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DmaMap () operations of type MapOperationBusMasterCommonBuffer should return a mapping that is coherent between the CPU and the device. For this reason, the API only allows DmaMap () to be called with this operation type if the memory to be mapped was allocated by DmaAllocateBuffer (), which in this implementation guarantees the coherency by using uncached mappings on the CPU side. This means that, if we encounter a cached mapping in DmaMap () with this operation type, the code is either broken, or someone is violating the API, but simply proceeding with a double buffer makes no sense at all, and can only cause problems. So instead, actively reject this operation type for cached memory mappings. Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>