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

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Date: Wed, 17 Jan 2024 14:23:33 -0800 (PST)
From: "Christoph Lameter (Ampere)" <cl@...ux.com>
To: Baruch Siach <baruch@...s.co.il>
cc: Christoph Hellwig <hch@....de>, 
    Marek Szyprowski <m.szyprowski@...sung.com>, 
    Rob Herring <robh+dt@...nel.org>, Frank Rowand <frowand.list@...il.com>, 
    Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
    Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev, 
    devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
    linux-kernel@...r.kernel.org, 
    Petr Tesařík <petr@...arici.cz>, 
    Ramon Fried <ramon@...reality.ai>
Subject: Re: [PATCH RFC 1/4] of: get dma area lower limit

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?