Date: Mon, 09 Feb 2009 11:10:40 -0800 From: Sam Leffler <sam@freebsd.org> To: "M. Warner Losh" <imp@bsdimp.com> Cc: scottl@samsco.org, src-committers@freebsd.org, jhb@freebsd.org, svn-src-all@freebsd.org, xcllnt@mac.com, svn-src-head@freebsd.org Subject: Re: svn commit: r188350 - in head/sys: amd64/amd64 arm/arm dev/usb2/core i386/i386 ia64/ia64 sys Message-ID: <49907FB0.9050901@freebsd.org> In-Reply-To: <20090209.115932.1098202471.imp@bsdimp.com> References: <2EA5FEEB-E676-4D1B-9700-399C783F4590@mac.com> <20090209.113053.65361811.imp@bsdimp.com> <49907B32.8080307@samsco.org> <20090209.115932.1098202471.imp@bsdimp.com>
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M. Warner Losh wrote:
> 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.
>
Not sure what you mean my "address space reasons" but it bounces for PCI
dma above 64M on ixp4xx.
Sam
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