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[0/5] drm/vblank: Enforce all-or-nothing vblank support

Message ID 20260814203542.1405135-1-lyude@redhat.com (mailing list archive)
Headers
Series drm/vblank: Enforce all-or-nothing vblank support |

Message

Lyude Paul Aug. 14, 2026, 8:35 p.m. UTC 
One of the oddities with DRM that is leftover from the old days is how
vblank support is implemented by drivers. This has caused some minor
issues for the rust bindings for KMS which could be worked around.
However, it seems like a much better idea to fix this on the C side.
Those oddities are mainly:

* There's nothing in the vblank core that actually ensures that all
  CRTCs implement vblank support if one of them do. On Rust's side, we
  can't have this and have to ensure either the whole device has vblank
  support or not - otherwise it would be quite painful ensuring we don't
  expose vblank methods to drivers that can't use them.
  This additionally means such a situation would lead to UB.
* There is a second variable for keeping track of the number of CRTCs
  and it's kind of broken and confusing. Even worse, we currently allow
  the vblank API to allow a driver to specify a different number of
  CRTCs then it actually has.

So, let's just go ahead and fix this across the whole tree, and enforce
that a driver must either implement hardware vblank support for all
CRTCs or no CRTCs.

Lyude Paul (5):
  drm/vblank: Add drm_device.has_hw_vblank
  drm/vblank: Remove drm->num_crtcs
  drm/vblank: Remove num_crtcs argument from drm_vblank_init()
  drm/vblank: Use drm_for_each_crtc() in drm_vblank_init()
  drm/vblank: Require all CRTCs implement vblank support in
    drm_vblank_init()

 drivers/gpu/drm/adp/adp_drv.c                 |  2 +-
 drivers/gpu/drm/amd/amdgpu/amdgpu_vkms.c      |  2 +-
 drivers/gpu/drm/amd/amdgpu/dce_v10_0.c        |  2 +-
 drivers/gpu/drm/amd/amdgpu/dce_v6_0.c         |  2 +-
 drivers/gpu/drm/amd/amdgpu/dce_v8_0.c         |  2 +-
 .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c |  2 +-
 .../gpu/drm/arm/display/komeda/komeda_kms.c   |  2 +-
 drivers/gpu/drm/arm/hdlcd_drv.c               |  2 +-
 drivers/gpu/drm/arm/malidp_drv.c              |  2 +-
 drivers/gpu/drm/armada/armada_drv.c           |  2 +-
 drivers/gpu/drm/aspeed/aspeed_gfx_drv.c       |  2 +-
 drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.c  |  2 +-
 drivers/gpu/drm/drm_vblank.c                  | 60 ++++++++++++-------
 drivers/gpu/drm/exynos/exynos_drm_drv.c       |  2 +-
 drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c     |  2 +-
 drivers/gpu/drm/gma500/psb_drv.c              |  2 +-
 drivers/gpu/drm/gma500/psb_irq.c              |  4 +-
 .../gpu/drm/hisilicon/hibmc/hibmc_drm_drv.c   |  2 +-
 .../gpu/drm/hisilicon/kirin/kirin_drm_drv.c   |  2 +-
 drivers/gpu/drm/hyperv/hyperv_drm_modeset.c   |  2 +-
 .../drm/i915/display/intel_display_driver.c   |  3 +-
 drivers/gpu/drm/imx/dc/dc-kms.c               |  2 +-
 drivers/gpu/drm/imx/dcss/dcss-kms.c           |  2 +-
 drivers/gpu/drm/imx/ipuv3/imx-drm-core.c      |  2 +-
 drivers/gpu/drm/imx/lcdc/imx-lcdc.c           |  2 +-
 drivers/gpu/drm/ingenic/ingenic-drm-drv.c     |  2 +-
 drivers/gpu/drm/kmb/kmb_drv.c                 |  2 +-
 drivers/gpu/drm/logicvc/logicvc_mode.c        |  2 +-
 drivers/gpu/drm/loongson/lsdc_drv.c           |  2 +-
 drivers/gpu/drm/mcde/mcde_drv.c               |  2 +-
 drivers/gpu/drm/mediatek/mtk_drm_drv.c        |  2 +-
 drivers/gpu/drm/meson/meson_drv.c             |  2 +-
 drivers/gpu/drm/msm/msm_kms.c                 |  2 +-
 drivers/gpu/drm/mxsfb/lcdif_drv.c             |  2 +-
 drivers/gpu/drm/mxsfb/mxsfb_drv.c             |  2 +-
 drivers/gpu/drm/nouveau/nouveau_display.c     |  2 +-
 drivers/gpu/drm/omapdrm/omap_drv.c            |  2 +-
 drivers/gpu/drm/pl111/pl111_drv.c             |  2 +-
 drivers/gpu/drm/qxl/qxl_display.c             |  2 +-
 drivers/gpu/drm/radeon/atombios_crtc.c        |  4 +-
 drivers/gpu/drm/radeon/radeon_irq_kms.c       |  2 +-
 drivers/gpu/drm/radeon/radeon_legacy_crtc.c   |  4 +-
 drivers/gpu/drm/renesas/rcar-du/rcar_du_kms.c |  2 +-
 drivers/gpu/drm/renesas/rz-du/rzg2l_du_kms.c  |  2 +-
 .../gpu/drm/renesas/shmobile/shmob_drm_drv.c  |  2 +-
 drivers/gpu/drm/rockchip/rockchip_drm_drv.c   |  2 +-
 drivers/gpu/drm/sprd/sprd_drm.c               |  2 +-
 drivers/gpu/drm/sti/sti_compositor.c          |  2 +-
 drivers/gpu/drm/stm/ltdc.c                    |  2 +-
 drivers/gpu/drm/sun4i/sun4i_drv.c             |  2 +-
 drivers/gpu/drm/tegra/drm.c                   |  2 +-
 drivers/gpu/drm/tidss/tidss_kms.c             |  2 +-
 drivers/gpu/drm/tilcdc/tilcdc_drv.c           |  2 +-
 drivers/gpu/drm/tiny/bochs.c                  |  2 +-
 drivers/gpu/drm/tiny/cirrus-qemu.c            |  2 +-
 drivers/gpu/drm/tve200/tve200_drv.c           |  2 +-
 drivers/gpu/drm/vc4/vc4_kms.c                 |  6 +-
 drivers/gpu/drm/verisilicon/vs_drm.c          |  2 +-
 drivers/gpu/drm/virtio/virtgpu_display.c      |  2 +-
 drivers/gpu/drm/vkms/vkms_drv.c               |  3 +-
 drivers/gpu/drm/vmwgfx/vmwgfx_vkms.c          |  2 +-
 drivers/gpu/drm/xlnx/zynqmp_kms.c             |  2 +-
 include/drm/drm_device.h                      | 10 +++-
 include/drm/drm_vblank.h                      |  2 +-
 64 files changed, 114 insertions(+), 92 deletions(-)


