[RFC] change the dma_unmap_single for arm

[Russell King - ARM Linux]

On Wed, Oct 27, 2010 at 05:47:11PM -0400, Nicolas Pitre wrote:
> On Wed, 27 Oct 2010, Russell King - ARM Linux wrote:
> 
> > On Wed, Oct 27, 2010 at 04:01:37PM +0200, saeed bishara wrote:
> > > Hi,
> > >       latest versions invalidates the buffer in order to overcome the
> > > speculative prefetch. however, in some cases, specially network, the
> > > data that DMAed can be much less that the buffer size, so it would be
> > > more efficient - and correct for arm case - to use the received bytes
> > > count rather than the buffer size. however, the linux API restricts
> > > the size to be the same as the one passed to the dma_map_single -
> > > which is the buffer size.
> > >     the simplest solution that I can think of is to change the API
> > > restriction, and make the drivers pass the received bytes count
> > > instead of buffer size, the kernel also will provide a function to the
> > > drivers  to permit using this hack.
> > 
> > The reason this isn't done is because some platforms have IOMMUs
> > which have to be programmed (or deprogrammed) appropriately.  For
> > this, it is important that the parameters are the same as the
> > original map request.
> > 
> > Relaxing this restriction makes the API much more complex, and will
> > probably cause additional pain to debug where people 'forget' to
> > use the right buffer size(s).
> 
> What about an additional call for the unmap case where both the original 
> buffer size and the actual used size are provided?  Simple sanity check 
> would test for original_size >= used_size.  IOMMU would care about 
> original_size while cache maintenance cares about used_size.

Maybe, but it needs to be discussed more widely than just on lakml.