Re: [PATCH] libfdt: add address translation support

[David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>]

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On Mon, May 12, 2014 at 04:14:12PM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2014-05-12 at 16:10 +1000, Benjamin Herrenschmidt wrote:
> > On Mon, 2014-05-12 at 16:00 +1000, David Gibson wrote:
> > > I'm not all that impressed by this translation code - in particular
> > > the per-bus-type hooks don't make sense to me.  AFAIK the
> > > interpretation of ranges is not bus specific. I think it will also
> > > fail in some cases with #address-cells > 2, which is unfortunate.
> > > 
> > > I'm about to post my own implementation of address translation.
> > 
> > Well, I came up with the original code which did per-bus type hooks.
> > There are cases where it is needed.
> > 
> > Take PCI. The top word can contain completely unrelated stuff such
> > as the "prefetchable" attribute.
> > 
> > It's perfectly legal to put a prefetchable BAR under a non-prefetchable
> > bridge window.
> > 
> > However a translation scheme that doesn't know to know that bit
> > will fail.
> 
> Oh and the other way around with IO vs. memory btw.
> 
> > It's not far fetched, it happens on our machines today.
> > 
> > And that's just one of the problems I had back then...
> 
> Note that I'm just stating the problem :-) There may be a better way to
> address it. For example, for the one above specifically, well, we could
> have a per-bus-type list of masks to apply before translation (bits to
> ignore).
> 
> I wish in fact OF had done that to begin with like we have
> interrupt-map-mask, maybe we should introduce that concept to limit the
> damage to existing well known cases like PCI and have anything new just
> tell us what to use for the compare.
> 
> In fact, thinking more, the above mask trick might not fully work for
> PCI, we have to check the exact values, because I think the same field
> somewhat use an enumeration for 32-bit MEM, 64-bit MEM and IO ... oh
> well.

Hm, yeah.  At this point I'm thinking a per-bus "quirks" mechanism
around the basic universal translation logic.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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