drm/i915/guc/slpc: Allow SLPC to use efficient frequency
Host Turbo operates at efficient frequency when GT is not idle unless the user or workload has forced it to a higher level. Replicate the same behavior in SLPC by allowing the algorithm to use efficient frequency. We had disabled it during boot due to concerns that it might break kernel ABI for min frequency. However, this is not the case since SLPC will still abide by the (min,max) range limits. With this change, min freq will be at efficient frequency level at init instead of fused min (RPn). If user chooses to reduce min freq below the efficient freq, we will turn off usage of efficient frequency and honor the user request. When a higher value is written, it will get toggled back again. The patch also corrects the register which needs to be read for obtaining the correct efficient frequency for Gen9+. We see much better perf numbers with benchmarks like glmark2 with efficient frequency usage enabled as expected. v2: Address review comments (Rodrigo) v3: with efficient frequency being dynamic, it is possible that the req frequency may go beyond max freq. This will cause SLPC selftests to fail. Add a FIXME there to start the test with [RPn, RP0] instead and restore it afterwards. BugLink: https://gitlab.freedesktop.org/drm/intel/-/issues/5468 Cc: Rodrigo Vivi <rodrigo.vivi@intel.com> Signed-off-by:Vinay Belgaumkar <vinay.belgaumkar@intel.com> Signed-off-by: