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

Daniel Vetter daniel at ffwll.ch
Mon Jan 25 10:51:30 PST 2016 

On Mon, Jan 25, 2016 at 05:38:30PM +0100, Mario Kleiner wrote:
> 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:

This might be due to the legacy helpers, which just love to redundantly
disable stuff that's off already. The problem I see with no-oping these
out is that for atomic drivers (which really should get this right) this
might paper over bugs: E.g. when you forget to call _off() when disabling
the crtc, then calling _on() twice in a row is indeed a serious bug.
Similar when you forget to call _on() and have multiple _off() calls in a
row.

So not sure what to do here.
-Daniel

> 
> 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
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>
> >

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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