[PATCH RFC 0/3] riscv: Add DMA_COHERENT support

[Christoph Hellwig]

On Mon, Jun 07, 2021 at 02:41:14PM +0800, Guo Ren wrote:
> Double/Triple the size of physical memory regions can't be accepted by
> SOC vendors, because it wastes HW resources.
> Some cost-down soc interconnects only have 32bit~34bit width of
> physical address, are you sure you could force them to expand it? (I
> can't)
> 
> > or somewhat dynamic.
> How can HW implement with dynamic modifying PMA? What's the granularity?

I'm just stating the requirements from the Linux DMA perspective.  You
also do not need tripple the address space, just double.