svn commit: r188350 - in head/sys: amd64/amd64 arm/arm dev/usb2/core i386/i386 ia64/ia64 sys

[M. Warner Losh]

In message: <49907B32.8080307 at samsco.org>
            Scott Long <scottl at samsco.org> writes:
: M. Warner Losh wrote:
: > In message: <2EA5FEEB-E676-4D1B-9700-399C783F4590 at mac.com>
: >             Marcel Moolenaar <xcllnt at 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