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Date: Tue, 3 Oct 2017 22:27:08 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: David Laight <David.Laight@ACULAB.COM>
cc: 'Christoph Hellwig' <hch@lst.de>,
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Subject: RE: [PATCH 02/11] x86: make dma_cache_sync a no-op
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On Tue, 3 Oct 2017, David Laight wrote:

> From: Christoph Hellwig
> > Sent: 03 October 2017 11:43
> > x86 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.
> 
> I believe it is just about possible to require an explicit
> write flush on x86.
> ISTR this can happen with something like write combining.

As the changelog says: x86 only implements dma_alloc_coherent() which as
the name says returns coherent memory, i.e. type WB (write back), which is
not subject to dma_cache_sync() operations.

If the driver converts that memory to WC (write combine) on its own via
PAT/MTRR, then it needs to take care of flushing the write buffer on its
own. It's not convered by this interface.

Thanks,

	tglx