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

From: Michel DÃnzer
Date: Thu Jan 21 2016 - 01:39:11 EST

On 21.01.2016 14:31, Mario Kleiner wrote:
> On 01/21/2016 04:43 AM, Michel DÃnzer 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").

> 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. :)

Earthling Michel DÃnzer |
Libre software enthusiast | Mesa and X developer