A GFP_KERNEL request is safely implemented as GFP_ATOMIC as long as the caller 
   checks return for NULL.  for dma_alloc_coherent return checking is a 
   requirement because the system may return NULL anyway if it is out of mappings 
   even with a GFP_KERNEL flag.
   
Now what about the corollary, a platform that needs
to sleep to setup the cpu mapings in a race-free manner?
It could not ever honor GFP_ATOMIC in that case.
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