New USB stack and Zero copy.

[John Baldwin]

On Friday 06 July 2007 02:59:39 am Hans Petter Selasky wrote:
> On Friday 06 July 2007 01:35, John Baldwin wrote:
> > On Thursday 05 July 2007 04:25:17 pm John Baldwin wrote:
> > > On Thursday 05 July 2007 03:31:59 am Hans Petter Selasky wrote:
> > > > On Wednesday 04 July 2007 19:35, John-Mark Gurney wrote:
> > > > > Hans Petter Selasky wrote this message on Wed, Jul 04, 2007 at 09:01
> > >
> > > +0200:
> > > > > > Also: How is the easiest way to load memory pages into DMA ? And I
> >
> > want
> >
> > > > > > that the loadig works like this, that when the page must be 
bounced
> > > > > > it should not allocate a bounce buffer, hence I already have a
> > > > > > bounce buffer. I only need to know which pages I can forward
> > > > > > directly to the
> > >
> > > USB
> > >
> > > > > > hardware, and the rest I will bounce somewhere else.
> > > > >
> > > > > Why do you not want to let bus_dma do the bouncing for you?  If it's
> > > > > to save a copy to another buffer, why don't you load the final 
buffer
> > > > > into bus_dma?
> > > >
> > > > Because if I let bus_dma do the bounching, I cannot do this while
> > > > holding
> >
> > a
> >
> > > > mutex, hence allocating DMA'able memory on the fly is not so good.
> > >
> > > This is not a hard problem to solve, every other driver using bus_dma
> > > solves it.  Just make sure your driver is in a sane state and drop the
> > > lock while you let bus_dmamap_load() map/copy things for you.
> >
> > Bah, backwards (was thinking of the fact that if you get EINPROGRESS you
> > will have to drop the lock and just wait until the callback is called to
> > make further progress on the request).  bus_dmamap_load() already
> > _requires_ you to hold your mutex when you call it, so I don't really see
> > what the issue is, unless you are assuming BUS_DMA_NOWAIT behavior and
> > can't properly handle deferred callbacks.  You do have to drop the lock
> > around bus_dmamem_alloc(), but not bus_dmamap_load().
> 
> The thing is that allocating memory on the fly will be slow, and especially 
> when the xxxx_load() functions will allocate contiguous memory. This only 
> works fine when you load mbufs and things with size less than PAGE_SIZE 
> bytes ??

Huh?  The bounce pages are preallocated, so bus_dmamap_load() isn't going to 
be allocating things.

-- 
John Baldwin