AXI Memory Over TL Serial Link

docs/Advanced-Concepts/Chip-Communication.rst

@@ -7,7 +7,7 @@ There are two types of DUTs that can be made: `tethered` or `standalone` DUTs.
 A `tethered` DUT is where a host computer (or just host) must send transactions to the DUT to bringup a program.
 This differs from a `standalone` DUT that can bringup itself (has its own bootrom, loads programs itself, etc).
 An example of a tethered DUT is a Chipyard simulation where the host loads the test program into the DUTs memory and signals to the DUT that the program is ready to run.
-An example of a standalone DUT is a Chipyard simulation where a program can be loaded from an SDCard by default.
+An example of a standalone DUT is a Chipyard simulation where a program can be loaded from an SDCard out of reset.
 In this section, we mainly describe how to communicate to tethered DUTs.
 
 There are two ways the host (otherwise known as the outside world) can communicate with a tethered Chipyard DUT:
@@ -45,33 +45,21 @@ Using the Tethered Serial Interface (TSI)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 By default, Chipyard uses the Tethered Serial Interface (TSI) to communicate with the DUT.
-TSI protocol is an implementation of HTIF that is used to send commands to the
-RISC-V DUT. These TSI commands are simple R/W commands
-that are able to probe the DUT's memory space. During simulation, the host sends TSI commands to a
-simulation stub called ``SimSerial`` (C++ class) that resides in a ``SimSerial`` Verilog module
-(both are located in the ``generators/testchipip`` project). This ``SimSerial`` Verilog module then
-sends the TSI command recieved by the simulation stub into the DUT which then converts the TSI
-command into a TileLink request. This conversion is done by the ``SerialAdapter`` module
-(located in the ``generators/testchipip`` project). In simulation, FESVR
-resets the DUT, writes into memory the test program, and indicates to the DUT to start the program
-through an interrupt (see :ref:`customization/Boot-Process:Chipyard Boot Process`). Using TSI is currently the fastest
-mechanism to communicate with the DUT in simulation.
-
-In the case of a chip tapeout bringup, TSI commands can be sent over a custom communication
-medium to communicate with the chip. For example, some Berkeley tapeouts have a FPGA
-with a RISC-V soft-core that runs FESVR. The FESVR on the soft-core sends TSI commands
-to a TSI-to-TileLink converter living on the FPGA (i.e. ``SerialAdapter``). After the transaction is
-converted to TileLink, the ``TLSerdesser`` (located in ``generators/testchipip``) serializes the
-transaction and sends it to the chip (this ``TLSerdesser`` is sometimes also referred to as a
-serial-link or serdes). Once the serialized transaction is received on the
-chip, it is deserialized and masters a bus on the chip. The following image shows this flow:
-
-.. image:: ../_static/images/chip-bringup.png
-
-.. note::
-    The ``TLSerdesser`` can also be used as a slave (client), so it can sink memory requests from the chip
-    and connect to off-chip backing memory. Or in other words, ``TLSerdesser`` creates a bi-directional TileLink
-    interface.
+TSI protocol is an implementation of HTIF that is used to send commands to the RISC-V DUT.
+These TSI commands are simple R/W commands that are able to probe the DUT's memory space.
+During simulation, the host sends TSI commands to a simulation stub in the test harness called ``SimSerial``
+(C++ class) that resides in a ``SimSerial`` Verilog module (both are located in the ``generators/testchipip``
+project).
+This ``SimSerial`` Verilog module then sends the TSI command recieved by the simulation stub
+to an adapter that converts the TSI command into a TileLink request.
+This conversion is done by the ``SerialAdapter`` module (located in the ``generators/testchipip`` project).
+After the transaction is converted to TileLink, the ``TLSerdesser`` (located in ``generators/testchipip``) serializes the
+transaction and sends it to the chip (this ``TLSerdesser`` is sometimes also referred to as a digital serial-link or SerDes).
+Once the serialized transaction is received on the chip, it is deserialized and masters a TileLink bus on the chip
+which handles the request.
+In simulation, FESVR resets the DUT, writes into memory the test program, and indicates to the DUT to start the program
+through an interrupt (see :ref:`customization/Boot-Process:Chipyard Boot Process`).
+Using TSI is currently the fastest mechanism to communicate with the DUT in simulation and is also used by FireSim.
 
