Re: [PATCH 1/4] dma-mapping: add a new dma_need_sync API

[Christoph Hellwig]

On Mon, Jul 06, 2020 at 12:42:27PM -0700, Jonathan Lemon wrote:
> On Mon, Jun 29, 2020 at 03:03:56PM +0200, Christoph Hellwig wrote:
> > Add a new API to check if calls to dma_sync_single_for_{device,cpu} are
> > required for a given DMA streaming mapping.
> > 
> > +::
> > +
> > +	bool
> > +	dma_need_sync(struct device *dev, dma_addr_t dma_addr);
> > +
> > +Returns %true if dma_sync_single_for_{device,cpu} calls are required to
> > +transfer memory ownership.  Returns %false if those calls can be skipped.
> 
> Hi Christoph -
> 
> Thie call above is for a specific dma_addr.  For correctness, would I
> need to check every addr, or can I assume that for a specific memory
> type (pages returned from malloc), that the answer would be identical?

You need to check every mapping.  E.g. this API pairs with a
dma_map_single/page call.  For S/G mappings you'd need to call it for
each entry, although if you have a use case for that we really should
add a dma_sg_need_sync helper instea of open coding the scatterlist walk.