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

[Christophe LEROY]

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
> worked.

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.

Christophe