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Re: AmigaONE && Debian (unstable?)

On Tue, 2004-06-08 at 03:32, Ole-Egil Hvitmyren wrote:

> I don't see ANY damn reason why you need to drag that debate in here, 
> but let's just say certain other OSes doesn't have a problem marking DMA 
> buffers as non-cacheable.

Hehehehehe, that's getting funny :) (Note that I don't know anything
about the specific problem you are dealing with here ...)

Ok, so Linux do support non-coherent DMA quite well, it's atcually
widely used by various sorts of embedded PPC CPUs like 4xx, 8xx, ...

_HOWEVER_, doing that in a northbridge for anything but an embedded CPU,
especially a CPU of the 6xx/7xxx family is just insane. It's basically
incompetent northbridge design.

Linux uses BATs to map the entire linar memory aperture, on those CPUs,
which is a significant performance gain, and allows to simplify some low
level memory management issues. However, for various reasons I can
explain separately, that means that on those CPU, we cannot easily map
arbitrary pieces of memory as non-cacheable in a reliable way (and yes,
that is a problem with AGP on some machines).
> It's stated pretty explicitly in the northbridge documentation that this 
> is how it needs to work, saying the hardware is buggy because it follows 
> its own documentation seems a TAD silly to me.

No, that means the HW is a Piece Of Shit !

> But you can't help it, can you? Every time someone mentions the AmigaOne 
> it has to be "not very stable the time (two years ago, was it?) I saw 
> it" and so on. Yes, we've had some Linux problems. Quite a few, 
> actually. But most of that seems to come from MAI and Eyetech wanting to 
> get everything for free, and not doing anything to actively support the 
> development of the Linux kernel.

I'm not from any of these but I've been one of the maintainers of the
PPC kernel for long enough to have a chance to play with a wide range of
northbridges. Cache coherency is a basic feature of anything claiming to
be used as a desktop machine.


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