The AMBA LTI (Local Translation Interface) defines the point-to-point protocol between an I/O device and the TLBU (Translation Buffer Unit) of an Arm System Memory Management (SMMU). The LTI protocol is used by systems built based on the Arm System SMMUv3 architecture specification.
The LTI protocol defines three different channels to request a translation for each transaction coming from an I/O Device. Additionally, it defines interface management signals and credit signals for flow control. LA and LR channels can have multiple virtual channels, which enable one VC to progress when another is blocked to avoid deadlock scenarios and can result in higher bandwidth.
LTI Channel
The LTI interface contains the following channels:
- LA Request channel: Address and attributes that require translation are sent to the TBU.
- LR Response channel: Provides the translated address and attributes to the LTI device.
- LC Completion channel: LTI devices must provide information about completion to the TBU.
As shown in the following image, the LTI Manager sends the request on the LA channel for each transaction from the device, which requires a translated address. LTI-TBU component passes the translated address and response on the LR channel. LTI enables devices to directly request a translation for each transaction while leaving the TBU to manage the Translation Lookaside Buffer (TLB). If LTI-TBU does not have a translated address, it sends a request to the TCU component using DTI protocol and forwards the translated address to the LTI Manager. For each translation response, the LTI Manager sends a completion message using the LC channel.
LTI Transaction Flow
A complete LTI transaction consists of a message on all three channels as shown below figure.
- The manager sends a request to the LA channel.
- The subordinate sends a response on the LR channel, enabling the translated transaction to be issued downstream.
- The manager sends a completion on the LC channel, once the downstream transaction is complete.
The following diagram represents the timing for the transactions initiated from device flows through the system.
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