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From: Baruch Siach <baruch@tkos.co.il>
To: "Christoph Lameter (Ampere)" <cl@linux.com>
Cc: Christoph Hellwig <hch@lst.de>, Marek Szyprowski
 <m.szyprowski@samsung.com>, Rob Herring <robh+dt@kernel.org>, Frank Rowand
 <frowand.list@gmail.com>, Catalin Marinas <catalin.marinas@arm.com>, Will
 Deacon <will@kernel.org>, Robin Murphy <robin.murphy@arm.com>,
 iommu@lists.linux.dev, devicetree@vger.kernel.org,
 linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Petr
 =?utf-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>, Ramon Fried
 <ramon@neureality.ai>
Subject: Re: [PATCH RFC 1/4] of: get dma area lower limit
Date: Thu, 18 Jan 2024 12:59:58 +0200
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Hi Christoph

On Wed, Jan 17 2024, Christoph Lameter (Ampere) wrote:
> 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?

DMA to/from devices in bus with 'dma-ranges' property is limited to
address space described in 'dma-ranges'. The arm64 platform currently
uses 'dma-ranges' as a hint to set ZONE_DMA limits globally. This series
is meant to make ZONE_DMA limits adjustment code work better for
platforms where the lower DMA limit is above 4GB. This commit adds the
ability to extract the lower limit from 'dma-ranges'.

baruch

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