Date: Thu, 24 Apr 2008 17:38:09 +0200 From: Hans Petter Selasky <hselasky@c2i.net> To: John Baldwin <jhb@freebsd.org> 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: <200804241738.09805.hselasky@c2i.net> In-Reply-To: <200804240806.54354.jhb@freebsd.org> References: <20080421171305.GA19840@eecs.harvard.edu> <200804240916.39607.hselasky@c2i.net> <200804240806.54354.jhb@freebsd.org>
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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? --HPS
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