[PATCH 0/4] drm: create blend mode property on alpha-capable planes of simple drivers

[Qinyun Tan]

Since commit 860e748bddcc ("drm: ensure blend mode supported if pixel
format with alpha exposed"), drm_mode_config_validate() warns when a
plane exposes an alpha pixel format but does not create the "pixel
blend mode" property. Several simple/virtual drivers trip this on
driver load. I hit the ast one on an ASPEED AST2600 BMC:

  [PLANE:37:plane-1] pixel format with alpha exposed but blend mode not setup
  WARNING: drivers/gpu/drm/drm_mode_config.c:872 at drm_mode_config_validate+0x48f/0x510 [drm]
  ...
  Call Trace:
   drm_dev_register+0x1ce/0x290 [drm]
   ast_pci_probe+0x19d/0x3f0 [ast]
   local_pci_probe+0x41/0x90

These four drivers share the same gap; this series covers ast, qxl,
virtio-gpu and vboxvideo.

These planes have always blended with pre-multiplied alpha: the
hardware (or host-side compositor for the virtual drivers) is fixed
function with no configurable blend mode, and userspace has
historically assumed pre-multiplied blending when the property is not
attached, which is also the documented default of the property. So
each patch simply exposes the "pixel blend mode" property advertising
only DRM_MODE_BLEND_PREMULTI, making the existing semantics explicit
and silencing the warning. No functional change.

This follows the same approach as the already-merged i915 and nouveau
fixes:

  67e955e073c6 ("drm/i915/display: expose blend mode on alpha-capable planes")
  df0311845915 ("drm/nouveau/kms/nv50-: Unconditionally create blend_mode prop for wndws")

The ast patch is verified on AST2600 hardware: the warning is gone and
the cursor plane now exposes the "pixel blend mode" property with
Pre-multiplied as the only and current value. The qxl, virtio and
vboxvideo patches follow the identical pattern and are compile-tested.

Qinyun Tan (4):
  drm/ast: create blend mode property on cursor plane
  drm/qxl: create blend mode property on primary and cursor planes
  drm/virtio: create blend mode property on cursor plane
  drm/vboxvideo: create blend mode property on planes

 drivers/gpu/drm/ast/ast_cursor.c       | 3 +++
 drivers/gpu/drm/qxl/qxl_display.c      | 4 ++++
 drivers/gpu/drm/vboxvideo/vbox_mode.c  | 4 ++++
 drivers/gpu/drm/virtio/virtgpu_plane.c | 4 ++++
 4 files changed, 15 insertions(+)

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2.43.7