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

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

On Tue, May 26, 2026 at 01:14:55PM +0200, Maarten Lankhorst wrote:
> Hey,
> 
> Den 2026-05-25 kl. 14:30, skrev Ville Syrjälä:
> > On Mon, May 25, 2026 at 10:13:57AM +0200, Maarten Lankhorst wrote:
> >> Hey,
> >>
> >> Den 2026-05-22 kl. 21:39, skrev 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.
> >>>
> >>> 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.´
> >>
> >> I tested fbdev on a tiled DP-MST monitor.
> >> It works better than my kwin's wayland compositor, as it detects both tiles
> >> and presents a single image spanning both tiles.
> > 
> > IIRC we've occasionally seen cases where it picks a non-tiled mode
> > on the primary connector, and also still enables the second tile.
> > 
> >> Kwin sees both as separate
> >> monitors.
> >>
> >> I still see vertical tearing between both tiles, so it would be nice if
> >> intel/display would support atomic updates for both crtc's directly.
> >>
> >> The code's already there for bigjoiner, just needs to do the same for
> >> tile joiner updates when all tiled crtc's are in the atomic update.
> > 
> > We don't have any special code for atomic updates with joiner
> > currently. It just happens to work most of the time.
> > 
> > With joiner the pipes will be in sync/phase, so that helps a bit.
> > But we do also try to make the pipes in sync/phase also for tiled
> > display via the use of the port sync. So if you see a difference
> > in tearing between joiner vs. tiled then that likely means the
> > problem is in userspace (as in it submits separate commits for
> > each tile).
> 
> The only thing kernel should do is if crtc 1 and 2 are part of a
> commit and in sync is to perform the same as bigjoiner updates for
> crtc 1 & 2 simultaneously.

Nothing is performed simultaneously for joiner either. The only thing
that is guaranteed is that the joined pipes are updated back to back,
because the pipes are always consecutive. If you have consecutive pipes
used for a tiled display (or the pipes in between are not part of the
commit) then there is zero difference between joiner vs. tiled display
commit.

> 
> That shouldn't be too much to ask from the kernel, and not hard to
> implement.
> 
> 
> > I have occasionally pondered about hiding the tiled display stuff
> > completely from userspace and handling it in the kernel the same
> > was as joiner. But the problem is that we'd also need to cook up
> > a new EDID for the display that combines both tiles, and we'd
> > still need the second connector to be enabled internally (and 
> > hide that fact from userspace). None of that code exists
> > currently and wouldn't be entirely trivial to implement.
> > 

-- 
Ville Syrjälä
Intel