Since last year there has been basic overclocking support in the AMDGPU DRM driver that's the kernel driver to all newer Radeon GPUs -- GCN 1.2+ and optionally the experimental GCN 1.0/1.1 support. With there not yet being a "Catalyst Control Center" / "Radeon Software Settings" area for the modern AMD Linux graphics driver, the overclocking needs to be done from the command-line.
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