On Tue, Jul 07, 2020 at 08:11:09AM -0700, Jonathan Lemon wrote:

quoted

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.