Subject: Re: On the subject of bus_dma(9)
To: Matthew Jacob <>
From: Jason R Thorpe <>
List: tech-kern
Date: 03/07/2001 10:14:29
On Wed, Mar 07, 2001 at 08:55:33AM -0800, Matthew Jacob wrote:

 > I provided input on this, and there were many things you didn't agree with.
 > One of them was the Solaris DDI DMA model differences which, btw, handles this
 > stuff fine.

Where is the documentation on this interface that I can read?

 > >  > 3. Therefore, bus_dmamem_alloc'd memory must be assumed to be for
 > >  > BUS_DMA_COHERENT purposes because there may be no way to 'change'
 > >  > the identity of the memory alloc'd in bus_dmamem_alloc (it might be
 > >  > from a pool that *can't* be made byte-coherent).
 > > 
 > > bus_dmamem_alloc() for a particular bus_dma_tag_t *should never* allocate
 > > from memory which can not be made to DTRT for a particular bus.  All this
 > > does it point to a bug in the sparc64 bus_dmamem_alloc() routines.
 > No - this is inflammatory, and you should be ashamed for saying it.

What?  Nonsense.  If the memory returned by a bus_dmamem_alloc()
can't be DMA map'd in a coherent fashion *and* `sync' operations
can't work on that memory region, then I am 100% correct in saying
that bus_dmamem_alloc() failed to allocate "DMA safe memory" (which
is what the manual page documents it as allocating).

Let me clarify -- by definition of what bus_dmamem_alloc() is, it MUST
return memory for which at least one of the following statements is true:

	(1) The memory is DMA coherent.

	(2) Sync operations ensure that the device sees the correct
	    version of the page.

If neither of those are true for a region that bus_dmamem_alloc() returns,
then the region is *not* "DMA safe memory", and therefore bus_dmamem_alloc()
is buggy.

 > And, btw, just because you haven't handled peer-peer- you might at least
 > document the issue.

Well, "peer-peer" needs to be better-defined, as well.  If you're talking
about DMA'ing into a host memory region that another device accesses, then
"DMA safe memory" should pretty well take care of it.  If you're talking
about DMA'ing into a memory region that is present in *bus space* (e.g.
a frame grabber DMA'ing into the frame buffer on a graphics adapter), that's
a whole different issue (and a fairly complicated one).

 > So- please reconsider letting me update the man page with some clarifications.

Once I collect all the clarifications that need to be made (I kind of have
a running list going), I'll be happy to make the updates.

        -- Jason R. Thorpe <>