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drm/ast: create blend mode property on cursor plane

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Since commit 860e748bdd ("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 not the "pixel blend mode"
property. The ast cursor plane (ARGB4444, ARGB8888) trips this on
driver load:

  [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

Per Thomas Zimmermann's review, the ASPEED documentation describes the
hardware cursor as blending with straight (non-pre-multiplied) alpha,
which corresponds to DRM_MODE_BLEND_COVERAGE. Expose a "pixel blend
mode" property advertising only DRM_MODE_BLEND_COVERAGE to make the
hardware semantics explicit and silence the warning.

Fixes: 860e748bdd ("drm: ensure blend mode supported if pixel format with alpha exposed")
Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Qinyun Tan <qinyuntan@linux.alibaba.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260901083234.1828755-2-qinyuntan@linux.alibaba.com
This commit is contained in:
Qinyun Tan 2026-09-01 16:32:31 +08:00 • committed by Thomas Zimmermann
commit 313f798f52
1 changed files with 3 additions and 0 deletions
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