[RFC PATCH 0/6] drm: Add vblank timers for devices without interrupts

[Thomas Zimmermann]

Compositors often depend on vblanks to limit their display-update
rate. Without, they see vblank events ASAP, which creates high CPU
overhead. This is especially a problem with virtual devices with fast
framebuffer access.

The series moves vkms' vblank timer to DRM and converts a number of
drivers. The final patch rate-limits the output of DRM's framebuffer
console to vblank intervals. It has been taken from [1]. It reduces
the time for doing

  time find /usr/src/linux

from 24s to 20s with the patched bochs driver. Apparently the system
is spending less CPU overhead on display updates.

This is an RFC patchset to see if the approach is feasible. It's been
motivated by a recent discussion about hypervdrm [2] and other long-
standing bug reports. [3][4]

[1] https://patchwork.freedesktop.org/series/66442/
[2] https://lore.kernel.org/dri-devel/20250523161522.409504-1-mhklinux@outlook.com/T/#ma2ebb52b60bfb0325879349377738fadcd7cb7ef
[3] https://bugzilla.suse.com/show_bug.cgi?id=1189174
[4] https://invent.kde.org/plasma/kwin/-/merge_requests/1229#note_284606

Thomas Zimmermann (6):
  drm/vblank: Add vblank timer
  drm/vkms: Use vblank timer
  drm/simpledrm: Use vblank timer
  drm/bochs: Use vblank timer
  drm/hypervdrm: Use vblank timer
  drm/fb-helper: Synchronize dirty worker with vblank

 drivers/gpu/drm/Makefile                    |   3 +-
 drivers/gpu/drm/drm_fb_helper.c             |  20 ++++
 drivers/gpu/drm/drm_vblank_timer.c          | 100 ++++++++++++++++++++
 drivers/gpu/drm/hyperv/hyperv_drm.h         |   4 +
 drivers/gpu/drm/hyperv/hyperv_drm_modeset.c |  70 ++++++++++++++
 drivers/gpu/drm/sysfb/simpledrm.c           |  81 ++++++++++++++++
 drivers/gpu/drm/tiny/bochs.c                |  68 +++++++++++++
 drivers/gpu/drm/vkms/vkms_crtc.c            |  49 +++-------
 drivers/gpu/drm/vkms/vkms_drv.h             |   6 +-
 include/drm/drm_vblank_timer.h              |  26 +++++
 10 files changed, 386 insertions(+), 41 deletions(-)
 create mode 100644 drivers/gpu/drm/drm_vblank_timer.c
 create mode 100644 include/drm/drm_vblank_timer.h

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2.49.0