[PATCH v4 0/4] A DRM API for adaptive sync and variable refresh rate support

Nicholas Kazlauskas nicholas.kazlauskas at amd.com
Thu Oct 11 16:39:38 UTC 2018

These patches are part of a proposed new interface for supporting variable refresh rate via DRM properties.

=== Changes from v3 ===

drm changes:

* Docstring and formatting fixes

amd/display changes:

* Updated commit message and debug statements

=== Changes from v2 ===

The interface has changed substantially from the last revision and the cover letter has been updated accordingly.

drm changes:

* Most "variable_refresh" prefixes in code have been shortened to just "vrr". This was motivated after changes to the interface had function names close to 80 characters long. Comments from the mailing list were already shortening these in discussion as well.
* Documentation for "Variable Refresh" has been added to the KMS properties subsection for DRM driver developers.
* The drm_connector property "variable_refresh_capable" has been renamed to "vrr_capable".
* The drm_crtc property "variable_refresh" has been been renamed "vrr_enabled" to better reflect its usage.
* The drm_connector "variable_refresh_enabled" property has been removed. Managing this is now up to userspace when it sets "vrr_enabled".
* The "vrr_capable" property no longer has any state in drm_connector_state associated with it. The value can now be updated with the drm_connector_set_vrr_capable_property() function. This better matches the immutable flag on the property.
* The "variable_refresh_changed" flag has been removed from atomic helpers based on feedback from the mailing list and updated interface usage in the amd kernel driver.

amd/display changes:

* Updated interface usage based on the new properties
* Updated VRR infopacket handling based on new xf86-video-amdgpu usage

=== Adaptive sync and variable refresh rate ===

Adaptive sync is part of the DisplayPort specification and allows for graphics adapters to drive displays with varying frame timings.

Variable refresh rate (VRR) is essentially the same, but defined for HDMI.

=== Use cases for variable refresh rate ===

Variable frame (flip) timings don't align well with fixed refresh rate displays. This results in stuttering, tearing and/or input lag. By adjusting the display refresh rate dynamically these issues can be reduced or eliminated.

However, not all content is suitable for dynamic refresh adaptation. The user may experience "flickering" from differences in panel luminance at different refresh rates. Content that flips at an infrequent rate or demand is more likely to cause large changes in refresh rate that result in flickering.

Userland needs a way to let the driver know when the screen content is suitable for variable refresh rates.

=== DRM API to support variable refresh rates ===

This patch introduces a new API via atomic properties on the DRM connector and CRTC.

The drm_connector has one new optional boolean property:

* bool vrr_capable - set by the driver if the hardware is capable of supporting variable refresh rates

The drm_crtc has one new default boolean property:

* bool vrr_enabled - set by userspace indicating that variable refresh rate should be enabled

== Overview for DRM driver developers ===

Driver developers can attach the "vrr_capable" property by calling drm_connector_attach_vrr_capable_property(). This should be done on connectors that could be capable of supporting variable refresh rates (such as DP or HDMI).

The "vrr_capable" can then be updated accordingly by calling drm_connector_set_vrr_capable_property().

The "vrr_enabled" property can be checked from the drm_crtc_state object.

=== Overview for Userland developers ==

The "vrr_enabled" property on the CRTC should be set to true when the CRTC is suitable for variable refresh rates.

To demonstrate the suitability of the API for variable refresh and dynamic adaptation there are additional patches using this API that implement adaptive variable refresh across kernel and userland projects:

* DRM (dri-devel)
* amdgpu DRM kernel driver (amd-gfx)
* xf86-video-amdgpu (https://gitlab.freedesktop.org/xorg/driver/xf86-video-amdgpu)
* mesa (mesa-dev)

These patches enable adaptive variable refresh on X for AMD hardware. Support for variable refresh is disabled by default in xf86-video-amdgpu and will require additional user configuration.

The patches have been tested as working on upstream userland with the GNOME desktop environment under a single monitor setup. They also work on KDE in a single monitor setup with the compositor disabled.

The patches require that suitable applications flip via the Present extension to xf86-video-amdgpu for the entire Screen. Some compositors may interfere with this if they are unable to unredirect the window.

Full implementation details for these changes can be reviewed in their respective mailing lists and the xf86-video-amdgpu GitLab.

=== Previous discussions ===

These patches are based upon feedback from previous threads on the subject. These are linked below for reference:


Nicholas Kazlauskas (4):
  drm: Add vrr_capable property to the drm connector
  drm: Add vrr_enabled property to drm CRTC
  drm: Document variable refresh properties
  drm/amd/display: Set FreeSync state using drm VRR properties

 Documentation/gpu/drm-kms.rst                 |   7 +
 .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 255 +++++++++---------
 .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h |   7 +-
 drivers/gpu/drm/drm_atomic_uapi.c             |   4 +
 drivers/gpu/drm/drm_connector.c               |  71 +++++
 drivers/gpu/drm/drm_crtc.c                    |   2 +
 drivers/gpu/drm/drm_mode_config.c             |   6 +
 include/drm/drm_connector.h                   |  15 ++
 include/drm/drm_crtc.h                        |   9 +
 include/drm/drm_mode_config.h                 |   5 +
 10 files changed, 257 insertions(+), 124 deletions(-)


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