Re: [PATCH 03/25] dma-direct: take dma-ranges/offsets into account in resource mapping

[Serge Semin <fancer.lancer@gmail.com>]

On Wed, Apr 20, 2022 at 10:47:46AM +0200, Christoph Hellwig wrote:
> I can't really comment on the dma-ranges exlcusion for P2P mappings,
> as that predates my involvedment, however:

My example wasn't specific to the PCIe P2P transfers, but about PCIe
devices reaching some platform devices over the system interconnect
bus.

> 
> On Wed, Apr 20, 2022 at 11:32:07AM +0300, Serge Semin wrote:
> > See, if I get to map a virtual memory address to be accessible by any
> > PCIe peripheral device, then the dma_map_sg/dma_map_page/etc
> > procedures will take the PCIe host controller dma-ranges into account.
> > It will work as expected and the PCIe devices will see the memory what
> > I specified. But if I get to pass the physical address of the same
> > page or a physical address of some device of the DEVs space to the
> > dma_map_resource(), then the PCIe dma-ranges won't be taken into
> > account, and the result mapping will be incorrect. That's why the
> > current dma_map_resource() implementation seems very confusing to me.
> > As I see it phys_addr_t is the type of the Interconnect address space,
> > meanwhile dma_addr_t describes the PCIe, DEVs address spaces.
> > 
> > Based on what I said here and in my previous email could you explain
> > what do I get wrong?
> 

> You simply must not use dma_map_resource for normal kernel memory.
> So while the exclusion might be somewhat confusing, that confusion
> really should not matter for any proper use of the API.

What if I get to have a physical address of a platform device and want
have that device being accessed by a PCIe peripheral device? The
dma_map_resource() seemed very much suitable for that. But considering
what you say it isn't.

-Sergey