Re: [PATCH v2 4/4] drm/simple-kms: Let DRM core send VBLANK events by default

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Jan 17, 2020 at 08:17:10AM +0100, Thomas Zimmermann wrote:
> Hi
> 
> Am 17.01.20 um 00:59 schrieb Daniel Vetter:
> > On Thu, Jan 16, 2020 at 05:22:34PM +0000, Emil Velikov wrote:
> >> Hi all,
> >>
> >> On Thu, 16 Jan 2020 at 07:37, Thomas Zimmermann <tzimmermann@suse.de> wrote:
> >>
> >>>> diff --git a/drivers/gpu/drm/drm_atomic_state_helper.c b/drivers/gpu/drm/drm_atomic_state_helper.c
> >>>> index 7cf3cf936547..23d2f51fc1d4 100644
> >>>> --- a/drivers/gpu/drm/drm_atomic_state_helper.c
> >>>> +++ b/drivers/gpu/drm/drm_atomic_state_helper.c
> >>>> @@ -149,6 +149,11 @@ void __drm_atomic_helper_crtc_duplicate_state(struct drm_crtc *crtc,
> >>>>       /* Self refresh should be canceled when a new update is available */
> >>>>       state->active = drm_atomic_crtc_effectively_active(state);
> >>>>       state->self_refresh_active = false;
> >>>> +
> >>>> +     if (drm_dev_has_vblank(crtc->dev))
> >>>> +             state->no_vblank = true;
> >>>> +     else
> >>>> +             state->no_vblank = false;
> >>>>  }
> >>>>  EXPORT_SYMBOL(__drm_atomic_helper_crtc_duplicate_state);
> >>>
> >>> I think the if/else branches are in the wrong order.
> > 
> > Yeah fumbled that.
> > 
> >>> But generally speaking, is it really that easy? The xen driver already
> >>> has to work around simple-kms's auto-enabling of no_vblank (see patch
> >>> 4). Maybe this settings interferes with other drivers as well. At least
> >>> the calls for sending fake vblanks should be removed from all affected
> >>> drivers.
> > 
> > Hm xen is really special, in that it has a flip complete event, but not a
> > vblank. I think forcing drivers to overwrite stuff in that case makes
> > sense.
> > 
> >> I'm not sure if setting no_vblank based on dev->num_crtcs is the correct thing.
> >> From the original commit and associated description for no_vblank:
> >>
> >> In some cases CRTCs are active but are not able to generating events, at
> >> least not at every frame at it's expected to.
> >> This is typically the case when the CRTC is feeding a writeback connector...
> > 
> > Yeah, but Thomas' series here wants to extend that. And I think if we roll
> > this out the common case will be "no hw vblank", and the writeback special
> 
> Default values should usually be 0 for zalloc and static initializers.
> Should we rename no_vblank to has_vblank then?

Hm, imo feels like hw without vblank is still the uncommon case. I'd leave
this as-is, but also no objections if you feel like repainting :-)

> > case is going to be the exception to the exception. Yup, patch 1 that
> > updates the docs doesn't reflect that, which is why I'm bringing up more
> > suggestions here around code & semantics of all these pieces to make them
> > do the most reasonable thing for most of the drivers.
> > 
> >> Reflects the ability of a CRTC to send VBLANK events....
> >>
> >>
> >> The proposed handling of no_vblank feels a little dirty, although
> >> nothing better comes to mind.
> >> Nevertheless code seems perfectly reasonable, so if it were me I'd merge it.
> > 
> > The idea with setting it very early is that drivers can overwrite it very
> > easily. Feels slightly dirty, so I guess we could also set it somewhere in
> > the atomic_helper_check function (similar to how we set the various
> > crtc->*_changed flags, but we're not entirely consistent on these either).
> > 
> > For the overall thing what feels irky to me is making this no_vblank
> > default logic (however we end up computing it in the end, whether like
> > this or what I suggested) specific to simple pipe helpers feels kinda
> > wrong. Simple pipe tends to have a higher ratio of drivers for hw without
> > vblank support, but by far not the only ones. Having that special case
> > feels confusing to me (and likely will trip up some people, vblank and
> > event handling is already a huge source of confusion in drm).
> 
> Making it a default for simple KMS was only the start. I intended to
> cover all drivers at some point. I just didn't want to go through all
> drivers at once.
> 
> I guess for the patchset's v3 I'll audit all drivers for the use of
> no_blank and drm_crtc_send_vblank_event(); and convert the possible
> candidates.

Yeah it's a pain, thanks for volunteering. Just figured the half-step here
is too much in the uncanney valley. If we're going to polish this, let's
do it right (and we have plenty enough drivers to make sure what we pick
will be a solid choice I think).
-Daniel

> 
> Best regards
> Thomas
> 
> > 
> > One idea behind drm_dev_has_vblank() is also that we could formalize a bit
> > all that, at least for the usual case - xen and maybe others being some
> > exceptions as usual (hence definitely not something the core code should
> > handle).
> > 
> > Cheers, Daniel
> > 
> 
> -- 
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Felix Imendörffer
> 




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