 Using the Debug Module Interface (DMI)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -90,26 +78,25 @@ command into a TileLink request. This conversion is done by the DTM named ``Debu
 When the DTM receives the program to load, it starts to write the binary byte-wise into memory.
 This is considerably slower than the TSI protocol communication pipeline (i.e. ``SimSerial``/``SerialAdapter``/TileLink)
 which directly writes the program binary to memory.
-Thus, Chipyard removes the DTM by default in favor of the TSI protocol for DUT communication.
 
 Starting the TSI or DMI Simulation
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-All default Chipyard configurations use TSI to communicate between the simulation and the simulated SoC/DUT. Hence, when running a
-software RTL simulation, as is indicated in the :ref:`simulation/Software-RTL-Simulation:Software RTL Simulation` section, you are in-fact using TSI to communicate with the DUT. As a
-reminder, to run a software RTL simulation, run:
+All default Chipyard configurations use TSI to communicate between the simulation and the simulated SoC/DUT.
+Hence, when running a software RTL simulation, as is indicated in the
+:ref:`simulation/Software-RTL-Simulation:Software RTL Simulation` section, you are in-fact using TSI to communicate with the DUT.
+As a reminder, to run a software RTL simulation, run:
 
 .. code-block:: bash
 
    cd sims/verilator
    # or
    cd sims/vcs
 
-   make CONFIG=LargeBoomConfig run-asm-tests
-
-FireSim FPGA-accelerated simulations use TSI by default as well.
+   make CONFIG=RocketConfig run-asm-tests
 
-If you would like to build and simulate a Chipyard configuration with a DTM configured for DMI communication, then you must tie-off the TSI interface, and instantiate the `SimDTM`. Note that we use `WithTiedOffSerial ++ WithSimDebug` instead of `WithTiedOffDebug ++ WithSimSerial`.
+If you would like to build and simulate a Chipyard configuration with a DTM configured for DMI communication,
+then you must tie-off the serial-link interface, and instantiate the `SimDTM`.
 
 .. literalinclude:: ../../generators/chipyard/src/main/scala/config/RocketConfigs.scala
     :language: scala
@@ -129,14 +116,110 @@ Then you can run simulations with the new DMI-enabled top-level and test-harness
 Using the JTAG Interface
 ------------------------
 
-The main way to use JTAG with a Rocket Chip based system is to instantiate the Debug Transfer Module (DTM)
-and configure it to use a JTAG interface. The default Chipyard designs instantiate the DTM and configure it
-to use JTAG. You may attach OpenOCD and GDB to any of the default JTAG-enabled designs.
+Another way to interface with the DUT is to use JTAG.
+Similar to the :ref:`Advanced-Concepts/Chip-Communication:Using the Debug Module interface (DMI)` section, in order to use the JTAG protocol,
+the DUT needs to contain a Debug Transfer Module (DTM) configured to use JTAG instead of DMI.
+Once the JTAG port is exposed, the host can communicate over JTAG to the DUT through a simulation stub
+called ``SimJTAG`` (C++ class) that resides in a ``SimJTAG`` Verilog module (both reside in the ``generators/rocket-chip`` project).
+This simulation stub creates a socket that OpenOCD and GDB can connect to when the simulation is running.
+The default Chipyard designs instantiate the DTM configured to use JTAG (i.e. ``RocketConfig``).
+
+.. note::
+    As mentioned, default Chipyard designs are enabled with JTAG.
+    However, they also use TSI/Serialized-TL with FESVR in case the JTAG interface isn't used.
+    This allows users to choose how to communicate with the DUT (use TSI or JTAG).
 
