What's up with cpu_is_noncoherent_r10000() ?

[David Daney <ddaney@avtrex.com>]

I am bringing up the git HEAD on an old ATI Xilleon X226.  This nice 
system claims to be 4KEc, but for some reason doesn't support mips32r2, 
but I digress.

Among its other problems this is a CONFIG_DMA_NONCOHERENT system, so 
drivers like net/e100.c do not function properly if the cache is not 
appropriately flushed/invalidated when they are doing DMA.  Fortunately 
the authors of said driver have used 
pci_dma_sync_single_for_{cpu,device} in what seems like the appropriate 
  manner.

pci_dma_sync_single_for_device() ends up in dma_sync_single_for_device() 
(in mm/dme-default.c) and is does the cache flush as expected.  The 
problem is with dma_sync_single_for_cpu() which for some reason only 
does the cache flush/invalidate  if cpu_is_noncoherent_r10000() returns 
true (which it does only for R10K CPUs).  When I hack it up so that it 
returns true unconditionally, e100 starts functioning normally for me. 
This leads me to think that the cache operation should be done for all 
CONFIG_DMA_NONCOHERENT systems not just R10K based systems.

What is the reasoning for only doing the cache operation on  R10K based 
systems?

David Daney