The Pi is running on a set top box chip that has been "graciously" thrown over the wall by Broadcom. Broadcom are not friends of the open source community.

>These manufacturers should be developing proper GPU drivers for mainline with full KMS/DRM/mesa support before they even sell their boards to the public claiming Linux support.

Broadcom or the Pi Foundation? One doesn't care and one doesn't have the resources.

At least the Raspberry PI has documentation for its GPU.

The rant should be directed at ARM itself for not providing documentation for Mali.

There is a mainline driver for the RPi with full Mesa support. On e.g. Arch Linux ARM, you can just install a wayland compositor and run it.

It's just the conservatism of distros like Raspbian. 32-bit, old kernel, proprietary blobs, old packages (stable debian).

Some clones are better than others. This kernel https://github.com/TinkerBoard/debian_kernel supports DRM/KMS quite well.

Don’t know about Linux desktop, but for my embedded use case where I build stuff directly on top of drm, kms and gles, it works fine driving 2 displays, one of them is 4k.