 Debugging with JTAG
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~
+
+Roughly the steps to debug with JTAG in simulation are as follows:
+
+1. Build a Chipyard JTAG-enabled RTL design. Remember default Chipyard designs are JTAG ready.
+
+.. code-block:: bash
+
+    cd sims/verilator
+    # or
+    cd sims/vcs
+
+    make CONFIG=RocketConfig
+
+2. Run the simulation with remote bit-bang enabled. Since we hope to load/run the binary using JTAG,
+   we can pass ``none`` as a binary (prevents FESVR from loading the program). (Adapted from: https://github.com/chipsalliance/rocket-chip#3-launch-the-emulator)
+
+.. code-block:: bash
+
+    # note: this uses Chipyard make invocation to run the simulation to properly wrap the simulation args
+    make CONFIG=RocketConfig BINARY=none SIM_FLAGS="+jtag_rbb_enable=1 --rbb-port=9823" run-binary
 
-Please refer to the following resources on how to debug with JTAG.
+3. `Follow the instructions here to connect to the simulation using OpenOCD + GDB. <https://github.com/chipsalliance/rocket-chip#4-launch-openocd>`__
+
+.. note::
+    This section was adapted from the instruction in Rocket Chip and riscv-isa-sim. For more information refer
+    to that documentation: `Rocket Chip GDB Docs <https://github.com/chipsalliance/rocket-chip#-debugging-with-gdb>`__,
+    `riscv-isa-sim GDB Docs <https://github.com/riscv/riscv-isa-sim#debugging-with-gdb>`__
+
+Example Test Chip Bringup Communication
+---------------------------------------
+
+Intro to Typical Chipyard Test Chip
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Most, if not all, Chipyard configurations are tethered using TSI (over a serial-link) and have access
+to external memory through an AXI port (backing AXI memory).
+The following image shows the DUT with these set of default signals:
+
+.. image:: ../_static/images/default-chipyard-config-communication.png
+
+In this setup, the serial-link is connected to the TSI/FESVR peripherals while the AXI port is connected
+to a simulated AXI memory.
+However, AXI ports tend to have many signals associated with them so instead of creating an AXI port off the DUT,
+one can send the memory transactions over the bi-directional serial-link (``TLSerdesser``) so that the main
+interface to the DUT is the serial-link (which has comparatively less signals than an AXI port).
+This new setup (shown below) is a typical Chipyard test chip setup:
+
+.. image:: ../_static/images/bringup-chipyard-config-communication.png
+
+Simulation Setup of the Example Test Chip
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To test this type of configuration (TSI/memory transactions over the serial-link), most of the same TSI collateral
+would be used.
+The main difference is that the TileLink-to-AXI converters and simulated AXI memory resides on the other side of the
+serial-link.
+
+.. image:: ../_static/images/chip-bringup-simulation.png
+
+.. note::
+    Here the simulated AXI memory and the converters can be in a different clock domain in the test harness
+    than the reference clock of the DUT.
+    For example, the DUT can be clocked at 3.2GHz while the simulated AXI memory can be clocked at 1GHz.
+    This functionality is done in the harness binder that instantiates the TSI collateral, TL-to-AXI converters,
+    and simulated AXI memory.
+    See :ref:`Advanced-Concepts/Harness-Clocks:Creating Clocks in the Test Harness` on how to generate a clock
+    in a harness binder.
+
+This type of simulation setup is done in the following multi-clock configuration:
+
+.. literalinclude:: ../../generators/chipyard/src/main/scala/config/RocketConfigs.scala
+    :language: scala
+    :start-after: DOC include start: MulticlockAXIOverSerialConfig
+    :end-before: DOC include end: MulticlockAXIOverSerialConfig
+
+Bringup Setup of the Example Test Chip after Tapeout
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Assuming this example test chip is taped out and now ready to be tested, we can communicate with the chip using this serial-link.
+For example, some Berkeley tapeouts of Chipyard chips have an FPGA running a RISC-V soft-core that is able to speak to the DUT.
+This RISC-V soft-core would serve as the host of the test that will run on the DUT.
+This is done by the RISC-V soft-core running FESVR, sending TSI commands to a ``SerialAdapter`` / ``TLSerdesser`` programmed on the FPGA.
+Once the commands are converted to serialized TileLink, then they can be sent over some medium to the DUT
+(like an FMC cable or a set of wires connecting FPGA outputs to the DUT board).
+Similar to simulation, if the chip requests offchip memory, it can then send the transaction back over the serial-link.
+Then the request can be serviced by the channel of FPGA DRAM.
+The following image shows this flow:
+
+.. image:: ../_static/images/chip-bringup.png
 
