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[dv] Fix race condition in cs_registers testbench

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The `driver_tick` DPI call drove inputs directly but was being scheduled
in an undefined order with other always_ff blocks.  This results in a
race condition where some always_ff blocks see old inputs and others see
new in the same clock tick. Instead use values from `driver_tick` to
perform NBA updates and avoid the race condition.
This commit is contained in:
Greg Chadwick 2021-01-15 16:29:08 +00:00
parent 8defdc4d6c
commit c8c3c55071
1 changed files with 24 additions and 5 deletions
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29
dv/cs_registers/tb/tb_cs_registers.sv
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@ -33,6 +33,11 @@ module tb_cs_registers #(
logic illegal_csr_insn_o;
logic csr_access_d;
ibex_pkg::csr_num_e csr_addr_d;
logic [31:0] csr_wdata_d;
ibex_pkg::csr_op_e csr_op_d;
logic csr_op_en_d;
//-----------------
// Reset generation
//-----------------
@ -121,12 +126,26 @@ module tb_cs_registers #(
csr_addr_i,
csr_wdata_i,
csr_rdata_o);
reg_dpi::driver_tick("reg_driver",
csr_access_i,
csr_op_i,
csr_op_en_i,
csr_addr_i,
csr_wdata_i);
csr_access_d,
csr_op_d,
csr_op_en_d,
csr_addr_d,
csr_wdata_d);
// Use NBA to drive inputs to ensure correct scheduling.
// This always_ff block will be executed on the positive edge of the clock with undefined order
// vs all other always_ff triggered on the positive edge of the clock. If `driver_tick` drives
// the inputs directly some of the always_ff blocks will see the old version of the inputs and
// others will see the new version depending on scheduling order. This schedules all the inputs
// to be NBA updates to avoid the race condition (in effect acting like any other always_ff
// block with the _d values being computed via DPI rather than combinational logic).
csr_access_i <= csr_access_d;
csr_addr_i <= csr_addr_d;
csr_wdata_i <= csr_wdata_d;
csr_op_i <= csr_op_d;
csr_op_en_i <= csr_op_en_d;
end
endmodule