New USB stack and Zero copy.

John-Mark Gurney gurney_j at
Wed Jul 4 17:52:37 UTC 2007

Hans Petter Selasky wrote this message on Wed, Jul 04, 2007 at 09:01 +0200:
> I want to get rid of the copying between DMA'able memory and non-DMA'able
> memory.
> Currently I allocate N memory-pages for each USB transfer like separate pages.

How do you allocate these pages?  With malloc(9) or w/ bus_dmamem_alloc(9)?

> The bus-dma system then assigns all of these pages each their virtual
> address.
> What I see is that when I allocate more than PAGE_SIZE bytes using bus-dma, I
> get physically contiguous memory. I don't need that for the USB stack.

That is a limitation of how bus_dma currently allocates memory...  It
calls contigmalloc, which doesn't handle multi-segment memory allocations

> The question is:
> Should we change bus-dma to support so called scatter and gather allocations,
> where the physical allocation is non-contiguous, and the virtual allocation
> is contiguous accross all the scattered pages ?

You can already support this by using malloc, and loading that buffer
into your bus_dma map...  or using the passing in userland buffer, and
loading that into the map...

> 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?

  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."

More information about the freebsd-usb mailing list