[PATCH 0/9] drm: Limit DRM_IOCTL_WAIT_VBLANK to vblank interrupts

[Thomas Zimmermann <tzimmermann@suse.de>]

DRM's WAIT_VBLANK ioctl synchronizes user-space clients to display
refresh. This is meaningless with vblank timers, which run unrelated
to the hardware's vblank.

Disable the ioctl for simulated vblanks. Set DRM_VBLANK_FLAG_SIMULATED
for CRTCs with simulated vblank events in all such drivers. The vblank
timers of these devices still rate-limit the number of page-flip events
to match the display refresh.

According to maintainers, user-space compositors do not require the ioctl
for rate-limitting display output. Weston and Kwin rely on page-flip
events. Mutter uses and internal timer to limit the number of display
updates per second.

When testing with mutter and weston, the page-flip rate appears correct
with the patch set applied.

This change has been discussed at length on IRC recently.

https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&highlight_names=&date=2026-05-08&show_html=true
https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&highlight_names=&date=2026-05-12&show_html=true
https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&highlight_names=&date=2026-05-13&show_html=true

Thomas Zimmermann (9):
  drm/vblank: Add drmm_vblank_init() to indicate managed cleanup
  drm/vblank: Add DRM_VBLANK_FLAG_SIMULATED
  drm/amdgpu: vkms: Set DRM_VBLANK_FLAG_SIMULATED
  drm/bochs: Set DRM_VBLANK_FLAG_SIMULATED
  drm/cirrus: Set DRM_VBLANK_FLAG_SIMULATED
  drm/hypervdrm: Set DRM_VBLANK_FLAG_SIMULATED
  drm/qxl: Set DRM_VBLANK_FLAG_SIMULATED
  drm/virtgpu: Set DRM_VBLANK_FLAG_SIMULATED
  drm/vkms: Set DRM_VBLANK_FLAG_SIMULATED

 drivers/gpu/drm/amd/amdgpu/amdgpu_vkms.c    |  3 ++-
 drivers/gpu/drm/drm_vblank.c                | 19 ++++++++++++-------
 drivers/gpu/drm/drm_vblank_helper.c         |  2 +-
 drivers/gpu/drm/hyperv/hyperv_drm_modeset.c |  2 +-
 drivers/gpu/drm/qxl/qxl_display.c           |  2 +-
 drivers/gpu/drm/tiny/bochs.c                |  2 +-
 drivers/gpu/drm/tiny/cirrus-qemu.c          |  2 +-
 drivers/gpu/drm/virtio/virtgpu_display.c    |  2 +-
 drivers/gpu/drm/vkms/vkms_drv.c             |  4 ++--
 include/drm/drm_crtc.h                      |  2 +-
 include/drm/drm_device.h                    |  2 +-
 include/drm/drm_vblank.h                    | 15 ++++++++++++++-
 12 files changed, 38 insertions(+), 19 deletions(-)


base-commit: 121c16f9d8c56ea07263df84ab971cc10870fe88
-- 
2.54.0