ibex/vendor/lowrisc_ip/csr_utils

CSR utilities

This csr_utils folder intends to implement CSR related methods and test sequences for DV to share across all testbenches.

CSR utility package

csr_utils_pkg provides common methods and properties to support and manage CSR accesses and CSR related test sequences.

Global types and variables

All common types and variables are defined at this package level. Examples are:

  uint       outstanding_accesses        = 0;
  uint       default_timeout_ns          = 1_000_000;

Outstanding_accesses

csr_utils_pkg used an internal variable to store the number of accesses (read or write) that have not yet completed. This variable is shared among all methods of register reading and writing. Directly accessing this variable is discouraged. Instead, the following methods are used to control this variable to keep track of non-blocking accesses made in the testbench:

  function automatic void increment_outstanding_access();
    outstanding_accesses++;
  endfunction

  function automatic void decrement_outstanding_access();
    outstanding_accesses--;
  endfunction

  task automatic wait_no_outstanding_access();
    wait(outstanding_accesses == 0);
  endtask

  function automatic void clear_outstanding_access();
    outstanding_accesses = 0;
  endfunction

CSR spinwait

One of the commonly used tasks in csr_utils_pkg is csr_spinwait. This task can poll a CSR or CSR field continuously or periodically until it reads out the expected value. This task also has a timeout check in case due to DUT or testbench issue, the CSR or CSR field never returns the expected value. Example below uses the csr_spinwait to wait until the CSR fifo_status field fifo_full reaches value bit 1:

csr_spinwait(.ptr(ral.status.fifo_full), .exp_data(1'b0));

Read and check all CSRs

The purpose of the read_and_check_all_csrs task is to read all valid CSRs from the given uvm_reg_block and check against their expected values from RAL. This task is primarily implemented to use after reset, to make sure all the CSRs are being reset to the default value.

Under_reset

Due to csr_utils_pkg is not connected to any interface, methods inside this package are not able to get reset information. Current the under_reset bit is declared with two functions:

function automatic void reset_asserted();
  under_reset = 1;
endfunction

function automatic void reset_deasserted();
  under_reset = 0;
endfunction

This reset information is updated in dv_lib/dv_base_vseq.sv. When the apply_reset task is triggered, it will set and reset the under_reset bit via the functions above.

Global CSR util methods

Global methods for CSR and MEM attributes

This package provides methods to access CSR or Memory attributes, such as address, value, etc. Examples are:

• get_csr_addrs
• get_mem_addr_ranges
• decode_csr_or_field

Global methods for CSR access

The CSR access methods are based on uvm_reg methods, such as uvm_reg::read(), uvm_reg::write(), uvm_reg::update(). For all CSR methods, user can pass either a register or a field handle. Examples are:

• csr_rd_check: Given the uvm_reg or uvm_reg_field object, this method will compare the CSR value with the expected value (given as an input) or with the RAL mirrored value
• csr_update: Given the uvm_reg object, this method will update the value of the register in DUT to match the desired value

To enhance the usability, these methods support CSR blocking, non-blocking read/write, and a timeout checking.

• A blocking thread will not execute the next sequence until the current CSR access is finished
• A non-blocking thread allows multiple CSR accesses to be issued back-to-back without waiting for the response
• A timeout check will discard the ongoing CSR access by disabling the forked thread and will throw a UVM_ERROR once the process exceeds the max timeout setting

CSR sequence library

csr_seq_lib.sv provides common CSR related test sequences to share across all testbenches. These test sequences are based off the standard sequences provided in UVM1.2 RAL. The parent class (DUT-specific test or sequence class) that creates them needs to provide them with the DUT RAL model. The list of CSRs are then extracted from the RAL model to performs the checks. In addition, the test sequences provide an ability to exclude a CSR from writes or reads (or both) depending on the behavior of the CSR in the design. This is explained more in the CSR exclusion methodology section below. All CSR accesses in these sequences are made non-blocking to ensure back-to-back scenarios are exercised. Supported CSR test sequences are:

• csr_hw_reset: Write all CSRs with random values and then reset the DUT. After reset, read all CSRs and compare with expected values
• csr_rw: Write a randomly selected CSRs, then read out the updated CSR or CSR field and compare with expected value
• csr_bit_bash: Randomly select a CSR and write 1's and 0's to every bit, then read the CSR to compare with expected value
• csr_aliasing: Randomly write a CSR, then read all CSRs to verify that only the CSR that was written was updated
• mem_walk: Write and read all valid addresses in the memory. Compare the read results with the expected values

CSR exclusion methodology

The CSR test sequences listed above intend to perform a sanity check to CSR read/write accesses, but do not intend to check specific DUT functionalities. Thus the sequences might need to exclude reading or writing certain CSRs depending on the specific testbench. csr_excl_item is a class that supports adding exclusions to CSR test sequences. Examples of useful functions in this class are:

• add_excl: Add exclusions to the CSR test sequences. This function has two inputs:

  • Exclusion scope: A hierarchical path name at all levels including block, CSR, and field. This input supports * and ? wildcards for glob style matching
  • CSR_exclude type: An enumeration defined as below:

    typedef enum bit[2:0] {
      CsrNoExcl         = 3'b000, // no exclusions
      CsrExclInitCheck  = 3'b001, // exclude csr from init val check
      CsrExclWriteCheck = 3'b010, // exclude csr from write-read check
      CsrExclCheck      = 3'b011, // exclude csr from init or write-read check
      CsrExclWrite      = 3'b100, // exclude csr from write
      CsrExclAll        = 3'b111  // exclude csr from init or write or write-read check
    } csr_excl_type_e;
    

  One example to use this function in HMAC to exclude all CSRs or fields with names starting with "key":

  csr_excl.add_excl({scope, ".", "key?"}, CsrExclWrite);
  

• has_excl: Check if the CSR has a match in the existing exclusions loopup, and is not intended to use externally

CSR sequence framework

The [cip_lib]({{< relref "hw/dv/sv/cip_lib/doc" >}}) includes a virtual sequence named cip_base_vseq, that provides a common framework for all testbenchs to run these CSR test sequences and add exclusions.