[PATCH RFC 13/25] drm: Add VRR target frame rate properties

[Nicolas Frattaroli]

On Tuesday, 29 September 2026 16:34:58 Central European Summer Time Leo Li wrote:
> 
> On 2026-09-28 04:10, Michel Dänzer wrote:
> >>> You can picture VRR limiting as always being active, but with a limit rational
> >>> of 0 it uses the display's limit as per the EDID, which is what unlimited game
> >>> mode is. So with how it's implemented right now in hdmi_validate_vrr(), your
> >>> example would set a maximum target, but leave the minimum at whatever the
> >>> display defaults to.
> >>>
> >>> Now that I'm thinking through this, a possible problem is that
> >>> drm_crtc_helper_vrr_is_fixed_rate() operates on the user supplied limits, but
> >>> if the display supplied lower limit is equal to the user supplied upper limit,
> >>> then we have a fixed rate scenario without recognising it as such. I think I
> >>> need to have a ponder on what the least surprising behaviour for userspace
> >>> is in that instance. The display limit stuff gets a bit complex due to
> >>> CinemaVRR and QMS TFRmin/TFRmax.
> >>>
> >>> I'll improve the documentation on the next revision to make the meanings more
> >>> explicit.
> >> Perhaps a simple way is to require simultaneous setting MIN and MAX pairs?
> >> IOW, require userspace to set MIN and MAX simultaneously to >0, or =0. For example:
> >>
> >> if ((vrr_min_n == 0 || vrr_min_d == 0 ||
> >>      vrr_max_n == 0 || vrr_max_d == 0) &&
> >>     (vrr_min_n > 0 || vrr_max_n > 0))
> >> 	return -EINVAL;
> >> That way, it's never ambiguous what userspace has requested for the range.
> >> They can copy the EDID supported range if they don't care about limiting one side, rather than leaving it at 0.
> > Determining the actual limits can be non-trivial (though I guess that might be fine as long as libdisplay-info can work them out), if user space gets them wrong, it might accidentally apply a narrower limit than intended.
> > 
> > 
> >> It's then also clear if they requested a static Hz.
> > I do see the benefit of your suggestion for this though.
> 
> Xaver and I were chatting about this at XDC, and yeah it'll be difficult
> to match KMD's monitor range, especially if KMD decides to patch it with
> quirks and whatnot.
> 
> Since we are handing compositors control over vrr range, does it sound
> sensible to expose KMD's monitor range as a read-only property pair on
> the drm connector? We probably don't need a num/den pair for it, it's
> not like panels advertise fractional VRR ranges (right?).

Sounds good to me.

And yeah, we don't really need num/denom for it; the safe assumption is that the
minimum range is expressed with a denominator of 1.001 whereas the maximum range
is expressed with a numerator of 1. That's sort of non-obvious for userspace
though, so I'll need to do some thinking around the specified behaviour.

This sounds like a mainly theoretical concern but it's a real one, the most
common 1.001 rates we'll run into are likely 24/1.001 or 30/1.001 and those
are also very reasonable for a monitor to have as a lower limit. And when
they specify that lower limit, they'll do it as just the integer rounded
value, but seemingly expect them to be understood as the 1.001 value for
the lower limit.

> Fun fact: vrr_range is exposed today over debugfs for IGT testing
> https://elixir.bootlin.com/linux/v7.3-rc5/source/drivers/gpu/drm/drm_debugfs.c#L586

Speaking of that, v2 will deduplicate my accidentally rebased-over
mostly duplicated EDID parsing and fix this function to report the
newly added vrr_min/vrr_max fields of the display_info. (Since
apparently monitor_range is populated by VESA and messing with it
may not be okay?)

Kind regards,
Nicolas Frattaroli

> 
> Thanks,
> Leo
> 
> > 
> > 
> > -- Earthling Michel Dänzer \ GNOME / Xwayland / Mesa developer https://redhat.com \ Libre software enthusiast
> 
>