One area that particularly stands out for Linux 5.15 are all of AMD's upstream contributions that happened to make it in this kernel. There is a lot of new enablement on the AMD side -- both for CPUs and Radeon graphics -- but also improving existing hardware support.
Basically, a lot is going on in the AMD Linux space at the moment. Linux 5.15-rc1 should be out this evening while the stable kernel release will be out about two months from now.
See Linux 5.15 Is A Very Exciting Kernel For AMD
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