New USB stack and Zero copy.

[Hans Petter Selasky]

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:
> > 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)?

bus_dmamem_alloc().

>
> > 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
> yet..
>
> > 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?

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.

--HPS