bus_dma (9). What exactly means "Loading of memory allocation" ?

[John Baldwin]

On Monday 02 February 2009 11:23:33 am Alexej Sokolov wrote:
> Hi,
> thanx for your answer.  I checked the source code of the *dma() functions.
> If I understand it correctly, "loading of memory allocation" means the
> following:
> 
> 1. At first memory allocation should be done: bufp = *alloc(sizeof ....)
> 2. then in ... _bus_dmamap_load_buffer() we get physical addres of allocated
> buffer:
>      if (pmap)
>                  curaddr = pmap_extract(pmap, vaddr);
>      else
>                  curaddr = pmap_kextract(vaddr);
> 
> ... then some "magic" with bouncing
> 
> 3. then physical address will passed to dmat->segments
>     segs[seg].ds_addr = curaddr;
>     segs[seg].ds_len = sgsize;
> 
> Ok, it all means: getting of physical address of allocated buffer. If
> physical space not accessble for device, allocating bounce buffers. Getting
> of physical addresses of allocated buffers. And then put these physical
> addresses and sizes of buffers in  dmat->segments array. <- loading of
> memory allocation (-:

Yes.  On architectures with an IOMMU, the load may also program entries into 
the IOMMU for the specified buffer and then populate the S/G array with the 
associated DMA addresses (sparc64 uses this).  I think the "load" name has 
more to do with this case in that you are "loading" a buffer into the DMA 
virtual address space (with IOMMUs you have a separate virtual address space 
for DMA that is not 1:1 with physical addresses as on i386 machines).  If you 
look at bus_dma as basically implementing an abstract IOMMU on all 
architectures then it might make a bit more sense.

-- 
John Baldwin