[openib-general] [PATCH v2 1/7] IB/core - Add DMA mapping functions to allow device drivers to interpose

[Ralph Campbell]

On Wed, 2006-12-06 at 13:33 +0200, Or Gerlitz wrote:
> Ralph Campbell wrote:
> > On Tue, 2006-12-05 at 23:21 +0200, Or Gerlitz wrote:
> >> On 12/5/06, Roland Dreier <rdreier at cisco.com> wrote:
> > I am not following what you two are saying.
> 
> > The ib_dma_mapping_ops functions as implemented by ib_ipath,
> > are redefining dma_addr_t as a kernel virtual address.
> > When ib_dma_map_single() is called, this is a NOP.
> > When ib_dma_map_sg() is called, the dma_map_sg() replacement needs
> > to convert a struct page pointer into a kernel virtual address.
> > When CONFIG_HIGHMEM is defined, some pages may not be mapped
> > into the kernel virtual address space so the driver needs to
> > call kmap().  Since the driver can't use the struct scattergather
> > to store the kmap() result, a separate table needs to be used
> > so the value can be returned by ib_sg_dma_address().
> 
> Indeed.
> 
> > Doing kmap_atomic() at the point where the kernel virtual
> > address is used is not practical since the driver is not
> > mapping dma_addr_t to struct page * although it is
> > possible to write it that way.  It would mean that
> > ib_map_single() would then be more complex in that a
> > kernel virtual address would need to be converted to a
> > struct page *.
> 
> Basically what Roland suggest is that you need to implement SW IOTLB 
> mapping from dma_addr_t (possibly offset-ed) to kv addr. And do the 
> actual kmap/unmap calls before/after you must touch the data.
> 
> Is this impossible?
> 
> Or.

It is not impossible, just inefficient.  Why add a mapping
table when it isn't needed?  If I needed to implement HIGMEM
support, I would probably make "dma_addr_t" be a physical
memory address, convert to PFN, find the struct page pointer,
and call kmap_atomic() or page_address().  Why go though all
that in the worst case CPU path when doing the conversion
to kernel virtual address outside the critical path is
feasible?