Re: [PATCH RESEND v2 0/5] Avoid requesting page from DMA zone when no managed pages

[Christoph Hellwig]

On Mon, Dec 13, 2021 at 03:39:25PM +0800, Baoquan He wrote:
> > > As said at above, ia64 and riscv don't have ZONE_DMA at all, they just
> > > cover low 4G with ZONE_DMA32 alone.
> > 
> > If you do not have devices that are crap and cannot address the full
> > memory then you dont need these special zones.
> 
> I am not a DMA expert, with my understanding, on x86_64 and arm64, we
> have PCIe devices which dma mask is 32bit

Yes, way to many, and they keep getting newly introduce as well.  Also
weirdo masks like 40, 44 or 48 bits.

> , means they can only address
> ZONE_DMA32.

Yes and no.  Offset between cpu physical and device address make this
complicated, even ignoring iommus.