{{% lowrisc-doc-hdr Comportable IP Testbench Architecture }}
Going along the lines of what it takes to design an IP that adheres to the Comportability Specifications, we attempt to standardize the DV methodology for developing the IP level testbench environment as well by following the same approach. This document describes the Comportable IP (CIP) library, which is a complete UVM enviromnent framework that each IP level environment components can extend from to get started with DV. The goal here is to maximize code reuse across all test benches so that we can improve the efficiency and time to market. The features described here are not exhaustive, so it is highly recommended to the reader that they examine the code directly. In course of development, we also periodically identify pieces of verification logic that might be developed for one IP but is actually a good candidate to be added to these library classes instead. This doc is instead intended to provide the user a foray into what these are and how are the meant to be used.
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The CIP library includes the base ral model, env cfg object, coverage object, virtual sequencer, scoreboard, env, base virtual sequence and finally the test class. To achieve run-time polymorphism, these classes are type parameterized to indicate what type of child objects are to be created. In the IP environments, the extended classes indicate the correct type parameters.
This class is intended to contain all of the settings, knobs, features, interface handles and downstream agent cfg handles. Features that are common to all IPs in accordance with the comportability spec are made a part of this base class, while the extended IP env cfg class will contain settings specific to that IP. An instance of the env cfg class is created in cip_base_test::build_phase and the handle is passed over uvm_config_db for the CIP env components to pick up. This allows the handle to the env cfg object to be available in the env's build_phase. Settings in the env cfg can then be used to configure the env based on the test needs.
A handle to this class instance is passed on to the scoreboard, virtual sequencer and coverage objects so that all such common settings and features are instantly accessible everywhere.
This class is type parameterized in the following way:
class cip_base_env_cfg #(type RAL_T = dv_base_reg_block) extends uvm_object;
The IP env cfg class will then extend from this class with the RAL_T parameter set to the actual IP RAL model class. This results in IP RAL model getting factory overridden automatically in the base env cfg itself during creation, so there is no need for manual factory override. We follow the same philosophy in all CIP library classes.
The following is a list of common features and settings:
pins_if #(NUM_MAX_INTERRUPTS=64) interface instance created in the tb to hookup the DUT interrupts. The actual number of interrupts might be much less than 64, but that is ok - we just connect as many as the DUT provides. The reason for going with a fixed width pins_if is to allow the intr_vif to be available in this base env cfg class (which does not know how many interrupt each IP DUT provides).pins_if #(NUM_MAX_ALERTS=64) interface instance created in the tb to hookup the DUT alerts, similar to intr_vif.pins_if #(1) interface instance created in the tb to hookup the DUT input devmode.dv_base_reg_block. In the base class, this is created using the RAL_T class parameter which the extended IP env cfg class sets correctly.Apart from these, there are several common settings such as zero_delays, clk_freq_mhz, which are randomized as well as knobs such as en_scb and en_cov to turn on/off scoreboard and coverage collection respectively.
The base class provides a virtual method called initialize() which is called in cip_base_test::build_phase to create some of the objects listed above. If the extended IP env cfg class has more such objects added, then the initialize() method is required to be overridden to create those objects as well.
We make all downstream interface agent cfg handles as a part of IP extension of cip_base_env_cfg so that all settings for the env and all downstream agents are avaiable within the env cfg handle. Since the env cfg handle is passed to all cip components, all those settings are also accesible.
This is the base coverage object that contain all functional coverpoints and covergroups. The main goal is to have all functional coverage elements implemented in a single place. This class is extended from uvm_component so that it allows items to be set via 'uvm_config_db using the component's hierarchy. This is created in cip_base_env and a handle to it is passed to the scoreboard and the virtual sequencer. This allows coverage to be sampled in scoreboard as well as the test sequences.
This class is type parameterized with the env cfg class type CFG_T so that it can derive coverage on some of the env cfg settings.
class cip_base_env_cov #(type CFG_T = cip_base_env_cfg) extends uvm_component;
This is the base virtual sequencer class that contains a handle to the tl_sequencer to allow layered test sequences to be created. The extended IP virtual sequencer class will include handles to the IP specific agent sequencers. G This class is type-parameterized with the env cfg class type CFG_T and coverage class type COV_T so that all test sequences can access the env cfg settings and sample the coverage via the p_sequencer handle.
class cip_base_virtual_sequencer #(type CFG_T = cip_base_env_cfg,
type COV_T = cip_base_env_cov) extends uvm_sequencer;
This is the base scoreboard component that already connects with the TileLink agent monitor to grab tl packets via analysis port at the address and the data phases. It provides a virtual task called process_tl_access that the extended IP scoreboard needs to implement. Please see code for additional details. The extended IP scoreboard class will connect with the IP-specific interface monitors if applicable to grab items from those analysis ports.
This class is type-parameterized with the env cfg class type CFG_T, ral class type RAL_T and the coverage class type COV_T.
class cip_base_scoreboard #(type RAL_T = dv_base_reg_block,
type CFG_T = cip_base_env_cfg,
type COV_T = cip_base_env_cov) extends uvm_component;
There are several virtual tasks and functions that are to be overridden in extended IP scoreboard class. Please take a look at the code for more details.
