Re: [PATCH RFC 1/4] of: get dma area lower limit

[Christoph Lameter (Ampere)]

On Wed, 27 Dec 2023, Baruch Siach wrote:

> of_dma_get_max_cpu_address() returns the highest CPU address that
> devices can use for DMA. The implicit assumption is that all CPU
> addresses below that limit are suitable for DMA. However the
> 'dma-ranges' property this code uses also encodes a lower limit for DMA
> that is potentially non zero.

All of memory can be used for DMA by default (==ZONE_NORMAL). ZONE_DMA 
defines a special range for devices that are unable to perform DMA to all 
of memory. Usually due to the lack of address bit support.

So I guess that the platform in question here has as a general limit as to 
what address spaces I/O devices can do DMA to?