Re: [PATCH 07/11] powerpc: make dma_cache_sync a no-op

From: Christophe LEROY
Date: Wed Oct 04 2017 - 04:29:22 EST

Le 03/10/2017 Ã 13:43, Christoph Hellwig a ÃcritÂ:
On Tue, Oct 03, 2017 at 01:24:57PM +0200, Christophe LEROY wrote:
powerpc does not implement DMA_ATTR_NON_CONSISTENT allocations, so it
doesn't make any sense to do any work in dma_cache_sync given that it
must be a no-op when dma_alloc_attrs returns coherent memory.
What about arch/powerpc/mm/dma-noncoherent.c ?

Powerpc 8xx doesn't have coherent memory.

It doesn't implement the DMA_ATTR_NON_CONSISTENT interface either,
so if it really doesn't have a way to provide dma coherent allocation
(although the code in __dma_alloc_coherent suggests it does provide
dma coherent allocations) I have no idea how it could ever have

Yes indeed it provides coherent memory by allocation non cached memory.

And drivers aiming at using non coherent memory do it by using kmalloc() with GFP_DMA then dma_map_single().
Then they use dma_sync_single_for_xxx(), which calls __dma_sync() on the 8xx and is a nop on other powerpcs.