In the past, -mm tree were used to also test subsystem patches, but this
function is now done via the
-`linux-next <https://www.kernel.org/doc/man-pages/linux-next.html>`
+`linux-next` (https://www.kernel.org/doc/man-pages/linux-next.html)
tree. The Subsystem maintainers push their patches first to linux-next,
and, during the merge window, sends them directly to Linus.