This is the base UVM env that puts all of the above components together and creates and makes connections across them. In the build phase, it retrieves the env cfg class type handle from uvm_config_db as well as all the virtual interfaces (which are actually part of the env cfg class). It then uses the env cfg settings to modify the downstream agent cfg settings as required. All of the above components are created based on env cfg settings, along with the TileLink host agent. In the connect phase, the scoreboard connects with the monitor within the TileLink agent to be able to grab packets from the TL interface during address and the data phases. In the end of elaboration phase, the ral model within the env cfg handle is locked and the ral sequencer and adapters are set to be used with the TileLink interface.
This class is type parameterized with env cfg class type CFG_T, coverage class type COV_T, virtual sequencer class type VIRTUAL_SEQUENCER_T and scoreboard class type SCOREBOARD_T.
class cip_base_env #(type CFG_T = cip_base_env_cfg,
type VIRTUAL_SEQUENCER_T = cip_base_virtual_sequencer,
type SCOREBOARD_T = cip_base_scoreboard,
type COV_T = cip_base_env_cov) extends uvm_env;
This is the base virtual sequence class that will run on the cip virtual sequencer. This base class provides ‘sequencing’ set of tasks such as dut_init() and dut_shutdown() which are called within pre_start and post_start respectively. This sequence also provides an array of sub-sequences some of which are complete tests within themselves, but implemented as tasks. The reason for doing so is SystemVerilog does not support multi-inheritance so all sub-sequences that are identified as being common to all IP benches implemented as tasks in this base virtual sequence class. Some examples:
add_csr_exclusions.This class is type parameterized with the env cfg class type CFG_T, ral class type RAL_T and the virtual sequencer class type VIRTUAL_SEQUENCER_T so that the env cfg settings, the ral CSRs are accessible and the p_sequencer type can be declared.
class cip_base_vseq #(type RAL_T = dv_base_reg_block,
type CFG_T = cip_base_env_cfg,
type COV_T = cip_base_env_cov,
type VIRTUAL_SEQUENCER_T = cip_base_virtual_sequencer) extends uvm_sequence;
All virtual sequences in the extended IP will eventually extend from this class and can hence, call these tasks and functions directly as needed.
This basically creates the IP UVM env and its env cfg class instance. Any env cfg setting that may be required to be controlled externally via plusargs are looked up here, before the env instance is created. This also sets a few variables that pertain to how / when should the test exit on timeout or failure. In the run phase, the test calls run_seq which basically uses factory to create the virtual sequence instance using the UVM_TEST_SEQ string that is passed via plusarg. As a style guide, it is preferred to encapsulate a complete test within a virtual sequence and use the same UVM_TEST plusarg for all of the tests (which points to the extended IP test class), and only change the UVM_TEST_SEQ plusarg.
This class is type parameterized with the env cfg class type CFG_T and the env class type ENV_T so that when extended IP test class creates the env and env cfg specific to that IP.
class cip_base_test #(type CFG_T = cip_base_env_cfg,
type ENV_T = cip_base_env) extends uvm_test;
Let's say we are verifying an actual comportable IP uart which has uart_tx and uart_rx interface. User then develops the uart_agent to be able to talk to that interface. User invokes the ralgen tool to generate the uart_reg_block, and then starts developing UVM environment by extending from the CIP library classes in the following way.
class uart_env_cfg extends cip_base_env_cfg #(.RAL_T(uart_reg_block));
User adds the uart_agent_cfg object as a member so that it remains as a part of the env cfg and can be accessed everywhere. In the base class's initialize() function call, an instance of uart_reg_block is created, not the dv_base_reg_block, since we override the RAL_T type.
class uart_env_cov extends cip_base_env_cov #(.CFG_T(uart_env_cfg));
User adds uart IP specific coverage items and uses the cov handle in scoreboard and test sequences to sample the coverage.
class uart_virtual_sequencer extends cip_base_virtual_sequencer #(.CFG_T(uart_env_cfg),
.COV_T(uart_env_cov));
User adds the uart_sequencer handle to allow layered test sequences to send traffic to / from TileLink as well as uart interfaces.
class uart_scoreboard extends cip_base_scoreboard #(.CFG_T(uart_env_cfg),
.RAL_T(uart_reg_block),
.COV_T(uart_env_cov));
User adds analysis ports to grab packets from the uart_monitor to perform end-to-end checking.
class uart_env extends cip_base_env #(.CFG_T (uart_env_cfg),
.COV_T (uart_env_cov),
.VIRTUAL_SEQUENCER_T (uart_virtual_sequencer),
.SCOREBOARD_T (uart_scoreboard));
User creates uart_agent object in the env and use it to connect with the virtual sequencer and the scoreboard. User also uses the env cfg settings to manipulate the uart agent cfg settings if required.
class uart_base_vseq extends cip_base_vseq #(.CFG_T (uart_env_cfg),
.RAL_T (uart_reg_block),
.COV_T (uart_env_cov),
.VIRTUAL_SEQUENCER_T (uart_virtual_sequencer));
User adds a base virtual sequence as a starting point and adds common tasks and functions to perform uart specific operations. User then extends from uart_base_vseq to add layered test sequences.
class uart_base_test extends cip_base_test #(.ENV_T(uart_env), .CFG_T(uart_env_cfg));
User sets UVM_TEST plus arg to uart_base_test so that all tests create the UVM env that is automatically tailored to UART IP. Each test then sets the UVM_TEST_SEQ plusarg to run the specific test sequence, along with additional plusargs as required.
The block diagram above shows the CIP testbench architecture, that puts together the static side tb which instantiates the dut, and the dynamic side, which is the UVM environment extended from CIP library. The diagram lists some common items that need to be instantiated in tb and set into uvm_config_db for the testbench to work.