On Tue, Jul 07, 2020 at 08:11:09AM -0700, Jonathan Lemon wrote:
> > 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.
dma_need_sync doesn't tell you if the unmap needs the dma_addr_t.
I've been think about replacing CONFIG_NEED_DMA_MAP_STATE with a runtime
for a while, which would give you exattly what you need. For now it
isn't very useful as there are very few configs left that do not have
CONFIG_NEED_DMA_MAP_STATE set.
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