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 we´ll 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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