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

[John Baldwin]

On Monday 09 February 2009 12:51:28 pm Marcel Moolenaar wrote:
> 
> 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?

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.

-- 
John Baldwin