* Re: ZONE_DMA (was: Re: 2.6.19 -mm merge plans)
[not found] ` <20060920172253.f6d11445.akpm@osdl.org>
@ 2006-09-21 0:31 ` Christoph Lameter
2006-09-21 1:03 ` Alan Cox
0 siblings, 1 reply; 3+ messages in thread
From: Christoph Lameter @ 2006-09-21 0:31 UTC (permalink / raw)
To: Andrew Morton
Cc: Martin J. Bligh, linux-kernel, linux-arch, Christoph Hellwig
On Wed, 20 Sep 2006, Andrew Morton wrote:
> > Would it not make sense to define what ZONE_DMA actually means
> > consistently before trying to change it? The current mess across
> > different architectures seems like a disaster area to me.
> >
> > What DOES requesting ZONE_DMA from a driver actually mean?
ZONE_DMA is a memory area that is needed by an arch for devices that
cannot do DMA to all of memory. The high boundary is set by
MAX_DMA_ADDRESS.
> My concern about these patches is that they'll only be useful for
> self-compiled kernels, because distros will be forced to enable ZONE_DMA
> for evermore anyway.
We already have 4 arches now that do not need ZONE_DMA at all.
ZONE_DMA does not have a bright future with IOMMUs and other things
around. None of my system uses ZONE_DMA and I have a variety of them.
And yes if we do not have this facility in the kernel then distros cannot
pick it up. At least on IA64 I know that hardware from the major vendors
has not been needing ZONE_DMA for a while now.
Also ZONE_DMA is a very bad concept. Multiple drivers may have different
address requirements. What we need is some way for a driver to tell the
kernel what the required range of addresses is. If a device is only
capable of handling 30 valid address bits then we may have to use
ZONE_DMA and only allow the use of the lower 16MB. It would be better to
develop a different mechanism.
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: ZONE_DMA (was: Re: 2.6.19 -mm merge plans)
2006-09-21 0:31 ` ZONE_DMA (was: Re: 2.6.19 -mm merge plans) Christoph Lameter
@ 2006-09-21 1:03 ` Alan Cox
2006-09-21 15:59 ` James Bottomley
0 siblings, 1 reply; 3+ messages in thread
From: Alan Cox @ 2006-09-21 1:03 UTC (permalink / raw)
To: Christoph Lameter
Cc: Andrew Morton, Martin J. Bligh, linux-kernel, linux-arch,
Christoph Hellwig
Ar Mer, 2006-09-20 am 17:31 -0700, ysgrifennodd Christoph Lameter:
> ZONE_DMA does not have a bright future with IOMMUs and other things
> around. None of my system uses ZONE_DMA and I have a variety of them.
IOMMUs don't always help. They IOMMU aperture on AMD64 for example is
too high to help devices with below 32bit limits.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: ZONE_DMA (was: Re: 2.6.19 -mm merge plans)
2006-09-21 1:03 ` Alan Cox
@ 2006-09-21 15:59 ` James Bottomley
0 siblings, 0 replies; 3+ messages in thread
From: James Bottomley @ 2006-09-21 15:59 UTC (permalink / raw)
To: Alan Cox
Cc: Christoph Lameter, Andrew Morton, Martin J. Bligh, linux-kernel,
linux-arch, Christoph Hellwig
On Thu, 2006-09-21 at 02:03 +0100, Alan Cox wrote:
> IOMMUs don't always help. They IOMMU aperture on AMD64 for example is
> too high to help devices with below 32bit limits.
That's because it's not an IOMMU; it's a GART. A true IOMMU separates
the machine physical and bus physical address spaces ... a GART merely
remaps a hole in physical address space.
James
^ permalink raw reply [flat|nested] 3+ messages in thread
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2006-09-21 0:31 ` ZONE_DMA (was: Re: 2.6.19 -mm merge plans) Christoph Lameter
2006-09-21 1:03 ` Alan Cox
2006-09-21 15:59 ` James Bottomley
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