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

[Thomas Zimmermann]

Hi

Am 22.05.26 um 21:39 schrieb Ville Syrjälä:
> 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.

I know, but we have to support it to some extent. The current mirroring 
logic at least works, except that it assumes CRTC 0 to be present.

Best regards
Thomas

> 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)

--
--
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)