linux-kernel - Re: [RFC v2 1/2] dt-bindings: iommu: Document iova-best-fit property for IOMMU masters

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Message-ID: <20221213163506.GA2011062-robh@kernel.org>
Date:   Tue, 13 Dec 2022 10:35:06 -0600
From:   Rob Herring <robh@...nel.org>
To:     Georgi Djakov <quic_c_gdjako@...cinc.com>
Cc:     krzysztof.kozlowski+dt@...aro.org, devicetree@...r.kernel.org,
        joro@...tes.org, will@...nel.org, robin.murphy@....com,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
        djakov@...nel.org
Subject: Re: [RFC v2 1/2] dt-bindings: iommu: Document iova-best-fit property
 for IOMMU masters

On Mon, Dec 12, 2022 at 01:51:37PM -0800, Georgi Djakov wrote:
> Document the "iova-best-fit" device-tree property, which is used to
> describe that the iommu master is constrained on memory and the system
> must put more effort when allocating IOVAs to avoid holes/gaps in
> memory. This improves the memory utilization and helps with memory
> fragmentation issues in some cases, but it could take longer to allocate
> an IOVA compared with the default "first-fit" algorithm.
> 
> Signed-off-by: Georgi Djakov <quic_c_gdjako@...cinc.com>
> ---
>  Documentation/devicetree/bindings/iommu/iommu.txt | 4 ++++
>  1 file changed, 4 insertions(+)

No new properties in common .txt files. Schemas only.

However, this looks like kernel policy which doesn't belong in DT.

> diff --git a/Documentation/devicetree/bindings/iommu/iommu.txt b/Documentation/devicetree/bindings/iommu/iommu.txt
> index 26ba9e530f13..ca1b4813c5bf 100644
> --- a/Documentation/devicetree/bindings/iommu/iommu.txt
> +++ b/Documentation/devicetree/bindings/iommu/iommu.txt
> @@ -88,6 +88,10 @@ prevent any driver from properly setting up the translations.
>  
>  Optional properties:
>  --------------------
> +- iova-best-fit: When present, the best-fit algorithm will be used, instead
> +  of first-fit. This reduces memory fragmentation when allocating IOVAs in
> +  some cases, but may also increase the time it takes to allocate an IOVA.
> +
>  - pasid-num-bits: Some masters support multiple address spaces for DMA, by
>    tagging DMA transactions with an address space identifier. By default,
>    this is 0, which means that the device only has one address space.
>