Re: [PATCH 1/5] PowerPC 74xx: Katana Qp device tree

Mark A. Greer wrote:

On Tue, Dec 04, 2007 at 07:52:50AM +1100, Benjamin Herrenschmidt wrote:
  

quoted

quoted

	#address-cells = <1>;
+	#size-cells = <1>;
+	model = "Katana-Qp"; /* Default */
+	compatible = "emerson,Katana-Qp";
+	coherency-off;
+
      

What do that mean (coherency-off) ?

Somebody is trying again to use a 74xx with non cache coherent DMA ?
    

Hi Ben.

I suspect Andrei got that from the prpmc2800 dts which I made so I'll
jump in.  (BTW, this is the same debate we have every year or two. :)

By looking at the dts, that board has an mv64460 which has a couple
issues when it comes to coherency (depending on the rev of the chip).

One is about not being able to use DCBST instructions with coherency on
and the other is about limiting the length of one of the traces (which
at least one board manufacturer that I know of refuses to implement).
The first one is supposed to be fixed by rev A1 of the part and the second
is supposed to be fixed by rev B0 of the part.  I don't know what rev(s)
are on the board(s) Andrei is using.  If its B0 or later, in theory, the
part should work with coherency on.

Andrei, have you tested with coherency on?
  

Yes, I tested it with "coherency on", but it didn't work.

I checked chip revisions on all boards I have and they all are >= 
mv64_4_60 B0.
Emerson team working on U-Boot for KatanaQP (David Jenkins copied in cc) 
notified that they have newer firmware version which makes "coherency 
on" functional.
At the moment I have no more details on it, but I'll investigate this 
issue soon and let you know all the details.

--
[snip errata]
--

So, the answer depends on what part & what rev of the part you have
(e.g., the pegasos doesn't use the MPSC and apparently has the other
issues worked around so it can turn on coherency but the prpmc2800
doesn't so it needs coherency off).

BTW, I haven't forgotten the inherent bug you described when coherency
is off (/me too lazy to find link to the email) but AFAIK I've never run
into it.  However, if I turn on coherency and stress the PCI bus, it
hangs (I can't even look at memory thru a bdi).

Mark
  

Thanks,
Andrei.