linux-kernel - Re: A question about ZONE

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Message-Id: <201010131657.18469.arnd@arndb.de>
Date:	Wed, 13 Oct 2010 16:57:17 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Casey Dahlin <cdahlin@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: A question about ZONE_DMA

On Wednesday 13 October 2010, Casey Dahlin wrote:
> So if I understand correctly, ZONE_DMA exists to separate and preserve the
> chunk of memory to which older ISA cards are capable of doing DMA. The question
> that arises for me is:
> 
> 1) Why do all 32-bit boxes have a ZONE_DMA when surely only a tiny and
> shrinking number of them have a need for it?
> 
> 2) Why do any 64-bit boxes have a ZONE_DMA? Is there really some godless
> monster out there who put an early ISA bridge and a 64-bit CPU on the same
> motherboard?!
> 
> Can someone shed light on what I'm missing?

There are some ISA devices that have become standard components, like floppy
drives. A lot of PCs still have connectors for floppies even though they
are hardly used these days.

Some architectures define ZONE_DMA in a different way, e.g. as the lower
2 GB because of device restrictions on 64 bit systems.

	Arnd
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