Re: Can we remove the Zone_DMA?

From: Daniel Hazelton
Date: Sun Apr 04 2010 - 01:16:51 EST

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.

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