Besides being very curious about the Opteron A1100 Seattle performance and hoping to be able to benchmark one soon, I've also been curious about the mainline kernel support for this 64-bit ARM SoC. Unfortunately, not all of the bits appear in place yet for running on a mainline kernel.
There have been some Seattle commits in the months prior to the mainline kernel such as for the amd-xgbe network driver, PCI changes, initial Seattle platform support, etc. However, posted just today was a patch on the mailing list for the AMD Seattle platform SATA controller. This AHCI Seattle driver is coming just shy of being able to make the Linux 4.5 kernel merge window.
Without the hardware in hand or any fresh source of information for details on the Linux kernel's support for Seattle, I'm afraid I can't comment much more at the moment. Then again, this isn't unusual in the ARM space with not all vendors working toward mainline platform/board support in a timely manner. Sadly, there's also been nothing to report on Coreboot support for the A1100, similar to the uncertain Coreboot future for Zen.
More A1100 Opteron Linux details as they become available... The general SoC details can be found via AMD.com.