Re: [RFC PATCH 1/3] dma-mapping: introduce a new dma api dma_addr_to_phys_addr()

[Robin Murphy]

On 2019-10-24 8:49 am, Laurentiu Tudor wrote:
> On 24.10.2019 05:01, hch@xxxxxx wrote:
> > On Wed, Oct 23, 2019 at 11:53:41AM +0000, Laurentiu Tudor wrote:
> > > We had an internal discussion over these points you are raising and
> > > Madalin (cc-ed) came up with another idea: instead of adding this prone
> > > to misuse api how about experimenting with a new dma unmap and dma sync
> > > variants that would return the physical address by calling the newly
> > > introduced dma map op. Something along these lines:
> > >     * phys_addr_t dma_unmap_page_ret_phys(...)
> > >     * phys_addr_t dma_unmap_single_ret_phys(...)
> > >     * phys_addr_t dma_sync_single_for_cpu_ret_phys(...)
> > > I'm thinking that this proposal should reduce the risks opened by the
> > > initial variant.
> > > Please let me know what you think.
> >
> > I'm not sure what the ret is supposed to mean, but I generally like
> > that idea better.
>
> It was supposed to be short for "return" but given that I'm not good at
> naming stuff I'll just drop it.

Hmm, how about something like "dma_unmap_*_desc" for the context of the 
mapped DMA address also being used as a descriptor token?

> > We also need to make sure there is an easy way
> > to figure out if these APIs are available, as they generally aren't
> > for any non-IOMMU API IOMMU drivers.
>
> I was really hoping to manage making them as generic as possible but
> anyway, I'll start working on a PoC and see how it turns out. This will
> probably happen sometime next next week as the following week I'll be
> traveling to a conference.

AFAICS, even a full implementation of these APIs would have to be 
capable of returning an indication that there is no valid physical 
address - e.g. if unmap is called with a bogus DMA address that was 
never mapped. At that point there'sseemingly no problem just 
implementing the trivial case on top of any existing unmap/sync 
callbacks for everyone. I'd imagine that drivers which want this aren't 
likely to run on the older architectures where the weird IOMMUs live, so 
they could probably just always treat failure as unexpected and fatal 
either way.

In fact, I'm now wondering whether it's likely to be common that users 
want the physical address specifically, or whether it would make sense 
to return the original VA/page, both for symmetry with the corresponding 
map calls and for the ease of being able to return NULL when necessary.

Robin.