[Fwd: Re: use of bus_dmamap_sync]

[John Baldwin]

On Wednesday 26 October 2005 04:47 am, Dinesh Nair wrote:
> On 10/26/05 10:39 Scott Long said the following:
> > Apparently the original poster sent his question to me in private, then
> > sent it again to the mailing list right as I was responding in private.
>
> apologies on that, scott. an initial search only turned up your message in
> the archives, but spreading it wider (not confining the google to
> lists.freebsd.org) brought up more hits, and that made me post it into
> -hackers.
>
> do bear with me as i try to understand this.
>
> > Below is my response.  Note that I edited it slightly to fix an error
> > that I found
> >
> >       bus_dmamap_sync(tag, map, BUS_DMASYNC_PREREAD);
> >       Ask hardware for data
> >       bus_dmamap_sync(tag, map, BUS_DMASYNC_POSTREAD);
> >
> >>     read from readbuf (i'm assuming that device has put data in
> >>                readbuf)
> >>     POSITION B
> >> }
>
> in other words, the PREREAD/POSTREAD wrap around the device's access to
> memory, and not the CPU's ?

Yes, scott's notes are more correct than mine here.

> >       bus_dmamap_sync(tag, map, BUS_DMASYNC_PREWRITE);
> >       notify hardware of the write
> >       bus_dmamap_sync(tag, map, BUS_DMASYNC_POSTWRITE);
> >
> > The point of the syncs is to do the proper memory barrier and cache
> > coherency magic between the CPU and the bus as well as do the memory
> > copies for bounce buffers.  If you are dealing with statically mapped
> > buffers, i.e. for an rx/tx descriptor ring, then you'll want code
>
> however, reading thru the syscall code, bus_dmamem_alloc() sets the dmamap
> to NULL, and if it's null, bus_dmamap_sync() is not called at all. would
> this mean that if memory is allocated by bus_dmamem_alloc(), it does not
> need to be synced with bus_dmamap_sync() ?

Perhaps on i386.  Each arch implements sync().  Argh, it does look like the 
memory barriers needed on e.g., Alpha aren't used with static buffers because 
of the map != NULL check in sys/busdma.h.  *sigh*  I guess archs that need 
membars even without bounce buffers need to always allocate and setup a 
bus_dmamap.  None of that matters for i386 though.

-- 
John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org