cam: big transfers

Kenneth D. Merry ken at
Wed Apr 9 13:09:31 PDT 2003

On Wed, Apr 09, 2003 at 14:01:58 -0600, Kenneth D. Merry wrote:
> On Wed, Apr 09, 2003 at 15:42:38 -0400, John S. Bucy wrote:
> > 
> > Is it possible to do large (>MAXPHYS) bus transfers from a userland process
> > via cam?
> Not currently, no.  You'd need to do some hacking around in
> cam_periph_mapmem() to allow for larger buffers.  It actually checks
> against DFLTPHYS (64K) at the moment, not MAXPHYS, since some adapters
> (e.g. the Adaptec 1542) can't handle more than 17 S/G segments.
> If you just need larger buffers, and not performance, you could malloc a
> new kernel-space buffer to hold the user data.

I forgot the finish the thought there.  You'd need to copy the data in and
out in that case.

> If you want performance as well, you could try writing your own routine
> that would map N pages from userland into the kernel, where N would be
> greater than MAXPHYS/PAGE_SIZE.
> If you just need say 256K transferred at a time, you could try bumping
> MAXPHYS to that and changing cam_periph_mapmem() to check against MAXPHYS
> as long as your hardware can support that.
> Ken
> -- 
> Kenneth Merry
> ken at
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Kenneth Merry
ken at

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