Messages in this thread | | | From | Andi Kleen <> | Subject | Re: [RFC 0/8] Optional ZONE_DMA | Date | Mon, 10 Jul 2006 02:02:09 +0200 |
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> > 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.
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