-* https://github.com/chipsalliance/rocket-chip#-debugging-with-gdb
-* https://github.com/riscv/riscv-isa-sim#debugging-with-gdb
+In fact, this exact type of bringup setup is what the following section discusses:
+:ref:`Prototyping/VCU118:Introduction to the Bringup Platform`.

docs/Advanced-Concepts/Harness-Clocks.rst

@@ -0,0 +1,38 @@
+.. _harness-clocks:
+
+Creating Clocks in the Test Harness
+===================================
+
+Chipyard currently allows the SoC design (everything under ``ChipTop``) to
+have independent clock domains through diplomacy.
+This implies that some reference clock enters the ``ChipTop`` and then is divided down into
+separate clock domains.
+From the perspective of the ``TestHarness`` module, the ``ChipTop`` clock and reset is
+provided from the harness clock and reset (called ``harnessClock`` and ``harnessReset``).
+In the default case, this ``harnessClock`` and ``harnessReset`` is directly wired to the
+clock and reset IO's of the ``TestHarness`` module.
+However, the ``TestHarness`` has the ability to generate a standalone clock and reset signal
+that is separate from the reference clock/reset of ``ChipTop``.
+This allows harness components (including harness binders) the ability to "request" a clock
+for a new clock domain.
+This is useful for simulating systems in which modules in the harness have independent clock domains
+from the DUT.
+
+Requests for a harness clock is done by the ``HarnessClockInstantiator`` class in ``generators/chipyard/src/main/scala/TestHarness.scala``.
+This class is accessed in harness components by referencing the Rocket Chip parameters key ``p(HarnessClockInstantiatorKey)``.
+Then you can request a clock and syncronized reset at a particular frequency by invoking the ``requestClockBundle`` function.
+Take the following example:
+
+.. literalinclude:: ../../generators/chipyard/src/main/scala/HarnessBinders.scala
+    :language: scala
+    :start-after: DOC include start: HarnessClockInstantiatorEx
+    :end-before: DOC include end: HarnessClockInstantiatorEx
+
+Here you can see the ``p(HarnessClockInstantiatorKey)`` is used to request a clock and reset at ``memFreq`` frequency.
+
+.. note::
+    In the case that the reference clock entering ``ChipTop`` is not the overall reference clock of the simulation
+    (i.e. not the clock/reset coming into the ``TestHarness`` module), the ``harnessClock`` and ``harnessReset`` can
+    differ from the implicit ``TestHarness`` clock and reset. For example, if the ``ChipTop`` reference is 500MHz but an
+    extra harness clock is requested at 1GHz, the ``TestHarness`` implicit clock/reset will be at 1GHz while the ``harnessClock``
+    and ``harnessReset`` will be at 500MHz.

docs/Advanced-Concepts/index.rst

@@ -14,4 +14,5 @@ They expect you to know about Chisel, Parameters, configs, etc.
    Debugging-BOOM
    Resources
    CDEs
+   Harness-Clocks
 

generators/chipyard/src/main/scala/ChipTop.scala

@@ -15,6 +15,9 @@ import barstools.iocell.chisel._
 
 case object BuildSystem extends Field[Parameters => LazyModule]((p: Parameters) => new DigitalTop()(p))
 
+trait HasReferenceClockFreq {
+  def refClockFreqMHz: Double
+}
 
 /**
  * The base class used for building chips. This constructor instantiates a module specified by the BuildSystem parameter,
@@ -24,15 +27,16 @@ case object BuildSystem extends Field[Parameters => LazyModule]((p: Parameters)
  */
 
 class ChipTop(implicit p: Parameters) extends LazyModule with BindingScope
-    with HasTestHarnessFunctions with HasIOBinders {
+    with HasTestHarnessFunctions with HasReferenceClockFreq with HasIOBinders {
   // The system module specified by BuildSystem
   lazy val lazySystem = LazyModule(p(BuildSystem)(p)).suggestName("system")
 
