Re: [PATCH v2 01/11] asm-generic: add dma_zone_size

[Nicolas Saenz Julienne]

On Mon, 2019-08-26 at 09:09 +0200, Christoph Hellwig wrote:
> On Tue, Aug 20, 2019 at 04:58:09PM +0200, Nicolas Saenz Julienne wrote:
> > Some architectures have platform specific DMA addressing limitations.
> > This will allow for hardware description code to provide the constraints
> > in a generic manner, so as for arch code to properly setup it's memory
> > zones and DMA mask.
> 
> I know this just spreads the arm code, but I still kinda hate it.

Rob's main concern was finding a way to pass the constraint from HW definition
to arch without widening fdt's architecture specific function surface. I'd say
it's fair to argue that having a generic mechanism makes sense as it'll now
traverse multiple archs and subsystems.

I get adding globals like this is not very appealing, yet I went with it as it
was the easier to integrate with arm's code. Any alternative suggestions?

> MAX_DMA_ADDRESS is such an oddly defined concepts.  We have the mm
> code that uses it to start allocating after the dma zones, but
> I think that would better be done using a function returning
> 1 << max(zone_dma_bits, 32) or so.  Then we have about a handful
> of drivers using it that all seem rather bogus, and one of which
> I think are usable on arm64.

Is it safe to assume DMA limitations will always be a power of 2? I ask as RPi4
kinda isn't: ZONE_DMA is 0x3c000000 bytes big, I'm approximating the zone mask
to 30 as [0x3c000000 0x3fffffff] isn't defined as memory so it's unlikely that
weÂll encounter buffers there. But I don't know how it could affect mm
initialization code.

This also rules out 'zone_dma_bits' as a mechanism to pass ZONE_DMA's size from
HW definition code to arch's.

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