Re: [PATCH 5/6] Optional ZONE_DMA for x86_64

Christoph Lameter
2006-09-12T17:28:50Z

On Tue, 12 Sep 2006, Andi Kleen wrote:

> > There are no drivers depending on CONFIG_ZONE_DMA32. If CONFIG_ZONE_DMA32
> > is not set then the system assumes that DMA devices are capable of
> > doing DMA to all of memory (which is mostly the case since most
> > x86_64 motherboards only allow a max of 4GB of memory and advanced
> > systems have DMA subsystems that handle I/O properly).
>
> Pretty much all have this problem (or do you know of a system with no
> USB controller?)

Most motherboards do not support more than 4GB ram.

> I think I would prefer to not make GFP_DMA32 optional.
> Adding a 4GB limit CONFIG to a 64bit kernel just seems somewhat
> dumb.

So is delivering motherboards for 64 bit with 4GB Ram.

> Also what happens when someone still sets GFP_DMA?
> iirc the SCSI stack tended to do that for some structures.
> Will it fail?

No it will be ignored.

> Making GFP_DMA optional would be an interesting
> experiment (I think it will break the floppies but other than that
> a modern system might be fine). But it would be better

Right some drivers are not available anymore that includes the floppy
driver.

> to warn the user stronger against it (most likely it will still
> break a lot of older addin cards) and tell them about
> the floppy.

I still have some dusty set of floppies in my attic. Tried to use floppy a
year to boot but it was such a hassle that I fell back to a usb stick.

You can still have your floppies. Just do not disable support for
ZONE_DMA. Its enabled by default.