base-commit: 568d4cb5dcc5ec4e22975699b6ea67a30806884c

Comments

Jani Nikula Aug. 17, 2026, 12:41 p.m. UTC | #1 
On Fri, 14 Aug 2026, Lyude Paul <lyude@redhat.com> wrote:
> One of the oddities with DRM that is leftover from the old days is how
> vblank support is implemented by drivers. This has caused some minor
> issues for the rust bindings for KMS which could be worked around.
> However, it seems like a much better idea to fix this on the C side.
> Those oddities are mainly:
>
> * There's nothing in the vblank core that actually ensures that all
>   CRTCs implement vblank support if one of them do. On Rust's side, we
>   can't have this and have to ensure either the whole device has vblank
>   support or not - otherwise it would be quite painful ensuring we don't
>   expose vblank methods to drivers that can't use them.
>   This additionally means such a situation would lead to UB.
> * There is a second variable for keeping track of the number of CRTCs
>   and it's kind of broken and confusing. Even worse, we currently allow
>   the vblank API to allow a driver to specify a different number of
>   CRTCs then it actually has.
>
> So, let's just go ahead and fix this across the whole tree, and enforce
> that a driver must either implement hardware vblank support for all
> CRTCs or no CRTCs.

For i915, the relevant init order is currently:

- drm_vblank_init()
- drm_mode_config_init()
- drm_crtc_init_with_planes() for each CRTC

For the patch series to work, drm_vblank_init() would have to happen
after mode config and CRTC init. Off-hand, I couldn't say if there are
dependencies that make this change difficult.

I'd be surprised if i915 were the only driver like this.


BR,
Jani.

