Re: [PATCH] drm/modeset-helper: Add blend mode property to the legacy primary plane

[Shixiong Ou]

Hi Thomas,

Thank you for your review.

Following your suggestion, I have withdrawn the original approach of
modifying the deprecated drm_crtc_init() helper. Instead, I have
prepared a new patch series that converts the remaining users of this
deprecated API (gma500, and radeon) to use  drm_crtc_init_with_planes() 
directly.

The new series is now ready and I will be submitting it shortly.
Best regards
Shixiong

On 2026/9/2 15:20, Thomas Zimmermann wrote:

Hi

Am 02.09.26 um 05:12 schrieb oushixiong1...@163.com:

From: Shixiong Ou <oushixi...@kylinos.cn>

The primary plane created by drm_crtc_init() exposes ARGB8888, which
has an alpha channel, but never gets a "pixel blend mode" property, so
drm_mode_config_validate() warns for every driver using this helper
(radeon, gma500, legacy amdgpu DCE). Advertise only
DRM_MODE_BLEND_PREMULTI, the mode userspace always assumed when the
property was absent.

I'd like to suggest a different strategy. The helper drm_crtc_init() is deprecated and there are only 3 drivers left that call it: amdgpu, gma500 and radeon.  It would be better to make a series for each that integrates the helper into the driver and adapts the format array to the hardware/driver's actual functionality. For gma500 at least, I'm pretty sure it does not support ARGB on its primary plane.

There's also work happening already for fixing amdgpu blend properties IIRC.

Best regards
Thomas

Fixes: 860e748bddcc ("drm: ensure blend mode supported if pixel format with alpha exposed")

Signed-off-by: Shixiong Ou <oushixi...@kylinos.cn>
---
  drivers/gpu/drm/drm_modeset_helper.c | 12 ++++++++++++
  1 file changed, 12 insertions(+)

diff --git a/drivers/gpu/drm/drm_modeset_helper.c b/drivers/gpu/drm/drm_modeset_helper.c

index e54584da4c3d..4c59e82af2f1 100644
--- a/drivers/gpu/drm/drm_modeset_helper.c
+++ b/drivers/gpu/drm/drm_modeset_helper.c
@@ -23,6 +23,7 @@
  #include <linux/export.h>
    #include <drm/drm_atomic_helper.h>
+#include <drm/drm_blend.h>
  #include <drm/drm_client_event.h>
  #include <drm/drm_fourcc.h>
  #include <drm/drm_framebuffer.h>

@@ -160,6 +161,17 @@ int drm_crtc_init(struct drm_device *dev, struct drm_crtc *crtc,

       */
      primary->format_default = true;
  +    /*
+     * safe_modeset_formats[] includes ARGB8888, which has an alpha
+     * channel. Advertise the blend mode userspace has always assumed
+     * for such planes when the "pixel blend mode" property was absent,
+     * to satisfy drm_mode_config_validate().
+     */
+    ret = drm_plane_create_blend_mode_property(primary,
+                           BIT(DRM_MODE_BLEND_PREMULTI));
+    if (ret)
+        goto err_drm_plane_cleanup;
+

      ret = drm_crtc_init_with_planes(dev, crtc, primary, NULL, funcs, NULL);

      if (ret)
          goto err_drm_plane_cleanup;