Messages in this thread | | | Date | Tue, 7 Jul 2020 08:11:09 -0700 | From | Jonathan Lemon <> | Subject | Re: [PATCH 1/4] dma-mapping: add a new dma_need_sync API |
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On Tue, Jul 07, 2020 at 08:47:30AM +0200, Christoph Hellwig wrote: > 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.
My use case is setting up a pinned memory area, and caching the dma mappings. I'd like to bypass storing the DMA addresses if they aren't needed. For example:
setup() { if (dma_need_sync(dev, addr, len)) { kvmalloc_array(...) cache_dma_mappings(...) }
dev_get_dma(page) { if (!cache) return page_to_phys(page)
return dma_cache_lookup(...)
The reason for doing it this way is that the page in question may be backed by either system memory, or device memory such as a GPU. For the latter, the GPU provides a table of DMA addresses where data may be accessed, so I'm unable to use the dma_map_page() API. -- Jonathan
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