A question about ZONE_DMA

A question about ZONE_DMA

[Casey Dahlin]

Hi all,

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?

--CJD

Re: A question about ZONE_DMA

[Oliver Neukum]

Am Mittwoch, 13. Oktober 2010, 16:52:41 schrieb Casey Dahlin:
> Hi all,
> 
> 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?

You don't know before you load the module of the driver that needs 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?!

Parports can use DMA and are generally ISA.

	Regards
		Oliver

Re: A question about ZONE_DMA

[Arnd Bergmann]

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

Re: A question about ZONE_DMA

[Casey Dahlin]

Thanks for the quick replies. My youth affords me the luxury of not knowing
much about floppy drives or parports :). All makes sense now.

--CJD

Re: A question about ZONE_DMA

[Andi Kleen]

Casey Dahlin <cdahlin@redhat.com> writes:

> Thanks for the quick replies. My youth affords me the luxury of not knowing
> much about floppy drives or parports :). All makes sense now.

In addition it is used as fallback for all kinds of other DMA which is
less than 32bits. e.g. there used to be popular systems with 31 or 29
bits addressing capability.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.

Re: A question about ZONE_DMA

[David Rientjes]

On Wed, 13 Oct 2010, Casey Dahlin wrote:

> Hi all,
> 
> 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?
> 

If you don't need it, disable CONFIG_ZONE_DMA, which is useful to prevent 
oom conditions when lowmem_reserve_ratio reserves memory in that zone that 
you'll never need to save.

If you do disable that option, you may also want to look at disabling 
CONFIG_GENERIC_ISA_DMA and CONFIG_ISA_DMA_API which you need to hack a 
little to get to compile.  I'll send a patch to fix that.