Re: [PATCH] dma: add new dma_mapping_ops API sync_page

[Remi Machet <rmachet@slac.stanford.edu>]

On Fri, 2008-10-10 at 14:46 +1100, Benjamin Herrenschmidt wrote: 
> > > A virtual address will typically be needed to perform the flush; why 
> > > pass the bus address?
> 
> > Because it is a sync API. You want to make sure that a physical memory
> > area is in sync with the caches, not the virtual address. This
> > distinction can become important in the event where the page is mapped
> > multiple times in the memory and the architecture does not take care of
> > synchronizing the multiple mapping, the dma_mapping_ops code should be
> > able to synchronize the multiple mapping. In most case it would be easy
> > of course to go from virtual address to the page address, but not if the
> > page is in high memory ...
> 
> Well, not really. IE, you are right that a dma_addr_t or a struct page
> is the way to go but for the wrong reasons :-)
> 
> All mappings should be coherent. The powerpc architecture is pretty
> strict with that. The only known violation is the instruction cache on
> 44x but that's irrelevant to your problem.
> 
> Thus, -any- virtual address will do.
Good! That certainly simplify the code.

> However, you may not have a virtual address. You may get into a
> situation where the page isn't in the linear mapping and needs to be
> kmap'ed for the sync to happen.
> 
> Now, using PCI_DRAM_OFFSET in bus_to_page() is incorrect here with
> Becky's new set of changes. You need to get the offset properly using
> the accessor she provides (I don't have the name off the top of my
> head).
> 
Totally agree with that. In the last set of patch I committed (which I
need to re-commit because I need to use vmalloc in dma-noncoherent.c) I
removed most of the reference to PCI_DRAM_OFFSET. The only reference to
it remaining is in virt_to_bus which is called by dma_cache_sync.

I did not see an accessor that can be used in dma-mapping.h (the
accessor API I have seen is private to dma.c and dma-noncoherent.c), I
would be happy to use it if there really is one though. I could add
another 2 APIs to dma_mapping_ops which converts a page to/from its bus
address, what do you think?

Remi