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Overlay plane alpha channel blending is incorrect

Brief summary of the problem:

Setup KMS to scanout a solid green buffer on the primary plane and the following image on the overlay plane:

gradient

Expected result:

out

Actual output:

pic

Even if the alpha in the gradient is linear, the hw has some sort of cut-out value: below a certain threshold pixels are completely transparent even though they have alpha != 0.

Hardware description:

  • CPU: Zen 2
  • GPU: 04:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] VanGogh (rev ae)
  • System Memory: N/A
  • Display(s): N/A
  • Type of Display Connection: eDP

System information:

  • Distro name and Version: ArchLinux
  • Kernel version: 5.13.0
  • Custom kernel: agd5f/amd-staging-drm-next
  • AMD package version: N/A

How to reproduce the issue:

Run tentative with the following config (may need to tweak the CRTC/plane IDs for your system):

# primary
plane 55 {
	CRTC_ID crtc 68
	CRTC_X value 0
	CRTC_Y value 0
	FB_ID fb {
		size 800x1280
		fill_color "#00FF00"
	}
}

# overlay
plane 60 {
	CRTC_ID crtc 68
	CRTC_X value 100
	CRTC_Y value 200
	CRTC_W value 256
	CRTC_H value 256
	SRC_X value 0
	SRC_Y value 0
	SRC_W value 256
	SRC_H value 256
	FB_ID fb {
		size 256x256
		fill_file gradient.ARGB8888-256x256.raw
		format ARGB8888
	}
}

Raw buffer used for the overlay: gradient.ARGB8888-256x256.raw (obtained with: contrib/drm-convert /path/to//gradient.png ARGB8888 run in the tentative source dir)