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

[Jani Nikula <jani.nikula@linux.intel.com>]

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

-- 
Jani Nikula, Intel