Re: [RFC] dma: passing "attributes" to dma_map_* routines

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2007-12-18 at 12:07 -0800, akepner@sgi.com wrote:
> On Tue, Dec 18, 2007 at 05:50:42PM +0100, Stefan Richter wrote:
> 
> > Do I understand correctly?:  A device and the CPUs communicate via two
> > separate memory areas:  A data buffer and a status FIFO.  The NUMA
> > interconnect may reorder accesses of the device to the areas.  (Write
> > accesses?  Read accesses?  Both?)
> 
> Yes, I think you understand. Reorderings are possible on reads and
> writes. Things get synced up by either an interrupt or a write to
> a memory region with a "barrier attribute". Memory allocated with
> dma_alloc_coherent() gets the barrier attribute. The idea here is
> to allow memory allocated with plain old malloc() or whatever to
> get the same attribute.
> 
> > 
> > To ensure synchronization between device and CPUs, you want to mark a
> > memory area which is to be dma-mapped with a flag which says:  "Writes
> > to the memory region will cause in-flight DMA to be flushed".  Whose
> > writes?  A write access from the device or a write access from a CPU?
> 
> A write from the device, e.g., when the device writes to indicate
> "data DMA is complete".

Can't you just have a primitive to sync things up that you call
explicitely from your driver after fetching a new status entry ?

Ben.