Date: Mon, 3 Aug 2009 17:01:37 +0200 From: Rafal Jaworowski <raj@semihalf.com> To: Hans Petter Selasky <hselasky@c2i.net> Cc: arm@freebsd.org, usb@freebsd.org Subject: Re: About the "USB Cache and busdma usage in USB" thread Message-ID: <8DC8C704-F84D-4A60-A11B-2F877EB903C9@semihalf.com> In-Reply-To: <20090724.233404.-399282844.imp@bsdimp.com> References: <3E1658AF-67C6-4E61-B6E7-BEF528C3FF4D@mac.com> <200907232209.47729.hselasky@c2i.net> <20090724.233404.-399282844.imp@bsdimp.com>
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On 2009-07-25, at 05:34, M. Warner Losh wrote: > In message: <200907232209.47729.hselasky@c2i.net> > Hans Petter Selasky <hselasky@c2i.net> writes: > : On Thursday 23 July 2009 20:53:06 Marcel Moolenaar wrote: > : > All, > : > > : > I went over the thread and this is what I have to say about it: > : > > : > Using busdma to manage/control CPU caches is wrong for the > : > following simple reason: bus_dmamap_sync() has the side-effect > : > of copying to and from the bounce buffer (if applicable). > : > > : > CPU caches should be kept coherent by using an appropriate API. > : > We already have cpu_flush_dcache(). All we have to do is add > : > cpu_inval_dcache() and let the MD code determine how best to > : > do this -- even if they decide to use busdma. > : > > : > In general: D-cache and I-cache control/handling should not be > : > hidden from MI code. It should not be treated as an artifact of > : > some platform. It should not be implemented by banking on some > : > side-effect of other function(s). We only achieve efficient > : > cache control if MI code calls appropriate APIs so that we can > : > precisely express what we need to achieve at that point. > : > > : > For example: when we write a breakpoint into the text segment > : > of some process by using ptrace(2), the ptrace(2) code must > : > call an appropriate API to make sure that the I-cache is made > : > coherent with memory. This may require a previous D-cache > : > flush! We should not kluge uiomove(9) like we did on PowerPC > : > to deal with this. Note ARM and ia64 are still broken in this > : > respect. > : > : Hi, > : > : I would be fine with a solution where cpufunctions are used > directly in USB. > : The only problem is that if bounce pages are used, which happens > in the case > : of loading kernel virtual data into DMA, then busdma sync calls > would still be > : required. > > They are needed on i386 kernels with more than 4GB of ram... Or ram > located above 4GB... Hans, So how do you want to proceed with these cache sync issues? We need to fix this before 8.0. Rafal
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