Re: [PATCH] drm/client: check whether CRTC is active before waiting for vblank

[Thomas Zimmermann]

Hi

Am 22.05.26 um 16:01 schrieb Icenowy Zheng:
> 在 2026-05-22五的 15:43 +0200，Thomas Zimmermann写道：
> > Hi
> >
> > Am 22.05.26 um 15:28 schrieb Ville Syrjälä:
> > [...]
> > > > > > But why does your HW use CRTC 1 in the first place.
> > > > > Could be eg. the enabled outputs can't be driven with CRTC 0.
> Yes, for many embedded display solutions the CRTC-connector map is
> totally fixed.
>
> > > > > I guess what you want to do is pick the first crtc from
> > > > > modesets[]
> > > > > which is enabled. Or perhaps even "pick the Nth enabled crtc
> > > > > from
> > > > > modesets[] based on the ioctl argument".
> > > > The enable-status of each CRTC could change later on, which might
> > > > lead
> > > > to problems.
> > > Sound like a locking issue if someone is changing the configuration
> > > at the same time we're trying to do the vblank wait here.
> > I mean that the connected outputs could change at a later point or we
> > could have multiple CRTCs in use. Today, someone in #intel-gfx
> > reported
> > a problem with panning if multiple CRTCs are in use.
> >
> > Therefore picking a CRTC freely could be a problem. Let's say we
> > configure modes from one CRTC, but later wait/pan/flush with another
> > CRTC. I would not trust this to work correctly.
> >
> > Hence, my suggestion is to select a primary CRTC during the fbdev
> > client's probe and use it for all later operations until the next
> > probe
> > happens.  All other CRTCs would mirror the primary one.
> What will happen if the "primary CRTC" is then disabled because of no
> connected connectors can be driven with it?

This happens during a client hotplug event. We'd re-detect all 
connectors, pick the CRTC/output with the lowest spec as new primary and 
mirror it to all other connected outputs.

Best regards
Thomas

> Thanks,
> Icenowy
>
> > Best regards
> > Thomas
> >
> >
> > > > Picking the one CRTC/output with the lowest spec and
> > > > mirroring it to the others might work. This CRTC would then be
> > > > the one
> > > > to wait for.
> > > >
> > > > Best regards
> > > > Thomas
> > > >
> > > > --
> > > > --
> > > > Thomas Zimmermann
> > > > Graphics Driver Developer
> > > > SUSE Software Solutions Germany GmbH
> > > > Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
> > > > GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809,
> > > > AG Nürnberg)

--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)