On Fri, Apr 27, 2018 at 11:07:07AM -0500, Christopher Lameter wrote: > Well it looks like what we are using it for is to force allocation from > low physical memory if we fail to obtain proper memory through a normal > channel. The use of ZONE_DMA is only there for emergency purposes. > I think we could subsitute ZONE_DMA32 on x87 without a problem. > > Which means that ZONE_DMA has no purpose anymore. > > Can we make ZONE_DMA on x86 refer to the low 32 bit physical addresses > instead and remove ZONE_DMA32? > > That would actually improve the fallback because you have more memory for > the old devices. Some devices have incredibly bogus hardware like 28 bit addressing or 39 bit addressing. We don't have a good way to allocate memory by physical address other than than saying "GFP_DMA for anything less than 32, GFP_DMA32 (or GFP_KERNEL on 32-bit) for anything less than 64 bit". Even CMA doesn't have a "cma_alloc_phys()". Maybe that's the right place to put such an allocation API.
