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Re: [Xen-devel] ISA DMA

To: Jan Beulich <jbeulich@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] ISA DMA
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Wed, 21 Mar 2007 16:42:38 +0000
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On 21/3/07 15:24, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> x86-64 excludes i8237.o from being built, while i386 doesn't. Also, for
> domU-s, there's nothing being done to make request_dma() fail. Any particular
> reason for either of these?

Theoretically if we stopped Xen using low physical memory we could support
ISA DMA. I think that's the only obstacle to practical i8237 usage. Also in
that case probably request_dma() would be valid (in domUs too, if the domU
is a driver domain that has been assigned an ISA device and its
corresponding DMA channel).

Thinking more practically, these are simply not issues that anyone cares
much about and are arguably not worth fixing since the current behaviour is
arguably not really incorrect (unlikely to crash your kernel or anything
like that) and providing alternative implementation of request_dma() simply
increases our diff to no really useful purpose (we'll go from 'not working
with ISA devices' to 'not working with ISA devices' :-).

 -- Keir

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