Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

[Christoph Hellwig]

On Thu, Nov 29, 2018 at 05:33:03PM +0000, Brian Starkey wrote:
> This sounds very useful for ion, to avoid CPU cache maintenance as
> long as the buffer stays in device-land.
> 
> One question though: How would you determine "the last user to unmap"
> to know when to do the final "make visible to CPU" step?

I'd assume the user of the DMA API keeps track of it.  E.g. dmabuf
would be a pretty natural layer to implement common logic for this.