Re: [RFC] generic device DMA implementation

[Miles Bader]

David Gibson <[e-mail]> writes:
> >   * pci_map_single allocates a `shadow area' of consistent memory and
> >     pci_unmap_single deallocates it
> 
> That's a little misleading: all your memory is consistent, the point
> is that it is a shadow area of PCI-mappable memory.

Well I suppose that's true if you're using the term in a cache-related
sense (which I gather is the convention).

OTOH, if you think about it from the view point of the PCI framework,
the terminology actually does make sense even in this odd case --
`consistent' memory is indeed consistent (both CPU and device see a
single image), but other memory used to communicate with the driver is
`inconsistent' (CPU and device see different things until a sync
operation is done).

> The issue is that there are other constraints for DMAable memory and
> you want drivers to be able to easily mallocate with those
> constraints.
> 
> Actually, it occurs to me that PC ISA DMA is in a similar situation -
> there is a constraint on DMAable memory (sufficiently low physical
> address) which has nothing to do with consistency.

Indeed.  What I'm doing is basically bounce-buffers.

-Miles
-- 
`The suburb is an obsolete and contradictory form of human settlement'