Date: Tue, 25 Oct 2005 16:10:52 -0400 From: John Baldwin <jhb@freebsd.org> To: Dinesh Nair <dinesh@alphaque.com> Cc: freebsd-hackers@freebsd.org Subject: Re: correct use of bus_dmamap_sync Message-ID: <200510251610.53127.jhb@freebsd.org> In-Reply-To: <435E7D8C.90401@alphaque.com> References: <435E3003.4050609@alphaque.com> <200510251327.59965.jhb@freebsd.org> <435E7D8C.90401@alphaque.com>
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On Tuesday 25 October 2005 02:46 pm, Dinesh Nair wrote: > On 10/26/05 01:27 John Baldwin said the following: > > On Tuesday 25 October 2005 09:15 am, Dinesh Nair wrote: > >>(must i malloc space for them before passing them into those functions, > >> or will the call to bus_dmamem_alloc do it for me ?) > > > > bus_dmamem_alloc() will do it for you. > > thanx. > > > Probably not as the sync()'s don't really do anything with memory > > allocated via bus_dmamem_alloc(). The operations are named from the > > CPU's perspective, > > however, the man page at > http://www.freebsd.org/cgi/man.cgi?query=bus_dmamap_sync&apropos=0&sektion= >0&manpath=FreeBSD+5.4-stable&format=html says, > > "Although no explicit loading is required to access the memory referenced > by the returned map, the synchronization requirements as described in the > bus_dmamap_sync() section still apply." Yes, and on some archs the sync() operations do have memory barriers in place, but there isn't any bounce buffering with bus_dmamem_alloc() memory. > also, is bus_dmamap_load() required, since the same man page section above > says it isnt ? Well, you need it to get the physical address to pass to your device for it to do DMA against. > have things changed between freebsd 4.x (which i'm using) and freebsd 5.x ? I don't think so as far as the interface. > > thus when you send data to your device, that is a WRITE operation (even > > though your device is doing a DMA to read data), and when you get data > > back from your device, that is a READ operation (even though your device > > is doing a DMA to write the data into the buffer). > > thanx, the verbiage on the man page is slightly confusing with it's use of > CPU, giving the opposite impression. Yes, I know. :) -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" = http://www.FreeBSD.org
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