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Re: ip27 PCI devices

To: "Maciej W. Rozycki" <macro@linux-mips.org>
Subject: Re: ip27 PCI devices
From: Ralf Baechle <ralf@linux-mips.org>
Date: Tue, 12 Apr 2005 12:42:52 +0100
Cc: Paul Chapman <paul.chapman@BrockU.CA>, linux-mips@linux-mips.org
In-reply-to: <Pine.LNX.4.61L.0504121213400.18606@blysk.ds.pg.gda.pl>
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References: <1113251422.21580.33.camel@paul.dev.brocku.ca> <20050412105815.GC5573@linux-mips.org> <Pine.LNX.4.61L.0504121213400.18606@blysk.ds.pg.gda.pl>
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On Tue, Apr 12, 2005 at 12:18:15PM +0100, Maciej W. Rozycki wrote:

> > > I've been experimenting with trying various PCI cards I have lying
> > > around in my Origin 200, to see if I can make any of them work.
> > 
> > The current Linux implementation limits IP27 to cards with 64-bit
> > addressing capability.
> 
>  Do we have a problem with our implementation of PCI DMA masks or is the 
> low 4GB of PCI address space already consumed on this system?  The problem 
> is most 32-bit PCI cards unfortunately do not support DAC.

32-bit devices can only address a tiny fraction of the address space on
IP27.  To make matters more interesting, there is no memory at all in the
low 4GB of the crosstalk address space,  so 32-bit PCI has to rely on the
yet non-existing support for the IOMMU.  SGI trying to save a little too
much money on the external SRAM for the IOMMU in the Origin 200 finally
made it a hard to use in an OS, deadlock prone thing.

  Ralf

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