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

[Jonathan Lemon]

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