>
> Lyude Paul (5):
>   drm/vblank: Add drm_device.has_hw_vblank
>   drm/vblank: Remove drm->num_crtcs
>   drm/vblank: Remove num_crtcs argument from drm_vblank_init()
>   drm/vblank: Use drm_for_each_crtc() in drm_vblank_init()
>   drm/vblank: Require all CRTCs implement vblank support in
>     drm_vblank_init()
>
>  drivers/gpu/drm/adp/adp_drv.c                 |  2 +-
>  drivers/gpu/drm/amd/amdgpu/amdgpu_vkms.c      |  2 +-
>  drivers/gpu/drm/amd/amdgpu/dce_v10_0.c        |  2 +-
>  drivers/gpu/drm/amd/amdgpu/dce_v6_0.c         |  2 +-
>  drivers/gpu/drm/amd/amdgpu/dce_v8_0.c         |  2 +-
>  .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c |  2 +-
>  .../gpu/drm/arm/display/komeda/komeda_kms.c   |  2 +-
>  drivers/gpu/drm/arm/hdlcd_drv.c               |  2 +-
>  drivers/gpu/drm/arm/malidp_drv.c              |  2 +-
>  drivers/gpu/drm/armada/armada_drv.c           |  2 +-
>  drivers/gpu/drm/aspeed/aspeed_gfx_drv.c       |  2 +-
>  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_dc.c  |  2 +-
>  drivers/gpu/drm/drm_vblank.c                  | 60 ++++++++++++-------
>  drivers/gpu/drm/exynos/exynos_drm_drv.c       |  2 +-
>  drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.c     |  2 +-
>  drivers/gpu/drm/gma500/psb_drv.c              |  2 +-
>  drivers/gpu/drm/gma500/psb_irq.c              |  4 +-
>  .../gpu/drm/hisilicon/hibmc/hibmc_drm_drv.c   |  2 +-
>  .../gpu/drm/hisilicon/kirin/kirin_drm_drv.c   |  2 +-
>  drivers/gpu/drm/hyperv/hyperv_drm_modeset.c   |  2 +-
>  .../drm/i915/display/intel_display_driver.c   |  3 +-
>  drivers/gpu/drm/imx/dc/dc-kms.c               |  2 +-
>  drivers/gpu/drm/imx/dcss/dcss-kms.c           |  2 +-
>  drivers/gpu/drm/imx/ipuv3/imx-drm-core.c      |  2 +-
>  drivers/gpu/drm/imx/lcdc/imx-lcdc.c           |  2 +-
>  drivers/gpu/drm/ingenic/ingenic-drm-drv.c     |  2 +-
>  drivers/gpu/drm/kmb/kmb_drv.c                 |  2 +-
>  drivers/gpu/drm/logicvc/logicvc_mode.c        |  2 +-
>  drivers/gpu/drm/loongson/lsdc_drv.c           |  2 +-
>  drivers/gpu/drm/mcde/mcde_drv.c               |  2 +-
>  drivers/gpu/drm/mediatek/mtk_drm_drv.c        |  2 +-
>  drivers/gpu/drm/meson/meson_drv.c             |  2 +-
>  drivers/gpu/drm/msm/msm_kms.c                 |  2 +-
>  drivers/gpu/drm/mxsfb/lcdif_drv.c             |  2 +-
>  drivers/gpu/drm/mxsfb/mxsfb_drv.c             |  2 +-
>  drivers/gpu/drm/nouveau/nouveau_display.c     |  2 +-
>  drivers/gpu/drm/omapdrm/omap_drv.c            |  2 +-
>  drivers/gpu/drm/pl111/pl111_drv.c             |  2 +-
>  drivers/gpu/drm/qxl/qxl_display.c             |  2 +-
>  drivers/gpu/drm/radeon/atombios_crtc.c        |  4 +-
>  drivers/gpu/drm/radeon/radeon_irq_kms.c       |  2 +-
>  drivers/gpu/drm/radeon/radeon_legacy_crtc.c   |  4 +-
>  drivers/gpu/drm/renesas/rcar-du/rcar_du_kms.c |  2 +-
>  drivers/gpu/drm/renesas/rz-du/rzg2l_du_kms.c  |  2 +-
>  .../gpu/drm/renesas/shmobile/shmob_drm_drv.c  |  2 +-
>  drivers/gpu/drm/rockchip/rockchip_drm_drv.c   |  2 +-
>  drivers/gpu/drm/sprd/sprd_drm.c               |  2 +-
>  drivers/gpu/drm/sti/sti_compositor.c          |  2 +-
>  drivers/gpu/drm/stm/ltdc.c                    |  2 +-
>  drivers/gpu/drm/sun4i/sun4i_drv.c             |  2 +-
>  drivers/gpu/drm/tegra/drm.c                   |  2 +-
>  drivers/gpu/drm/tidss/tidss_kms.c             |  2 +-
>  drivers/gpu/drm/tilcdc/tilcdc_drv.c           |  2 +-
>  drivers/gpu/drm/tiny/bochs.c                  |  2 +-
>  drivers/gpu/drm/tiny/cirrus-qemu.c            |  2 +-
>  drivers/gpu/drm/tve200/tve200_drv.c           |  2 +-
>  drivers/gpu/drm/vc4/vc4_kms.c                 |  6 +-
>  drivers/gpu/drm/verisilicon/vs_drm.c          |  2 +-
>  drivers/gpu/drm/virtio/virtgpu_display.c      |  2 +-
>  drivers/gpu/drm/vkms/vkms_drv.c               |  3 +-
>  drivers/gpu/drm/vmwgfx/vmwgfx_vkms.c          |  2 +-
>  drivers/gpu/drm/xlnx/zynqmp_kms.c             |  2 +-
>  include/drm/drm_device.h                      | 10 +++-
>  include/drm/drm_vblank.h                      |  2 +-
>  64 files changed, 114 insertions(+), 92 deletions(-)
>
>
> base-commit: 568d4cb5dcc5ec4e22975699b6ea67a30806884c