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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, May 22, 2026 at 03:43:26PM +0200, Thomas Zimmermann wrote:
> 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.
> >>>
> >>> 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.

Actual mirroring may not be possible due to different modes supported
on each output. The whole multi-output fbdev thing in the drm fb helper
is kind of a hack that's rather hard to make work 100% sensibly.

For the panning possibly the only sensible thing is to use the max of
hdisplay/vdisplay of all the crtcs as the xres/yres so it's clear
how much things can actually be panned. Oh and tiled displays (assuming
we would actually want the fbdev stuff to tile correctly) make the
situation even more complicated. I think the current support for tiled
displays in the fb helper is semi-busted.

> 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)
> 

-- 
Ville Syrjälä
Intel