[PATCH v6 3/5] drm: Document variable refresh properties
Wentland, Harry
Harry.Wentland at amd.com
Wed Nov 7 14:57:03 UTC 2018
On 2018-11-06 3:24 p.m., Nicholas Kazlauskas wrote:
> These include the drm_connector 'vrr_capable' and the drm_crtc
> 'vrr_enabled' properties.
>
> Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas at amd.com>
> Cc: Harry Wentland <harry.wentland at amd.com>
> Cc: Manasi Navare <manasi.d.navare at intel.com>
> Cc: Pekka Paalanen <ppaalanen at gmail.com>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Michel Dänzer <michel at daenzer.net>
> ---
> Documentation/gpu/drm-kms.rst | 7 ++++
> drivers/gpu/drm/drm_connector.c | 61 +++++++++++++++++++++++++++++++++
> 2 files changed, 68 insertions(+)
>
> diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
> index 4b1501b4835b..8da2a178cf85 100644
> --- a/Documentation/gpu/drm-kms.rst
> +++ b/Documentation/gpu/drm-kms.rst
> @@ -575,6 +575,13 @@ Explicit Fencing Properties
> .. kernel-doc:: drivers/gpu/drm/drm_atomic_uapi.c
> :doc: explicit fencing properties
>
> +
> +Variable Refresh Properties
> +---------------------------
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_connector.c
> + :doc: Variable refresh properties
> +
> Existing KMS Properties
> -----------------------
>
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index 49290060ab7b..a6adf5450db3 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -1255,6 +1255,67 @@ int drm_mode_create_scaling_mode_property(struct drm_device *dev)
> }
> EXPORT_SYMBOL(drm_mode_create_scaling_mode_property);
>
> +/**
> + * DOC: Variable refresh properties
> + *
> + * Variable refresh rate capable displays can dynamically adjust their
> + * refresh rate by extending the duration of their vertical porch until
vertical porch -> vertical front porch
> + * page flip or timeout occurs. This can reduce or remove stuttering
> + * and latency in scenarios where the page flip does not align with the
> + * vblank interval.
> + *
> + * An example scenario would be an application flipping at a constant rate
> + * of 48Hz on a 60Hz display. The page flip will frequently miss the vblank
> + * interval and the same contents will be displayed twice. This can be
> + * observed as stuttering for content with motion.
> + *
> + * If variable refresh rate was active on a display that supported a
> + * variable refresh range from 35Hz to 60Hz no stuttering would be observable
> + * for the example scenario. The minimum supported variable refresh rate of
> + * 35Hz is below the page flip frequency and the vertical front porch can
> + * be extended until the page flip occurs. The vblank interval will be
> + * directly aligned to the page flip rate.
> + *
> + * Userspace control for variable refresh rate is supported via properties
> + * on the &drm_connector and &drm_crtc objects.
> + *
> + * "vrr_capable":
> + * Optional &drm_connector boolean property that drivers should attach
> + * with drm_connector_attach_vrr_capable_property() on connectors that
> + * could support variable refresh rates. Drivers should update the
> + * property value by calling drm_connector_set_vrr_capable_property().
> + *
> + * Absence of the property should indicate absence of support.
> + *
> + * "vrr_enabled":
> + * Default &drm_crtc boolean property that notifies the driver that the
> + * content on the CRTC is suitable for variable refresh rate presentation.
> + * The driver will take this property as a hint to enable variable
> + * refresh rate support if the receiver supports it, ie. if the
> + * "vrr_capable" property is true on the &drm_connector object. The
> + * veritcal front porch duration will be extended until page-flip or
veritcal -> vertical
> + * timeout when enabled.
> + *
> + * The minimum vertical front porch duration is defined as the vertical
> + * front porch duration for the current mode.
> + *
> + * The maximum vertical front porch duration is greater than or equal to
> + * the minimum vertical front porch duration. The duration is derived
> + * from the minimum supported variable refresh rate for the connector.
> + *
> + * The driver may place further restrictions within these minimum
> + * and maximum bounds.
> + *
> + * Some displays may exhibit noticeable differences in brightness when
> + * varying vertical front porch duration.
> + *
Maybe something like this makes sense here:
* Some displays may exhibit noticeable differences in brightness when
* varying vertical front porch duration drastically. It is therefore
* advised userspace only provide the "vrr_enabled" hint for content
* with a render rate that is expected to change gradually frame to frame,
* such as games.
> + * The semantics for the vertical blank timestamp differ when
> + * variable refresh rate is active. The vertical blank timestamp
> + * is defined to be an estimate using the current mode's fixed
> + * refresh rate timings. The semantics for the page-flip event
> + * timestamp remain the same.
> + */
> +
> /**
> * drm_connector_attach_vrr_capable_property - creates the
> * vrr_capable property
>
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