Date: Mon, 09 Feb 2009 11:18:29 -0800 From: Marcel Moolenaar <xcllnt@mac.com> To: John Baldwin <jhb@freebsd.org> Cc: svn-src-head@freebsd.org, Scott Long <scottl@samsco.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, "M. Warner Losh" <imp@bsdimp.com> Subject: Re: svn commit: r188350 - in head/sys: amd64/amd64 arm/arm dev/usb2/core i386/i386 ia64/ia64 sys Message-ID: <0AA26479-B36A-4412-BF2B-7D4C573BC266@mac.com> In-Reply-To: <200902091342.16078.jhb@freebsd.org> References: <200902082254.n18MsxVt037307@svn.freebsd.org> <200902090923.45887.jhb@freebsd.org> <2EA5FEEB-E676-4D1B-9700-399C783F4590@mac.com> <200902091342.16078.jhb@freebsd.org>
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On Feb 9, 2009, at 10:42 AM, John Baldwin wrote: >>> 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? > > All of the platforms that do bounce buffering will honor this. If a > given > platform doesn't support bounce buffering then I imagine it will fail > misaligned requests with an error. It would probably make it easier > to > support bounce buffering on more platforms if some of the support > code for > managing bounce zones could be moved into a subr_busdma.c or the > like since > much of it is copy and pasted. Very much agreed with the unification in subr_busdma.c. If the shared implementation then grows a IOMMU interface, then platforms with a IOMMU can avoid bounce buffering when possible (i.e. no re-alignment is needed or enough free IOMMU mappings exist to avoid the bounce buffering). -- Marcel Moolenaar xcllnt@mac.com
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