You have probably seen snippets of Chisel referencing keys, traits, and configs by this point. This section aims to elucidate the interactions between these Chisel/Scala components, and provide best practices for how these should be used to create a parameterized design and configure it.
We will continue to use the GCD example.
Keys specify some parameter which controls some custom widget. Keys should typically be implemented as Option types, with a default value of None that means no change in the system. In other words, the default behavior when the user does not explicitly set the key should be a no-op.
Keys should be defined and documented in sub-projects, since they generally deal with some specific block, and not system-level integration. (We make an exception for the example GCD widget).
../../generators/chipyard/src/main/scala/GCD.scala
The object within a key is typically a case class XXXParams, which defines a set of parameters which some block accepts. For example, the GCD widget's GCDParams parameterizes its address, operand widths, whether the widget should be connected by Tilelink or AXI4, and whether the widget should use the blackbox-Verilog implementation, or the Chisel implementation.
../../generators/chipyard/src/main/scala/GCD.scala
Accessing the value stored in the key is easy in Chisel, as long as the implicit p: Parameters object is being passed through to the relevant module. For example, p(GCDKey).get.address returns the address field of GCDParams. Note this only works if GCDKey was not set to None, so your Chisel should check for that case!
Typically, most custom blocks will need to modify the behavior of some pre-existing block. For example, the GCD widget needs the Top module to instantiate and connect the widget via Tilelink, generate a top-level gcd_busy port, and connect that to the module as well. Traits let us do this without modifying the existing code for the Top, and enables compartmentalization of code for different custom blocks.
Top-level traits specify that the Top has been parameterized to read some custom key and optionally instantiate and connect a widget defined by that key. Traits should not mandate the instantiation of custom logic. In other words, traits should be written with CanHave semantics, where the default behavior when the key is unset is a no-op.
Top-level traits should be defined and documented in subprojects, alongside their corresponding keys. The traits should then be added to the Top being used by Chipyard.
Below we see the traits for the GCD example. The Lazy trait connects the GCD module to the Diplomacy graph, while the Implementation trait causes the Top to instantiate an additional port and concretely connect it to the GCD module.
../../generators/chipyard/src/main/scala/GCD.scala
These traits are added to the default Top in Chipyard.
../../generators/chipyard/src/main/scala/Top.scala
Config fragments set the keys to a non-default value. Together, the collection of config fragments which define a configuration generate the values for all the keys used by the generator.
For example, the WithGCD config fragment is parameterized by the type of GCD widget you want to instantiate. When this config fragment is added to a config, the GCDKey is set to a instance of GCDParams, informing the previously mentioned traits to instantiate and connect the GCD widget appropriately.
../../generators/chipyard/src/main/scala/GCD.scala
We can use this config fragment when composing our configs.
../../generators/chipyard/src/main/scala/RocketConfigs.scala
Note
Readers who want more information on the configuration system may be interested in reading cdes.