ESI Global Services

This section not fully fleshed out and suffers from poor writing.

ESI will provide access to global resource via ESI Services. These “buses” define a typed interface and instances are instantiated globally – they should be accessible at all levels of the design hierarchy. ESI services are intended to provide access to truly global resources like PCIe, shared DMA engines, DRAM, network interfaces, etc. They are generic enough to be extended to non-physical interfaces like telemetry – if you have a centralized telemetry reporting engine, you probably want any module or submodule in the design to be able to access and report to it.

Status: planning

Service/Client & Directions ¶

ESI services have servers and clients which are roughly analogous to masters and slaves in traditional buses, but with slightly different meanings. Whereas master/slave denote who can initiate a transaction, servers/clients denote the many-to-many or one-to-many relationship. Specifically, servers are expected to have many clients. Any service or client can initiate a transaction, depending on the direction specified.

Whether a module implements a server or client must be explicitly declared in the module declaration. Directions (out/in) of data channels/MMIO are determined by that explicit declaration and the direction specified in the service definition. Four different directions are allowed in the service definition: ToServer, ToServers, ToClient, ToClients. The plurals indicate the messages sent should be broadcast to all servers or client. The singular form indicates that a specific server or client must be addressed when sending a message unless the server is single server and the sender is a client.

Service Definitions ¶

An ESI service is essentially an interface which modules are required to implement to connect to a given service. The first attribute which must be specified is whether multiple servers are allowed via the single or multi keywords. The second is a list of MMIO and/or channels and associated directions.

Message Sizes ¶

A ToServer data channel is free to block (backpressure) other clients while one is sending a message. As such, in most cases it is best to keep messages short. While the list type is allowed, it is easy to abuse.

Wire Level Signaling ¶

The ESI compiler is free to choose any communication substrate which implements the required functionality. A traditional interconnect is not guaranteed, though the ESI compiler will may select a traditional signaling protocol like Avalon-MM or AXI AMBA in many cases.

Examples ¶

(Pseudocode in C++-ish/Verilog-ish style since MLIR asm syntax TDB.)

// DMA_Request, Response are structs.

service HostMemory single { // Specify a service with one server
    ToServer MMIO<any> Memory; // An untyped MMIO wherein the server
                                 // is the 'slave'
    ToServer DMA_Request DmaReq; // DMA access
    ToClient DMA_Response DmaResp;
}

module NiosProcessor { // A NIOS core allowing memory access
    server HostMemory Mem; // A single master server can be implicitly
                           // instantiated and referred to be the
                           // module's instance name
}

service Telemetry multi { // Specify a service with multiple servers
    ToClients void RequestStatus; // Any server can request a status update
                                  // from every client
    ToServers Event<AllStatusesUnion> Events; // Clients can respond to all
                                              // the servers
}

Telemetry GlobalBroadcastTelemetry;

service RoutedNetwork multi { // A service with multiple servers
    ToServer NetworkPacket Send; // Clients must select a server
    ToClient NetworkPacket Recv; // Servers must select a particular client
    ToServer MMIO<RoutingConfig> Config; // A per-server configuration interface
}

RoutedNetwork Net;

module NetTelemetryProcessor { // A module which collects telemetry
                               // and sends it over the network
    server Telemetry TelemetryInput;
    client RoutedNetwork TelemetryOutput;
}

NetTelemetryProcessor netTelemProc (
    .TelemetryInput(GlobalBroadcastTelemetry),
    .TelemetryOutput(Net)
)