Unfortunately most mobile devices are not supported by mainline Linux (N900 is best supported, but not fully), usually only server/desktop/laptop vendors have any incentive to mainline their drivers.
The 2013 Nexus 7 was, at some point, 50 patches away from running mainline [1]. I know, single datapoint, but it gives me hope that there's some way out of the tabletification of computing.
https://wiki.debian.org/DebianKernel/ARMMP
Unfortunately most mobile devices are not supported by mainline Linux (N900 is best supported, but not fully), usually only server/desktop/laptop vendors have any incentive to mainline their drivers.