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
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?
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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