Date: Thu, 24 Apr 2008 16:01:51 -0400 From: John Baldwin <jhb@freebsd.org> To: Hans Petter Selasky <hselasky@c2i.net> Cc: freebsd-embedded@freebsd.org, freebsd-usb@freebsd.org, Geoffrey Mainland <mainland@eecs.harvard.edu> Subject: Re: Soekris 4826 USB failure on FreeBSD 7.0 Message-ID: <200804241601.51441.jhb@freebsd.org> In-Reply-To: <200804241738.09805.hselasky@c2i.net> References: <20080421171305.GA19840@eecs.harvard.edu> <200804240806.54354.jhb@freebsd.org> <200804241738.09805.hselasky@c2i.net>
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On Thursday 24 April 2008 11:38:09 am Hans Petter Selasky wrote: > On Thursday 24 April 2008, John Baldwin wrote: > > > > This would indicate a bug in the bus_dmamap_load() call (wrong length?) and > > that is going to hose you when you do the bus_dmamap_sync() for systems > > with bounce pages (not enough data will get copied back and forth?). You > > need to track down the real bug and fix it rather than adding a hack in > > your callback routine. > > Hi John, > > The minimum segment size is PAGE_SIZE bytes in my DMA-tag. Bus-dma starts > counting at the memory location of the allocation. And not the memory > location aligned to PAGE_SIZE bytes. > > Memory pages: |--------------|--------------| > My allocation: |---------XXXXXXXXXXXXX-------| > > Sometimes when I allocate DMA memory I can end up having an allocation > crossing two contiguous memory pages in physical memory. In those cases > bus_dma does not give me the segment address of the second page in the > segment list, because the allocation is less than PAGE_SIZE bytes. It fits > within the values specified in the DMA tag. But in other cases, where the > pages are not contiguous in RAM, bus_dma will return two different segment > addresses. > > The question is: Is this a bug or is it a feature? If you can't have a S/G element cross a page boundary, then use a 'boundary' of PAGE_SIZE when you create your tag. As it is, you are (correctly for the tag you are creating) getting a single S/G entry that spans the entire region. It sounds like you want two regions that is split on the page boundary. If so, that is what the 'boundary' argument to bus_dmatag_create() is for. -- John Baldwin
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