return an
>> error, so clients can't enable vblank irqs during the modeset - pageflip
>> ioctl and waitvblank ioctl would fail while a modeset happens -
>> hopefully userspace handles this correctly everywhere.
>
> We've fixed xf86-video-ati for this.
>
>
>> I'll hack up a patch for demonstration now.
>
> You're a bit late to that party. :)
>
> http://lists.freedesktop.org/archives/dri-devel/2015-May/083614.html
> http://lists.freedesktop.org/archives/dri-devel/2015-July/086451.html
>
>

Oops. Just sent out my little (so far untested) creations. Yes, they are 
essentially the same as Daniel's patches. The only addition is to also 
fix that other potential small race i describe by slightly moving the 
xxx_pm_compute_clocks() calls around. And a fix for drm_vblank_get/put 
imbalance in radeon_pm if vblank_on/off would be used.

-mario