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

[Nicolas Frattaroli <nicolas.frattaroli@collabora.com>]

On Wednesday, 23 September 2026 11:51:45 Central European Summer Time Michel Dänzer wrote:
> On 9/21/26 17:51, Nicolas Frattaroli wrote:
> > 
> > + * .. _VRR-MIN-NUMERATOR:
> > + *
> > + * "VRR_MIN_NUMERATOR":
> > + *	Default &drm_crtc integer property forming the numerator of a
> > + *	numerator/denominator pair of a frame rate to set as the minimum VRR
> > + *	target rate. Set to 0 to disable.
> > + *
> > + * "VRR_MIN_DENOMINATOR":
> > + *	Default &drm_crtc integer property forming the denominator of a
> > + *	numerator/denominator pair of a frame rate to set as the minimum VRR
> > + *	target rate. If :ref:`VRR_MIN_NUMERATOR <VRR-MIN-NUMERATOR>` is not
> > + *	zero, it must be non-zero.
> > + *	Otherwise, must also be zero.
> > + *
> > + * .. _VRR-MAX-NUMERATOR:
> > + *
> > + * "VRR_MAX_NUMERATOR":
> > + *	Default &drm_crtc integer property forming the numerator of a
> > + *	numerator/denominator pair of a frame rate to set as the maximum VRR
> > + *	target rate. Set to 0 to disable.
> > + *
> > + * "VRR_MAX_DENOMINATOR":
> > + *	Default &drm_crtc integer property forming the denominator of a
> > + *	numerator/denominator pair of a frame rate to set as the maximum VRR
> > + *	target rate. If :ref:`VRR_MAX_NUMERATOR <VRR-MAX-NUMERATOR>` is not
> > + *	zero, it must be non-zero. Otherwise, must also be zero.
> >   */
> 
> Is there a reason that DENOMINATOR must be 0 when the corresponding NUMERATOR is? 0 divided by any number is still 0.

No, I think that's an arbitrary convention I settled on and don't enforce. I
guess it should be "Otherwise, may also be zero", because the only situation
I'm making userspace avoid is x/0 where x != 0.

> > Either way, should these rules be enforced in drm_atomic_crtc_set_property?
> 
> Or rather in atomic_check.

Due to complicating factors like EDID, CinemaVRR, and QMS TFRmin/TFRmax, checking
the properties for sensible values is done in the HDMI state helpers at the moment.

If/when there is a similar mechanism for DP, we can probably factor the common
parts out. I really do hope all drivers (including those that don't use the HDMI
state helpers) can at least share the hdmi_validate_vrr() logic, but I haven't
factored this out into an exported function yet because I don't know how similar
the DisplayPort-equivalent mechanisms requirements are, or how much of the state
derivation non-state-helper drivers (i.e. i915 and amdgpu) need.

Kind regards,
Nicolas Frattaroli