Date: Mon, 09 Feb 2009 11:59:32 -0700 (MST) From: "M. Warner Losh" <imp@bsdimp.com> To: scottl@samsco.org Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, xcllnt@mac.com, src-committers@freebsd.org, jhb@freebsd.org Subject: Re: svn commit: r188350 - in head/sys: amd64/amd64 arm/arm dev/usb2/core i386/i386 ia64/ia64 sys Message-ID: <20090209.115932.1098202471.imp@bsdimp.com> In-Reply-To: <49907B32.8080307@samsco.org> References: <2EA5FEEB-E676-4D1B-9700-399C783F4590@mac.com> <20090209.113053.65361811.imp@bsdimp.com> <49907B32.8080307@samsco.org>
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In message: <49907B32.8080307@samsco.org> Scott Long <scottl@samsco.org> writes: : M. Warner Losh wrote: : > In message: <2EA5FEEB-E676-4D1B-9700-399C783F4590@mac.com> : > Marcel Moolenaar <xcllnt@mac.com> writes: : > : : > : On Feb 9, 2009, at 6:23 AM, John Baldwin wrote: : > : : > : > On Monday 09 February 2009 12:37:53 am Marcel Moolenaar wrote: : > : >> : > : >> On Feb 8, 2009, at 5:00 PM, Scott Long wrote: : > : >> : > : >>> Busdma allows you to request bouncing for realignment. : > : >> : > : >> How exactly? : > : > : > : > The 'align' parameter to bus_dma_tag_create(). If your hardware : > : > needs buffers : > : > to be aligned on a 4-byte boundary and you bus_dmamap_load() a buffer : > : > where 'addr % 4 != 0', then the buffer is bounced. Since by default : > : > the new : > : > buffer starts on a page boundary, it satifies the 'addr % 4'. : > : : > : But according to the man page, bounce buffering may not : > : be implemented or not be applicable to a platform. It : > : seems to me that you cannot depend on this side-effect : > : in a generic driver. Are you guys talking only in terms : > : of i386 or is this generally applicable? : > : > There's no bounce buffering on MIPS right now, for example... There : > likely should be, but it isn't there now. : > : > Warner : : So yes, it's only available really on amd64 and i386 at the moment. No : reason why it can't be available elsewhere, and I'm working on factoring : all of the bounce logic out into an MD file so it can be made available. : For architectures that use an IOMMU, there's probably no reason why : re-alignment can't use the IOMMU instead of bouncing. arm also does bounce buffering, but not for memory address space reasons. Warner
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