On db845c, we have seen an odd behavior with modal dialogs in AOSP, where the background which is normally darkened, was rendering as black, though it would occasionally flicker to the right thing. This cropped up after alpha support on planes landed in 5.15, so we assumed it was an edge case issue with the dpu1 driver.
But while working on a separate issue seen on x86 hardware: #46 (closed)
Roman noticed drm_hwcomposer always sets the blending type to NONE for the client layer. Which had gone un-noticed becase he didn't have any devices that exposed the NONE type.
Thus Roman implemented this patch, which sets up the client blend type to PREMULTIPLIED.
While it did not resolve the x86 issue above, it does resolve the incorrect rendering seen on db845c with alpha support enabled.