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

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Message-ID: <20221212215138.17897-1-quic_c_gdjako@quicinc.com>
Date:   Mon, 12 Dec 2022 13:51:37 -0800
From:   Georgi Djakov <quic_c_gdjako@...cinc.com>
To:     <robh+dt@...nel.org>, <krzysztof.kozlowski+dt@...aro.org>,
        <devicetree@...r.kernel.org>
CC:     <joro@...tes.org>, <will@...nel.org>, <robin.murphy@....com>,
        <iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>,
        <djakov@...nel.org>
Subject: [RFC v2 1/2] dt-bindings: iommu: Document iova-best-fit property for IOMMU masters

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(+)

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.