Re: [RFC 0/8] Optional ZONE_DMA

[Andi Kleen <ak@suse.de>]

> > On x86_64 systems also usually we do not need ZONE_DMA since there
> > are barely any ISA DMA devices around (or are you still using a floppy?).
> > So for most cases the zone can be dropped. Also if the x86_64 systems
> > has less than 4G RAM or DMA controllers that actually can do 64 bit
> > then we also do not need ZONE_DMA32. My x86_64 system has 1G of
> > memory therefore I can run with a single zone.
> 
> Keep in mind that:

Yes we can't really make it optional. There are reasons to use GFP_DMA
even without ISA. Also on x86-64 CONFIG_ISA is never set so it would
completely eliminate GFP_DMA, which we can't do.

That said however nearly users of GFP_DMA outside arch/* are wrong.
They should be all audited and convered to use the PCI DMA API instead.

> -LPC devices like the floppy controller, maybe enhanced parallel, etc. 
> may have 24-bit DMA restrictions even if there is no physical ISA bus.
> 
> -Even in totally ISA and LPC-free systems, some PCI devices (like those 
> that were a quick hack of an ISA device onto PCI) still have 24-bit 
> address restrictions. There are other devices that have sub-32-bit DMA 
> capabilities, like Broadcom wireless chips that only address 31 bits 
> (although I think they are fixing this in the driver). Without the DMA 
> zone there is no way to ensure that these requests can be satisfied.

There are also devices with 31 or 30 bits that also fall back to GFP_DMA
and a couple of other cases.


-Andi
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