You may have missed this news about AMD in amongst the Mantle announcements, support has been added for the VCE2 hardware encoding engine on newer AMD GCN based GPUs.  The open-source Radeon driver now supports GStreamer OpenMAX which can speed H.264 encoding in general but is truly optimized for encoding for mobile devices.  The current release is still a work in progress, the official release will come soon and you can track the progress by signing up to the mailing lists mentioned by Phoronix.  This is good news as previously the only open source hardware accelerated encoding was through Intel's GPU and VA-API.

"AMD is doing another large and important open-source graphics driver code drop this morning. This morning AMD is publishing their VCE code that allows for hardware-based video encoding. "

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