Linux 4.9 presents a lot to be thankful for in the open-source community, but when it comes to features we would have liked to see but didn't, here are some of them:
AMDGPU DAL - AMD's display abstraction layer code sadly isn't yet in the mainline kernel for AMDGPU DRM. This means that the latest AMD GPUs still don't have HDMI audio support, HDMI 2.0, FreeSync / Adaptive-Sync, and other modern display features being held up by DAL not yet being ready for mainline. It's still not clear when it will be ready for mainline but can at least be used by those with AMDGPU-PRO or building their own kernel off some old DAL staging code.
Turbo Boost Max 3.0 - The new Turbo Boost support for select Intel CPUs (e.g. Broadwell-E) isn't yet mainline for offering optimal performance. Patches are available and it looks like it might come for Linux 4.10, a number of months after the TBM 3.0 CPUs began appearing.
Nouveau Re-Clocking / Boost - The "boost patches" for hitting boost frequences on newer NVIDIA GPUs were queued up in Nouveau DRM, but that tree wasn't pulled for Linux 4.9. Linux 4.9 didn't end up seeing any Nouveau changes. Thus the open-source NVIDIA driver performance is still poor overall.
NVIDIA GTX 1000 Series 3D - Likewise, Nouveau doesn't yet have any support for the consumer NVIDIA GTX Pascal cards... This is being held up by NVIDIA not yet releasing the needed signed firmware images for the GTX 1050/1060/1070/1080 so there can be open-source NVIDIA driver support.
Btrfs RAID 5/6 Woes - There's yet to be a rework of the faulty RAID 5/6 Btrfs code. For now it's best not to use those Btrfs RAID levels on a production machine.
LLVM Clang Building - The mainline kernel still can't build cleanly under the LLVM Clang compiler. The LLVMLinux project slowed down a lot but hopefully developers will get back to working on this support in the not too distant future.
Better WiFi Support - There are some individual WiFi driver improvements in Linux 4.9, but "make WiFi fast" improvements are coming beginning with Linux 4.10.
ZFS - Of course, still held up by the ZFS file-system code being CDDL-licensed so it can't be merged into the mainline Linux kernel tree, but we can still hold out hope.... Right? At least the ZFSOnLinux (ZOL) project continues to thrive and provide promptly updated out-of-tree modules for new kernel releases.
Reiser4 - Since mentioning ZFS, might as well mention Reiser4. At least there's no licensing issues preventing Reiser4 from potentially merging... Reiser4 is still alive, just with very limited development resources. Just recently Reiser4 implemented mirror and failover support.
Bcachefs - Arguably much more promising than Reiser4 is Bcachefs as the file-system born out of the BCache project. Bcachefs is still in development and will hopefully be ready for merging into a future kernel release, maybe in 2017?
Any other features you would have liked to see in the mainline Linux 4.9 kernel? Share your thoughts by commenting on this article.