Date: Wed, 4 Jul 2007 10:35:31 -0700 From: John-Mark Gurney <gurney_j@resnet.uoregon.edu> To: Hans Petter Selasky <hselasky@c2i.net> Cc: freebsd-hackers@freebsd.org, freebsd-usb@freebsd.org Subject: Re: New USB stack and Zero copy. Message-ID: <20070704173531.GO1221@funkthat.com> In-Reply-To: <200707040901.33019.hselasky@c2i.net> References: <200707040901.33019.hselasky@c2i.net>
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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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