Re: IDE DMA on AXP & barriers

[Kurt Garloff <garloff@suse.de>]

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On Thu, Dec 06, 2001 at 12:59:35PM +0300, Ivan Kokshaysky wrote:
> On Thu, Dec 06, 2001 at 06:13:15AM +0100, Kurt Garloff wrote:
> > I would imagine that the following barriers are nacessary:
> > * After setting up the IDE DMA tables (PRD) and having the data there,
> >   we need to have a barrier before telling the controller to do DMA.
> 
> Actually, we have more than one - the memory barriers are hidden in outX()
> macros.

You seem to be right.
Here are the results of my investigation:
* IDE DMA on reads just works fine
* IDE DMA on writes does produce corruption of a couple of bytes at offsets
  of multiples of PAGE_SIZE (8k)
* If I limit the DMA SG segments to not cross the page boundaries, DMA write
  seems to work. (I see occasional DMA timeouts though, so it might be
  necessary to limit the SG table length, or maybe to disable all the
  debugging printks currently active.)

Expect a patch soon.

The sg tables are set up correctly, AFAICS.
The only thing which makes me wonder, is why the EOT bit is not set on the
last table entry. But it does not seem to make a difference for the CMD646.

So I  wonder where the bug actually is. Somewhere in hardware; but I wonder
whether the CMD646 or the 2117x (PYXIS/CIA) is to blame. The QLogicISP seems
to happily do BM-DMA, so I'd point to the CMD646. OTOH, the QLogic sits on
the 2nd PCI bus (32bit), which could make a difference as well.

Regards,
-- 
Kurt Garloff  <garloff@suse.de>                          Eindhoven, NL
GPG key: See mail header, key servers         Linux kernel development
SuSE GmbH, Nuernberg, DE                                SCSI, Security

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