From: Daniel Hazelton <dhazelton@enter.net>
To: "tek-life" <teklife.kernel@gmail.com>
Subject: Re: Can we remove the Zone_DMA?
Date: Sun, 4 Apr 2010 01:07:13 -0400
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On Sunday 04 April 2010 12:21:54 am tek-life wrote:
> I??m a newbie on the linux kernel. Now I am reading the source code of
> Linux . I have a question in the following about ZONE_DMA.
> 
> 
> In Linux , The Memory is divided to three zone. They are ZONE_DMA
> ??ZONE_NORMAL  and ZONE_HIGHMEM. From the book of "Undstand the Linux
> kernel ", the ZONE_DMA has the effect that the Direct Memory Access
> (DMA) processors for old ISA buses have a strong limitation: they are
> able to address only the first 16 MB of RAM. SO ,we must set a zone
> for  the DMA on ISA bus.  And I suspect that the hardware has
> developed so quickly .And  in this days the ISA has been weeded out.
> And so ,if we not defined the ZONE_DMA, is the system be effected? And
> why not remove ZONE_DMA from the kernel . If it cann??t to do so??the
> compatibility is the only reason?

While ISA is gone as a true peripheral interconnect for new systems it does, 
actually, still live on in a lot of systems that Linux still supports. While 
those systems, generally, are running the same kernel and userspace they were 
a decade ago I have no doubt that somebody might find an old machine and put 
Linux on it - just because they could.

And that also discounts the non-IBM PC machines that are out there that Linux 
also supports. While I don't know enough about them to say for sure, I am 
quite certain that at least some of them are still using the ISA bus.

DRH
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