-  // The implicitClockSinkNode provides the implicit clock and reset for the System
+  // The implicitClockSinkNode provides the implicit clock and reset for the system (connected by clocking scheme)
   val implicitClockSinkNode = ClockSinkNode(Seq(ClockSinkParameters(name = Some("implicit_clock"))))
 
   // Generate Clocks and Reset
-  p(ClockingSchemeKey)(this)
+  val mvRefClkFreq = p(ClockingSchemeKey)(this)
+  def refClockFreqMHz: Double = mvRefClkFreq.getWrappedValue
 
   // NOTE: Making this a LazyRawModule is moderately dangerous, as anonymous children
   // of ChipTop (ex: ClockGroup) do not receive clock or reset.

generators/chipyard/src/main/scala/Clocks.scala

@@ -7,7 +7,7 @@ import scala.collection.mutable.{ArrayBuffer}
 import freechips.rocketchip.prci._
 import freechips.rocketchip.subsystem.{BaseSubsystem, SubsystemDriveAsyncClockGroupsKey, InstantiatesTiles}
 import freechips.rocketchip.config.{Parameters, Field, Config}
-import freechips.rocketchip.diplomacy.{OutwardNodeHandle, InModuleBody, LazyModule}
+import freechips.rocketchip.diplomacy.{ModuleValue, OutwardNodeHandle, InModuleBody, LazyModule}
 import freechips.rocketchip.util.{ResetCatchAndSync}
 
 import barstools.iocell.chisel._
@@ -38,7 +38,7 @@ object GenerateReset {
 }
 
 
-case object ClockingSchemeKey extends Field[ChipTop => Unit](ClockingSchemeGenerators.dividerOnlyClockGenerator)
+case object ClockingSchemeKey extends Field[ChipTop => ModuleValue[Double]](ClockingSchemeGenerators.dividerOnlyClockGenerator)
 /*
   * This is a Seq of assignment functions, that accept a clock name and return an optional frequency.
   * Functions that appear later in this seq have higher precedence that earlier ones.
@@ -59,7 +59,7 @@ class ClockNameContainsAssignment(name: String, fMHz: Double) extends Config((si
 })
 
 object ClockingSchemeGenerators {
-  val dividerOnlyClockGenerator: ChipTop => Unit = { chiptop =>
+  val dividerOnlyClockGenerator: ChipTop => ModuleValue[Double] = { chiptop =>
     implicit val p = chiptop.p
 
     // Requires existence of undriven asyncClockGroups in subsystem
@@ -75,22 +75,25 @@ object ClockingSchemeGenerators {
     }
 
     val aggregator = LazyModule(new ClockGroupAggregator("allClocks")).node
+    // provides the implicit clock to the system
     (chiptop.implicitClockSinkNode
       := ClockGroup()
       := aggregator)
+    // provides the system clock (ex. the bus clocks)
     (systemAsyncClockGroup
       :*= resetSetter
       :*= ClockGroupNamePrefixer()
       :*= aggregator)
 
     val referenceClockSource =  ClockSourceNode(Seq(ClockSourceParameters()))
+    val dividerOnlyClkGenerator = LazyModule(new DividerOnlyClockGenerator("buildTopClockGenerator"))
+    // provides all the divided clocks (from the top-level clock)
     (aggregator
       := ClockGroupFrequencySpecifier(p(ClockFrequencyAssignersKey), p(DefaultClockFrequencyKey))
       := ClockGroupResetSynchronizer()
-      := DividerOnlyClockGenerator()
+      := dividerOnlyClkGenerator.node
       := referenceClockSource)
 
-
     InModuleBody {
       val clock_wire = Wire(Input(Clock()))
       val reset_wire = GenerateReset(chiptop, clock_wire)
@@ -105,6 +108,9 @@ object ClockingSchemeGenerators {
       chiptop.harnessFunctions += ((th: HasHarnessSignalReferences) => {
         clock_io := th.harnessClock
         Nil })
+
+      // return the reference frequency
+      dividerOnlyClkGenerator.module.referenceFreq
     }
   }
 }