Config in UVM_ How to use the DB mechanism

set Function and get The parameters of the function

config_db The mechanism is used in UVM Verify the parameters passed between platforms . They usually appear in pairs .set Send letter when function ,get The function is to receive .
For example, in a test case build_phase You can use the following methods to send letters ：
uvm_config_db#(int)::set(this, "env.i_agt.drv" "pre_num", 100);
The first parameter and the second parameter are combined to form the target path , Only the target that matches this path can receive . The first parameter must be a uvm_component Pointer to instance , The second parameter is the path relative to this instance . General , If the first parameter is set to this, Then the second parameter can be an empty string . The third parameter is set The third argument in the function , These two parameters must strictly match , The fourth parameter is the variable to be set .

stay top_tb Pass through config_db The mechanism set Function settings virtual interface when ,set The first function of the function is null. under these circumstances ,UVM Will automatically replace the first singular with uvm_root::get(), namely uvm_top. let me put it another way , The following two expressions are completely equivalent ：

uvm_config_db#(virtual my_if)::set(null, "uvm_test_top.env.i_agt.drv", "vif", input_vif);
uvm_config_db#(virtual my_if)::set(uvm_root::get(), "uvm_test_top.env.i_agt.drv", "vif", input_vif);

set And get The third argument in the function can be associated with get The fourth parameter in the function is different .

umv_config_db#(int)::set(this, "env.i_agt.drv", "p_num", 100);
umv_config_db#(int)::set(this, "", "p_num", pre_num);

Omit get sentence
src/ch3/section3.5/3.5.3/my_driver.sv

7 int pre_num;
8 `uvm_component_utils_begin(my_driver)
9 `uvm_field_int(pre_num, UVM_ALL_ON)
10 `uvm_component_utils_end
11
12 function new(string name = "my_driver", uvm_component parent = null);
13 super.new(name, parent);
14 pre_num = 3;
15 endfunction
16
17 virtual function void build_phase(uvm_phase phase);
18 `uvm_info("my_driver", $sformatf("before super.build_phase, the pre_num is %0d", pre_num), UVM_LOW)
19 super.build_phase(phase);
20 `uvm_info("my_driver", $sformatf("after super.build_phase, the pre_num is %0d", pre_num), UVM_LOW)
21 if(!uvm_config_db#(virtual my_if)::get(this, "", "vif", vif))
22 `uvm_fatal("my_driver", "virtual interface must be set for vif!!!")
23 endfunction

uvm_config_db#(int)::get(this, "", "pre_num", pre_num);
The key here is build_phase Medium super.build_phase sentence , When executed driver Of super.build_phase when , Automatically get sentence . Before this practice
Yes, it is ： First of all ,my_driver You have to use uvm_component_utils Macro registration ; second ,pre_num You have to use uvm_field_int Macro registration ; Third , Calling set function
When ,set The third argument to the function must be the same as the argument to get The names of the variables in the function are consistent , It has to be pre_num.

Multiple settings across levels

In the figure 3-4 in , If uvm_test_top and env All right driver Of pre_num Set the value of , stay uvm_test_top The setting statements in are as follows ：
file ：src/ch3/section3.5/3.5.4/normal/my_case0.sv

32 function void my_case0::build_phase(uvm_phase phase);
33 super.build_phase(phase);
…
39 uvm_config_db#(int)::set(this,
40 "env.i_agt.drv",
41 "pre_num",
42 999);
43 `uvm_info("my_case0", "in my_case0, env.i_agt.drv.pre_num is set to 999",UVM_LOW)

stay env The setting statement of is as follows ：
file ：src/ch3/section3.5/3.5.4/normal/my_env.sv

19 virtual function void build_phase(uvm_phase phase);
20 super.build_phase(phase);
…
31 uvm_config_db#(int)::set(this,
32 "i_agt.drv",
33 "pre_num",
34 100);
35 `uvm_info("my_env", "in my_env, env.i_agt.drv.pre_num is set to 100",UVM_LOW)
36 endfunction

that driver The value obtained in is 100 still 999 Well ？ The answer is 999.UVM The higher the level of regulation , Then the higher its priority . The hierarchy here refers to UVM
The position in the tree , The closer to the root node uvm_top, The higher the level .uvm_test_top The level of is higher than env Of , therefore uvm_test_top Medium set Priority of function
High class .
The above conclusion is in set The first argument to the function is this The time was established , But if set The first argument to the function is not this How will? ？ hypothesis uvm_test_top Of set The sentence is ：
file ：src/ch3/section3.5/3.5.4/abnormal/my_case0.sv
32 function void my_case0::build_phase(uvm_phase phase);
33 super.build_phase(phase);
…
39 uvm_config_db#(int)::set(uvm_root::get(),
40 “uvm_test_top.env.i_agt.drv”,
41 “pre_num”,
42 999);
43 `uvm_info(“my_case0”, “in my_case0, env.i_agt.drv.pre_num is set to 999”, UVM_LOW)

and env Of set The sentence is ：
file ：src/ch3/section3.5/3.5.4/normal/my_env.sv
19 virtual function void build_phase(uvm_phase phase);
20 super.build_phase(phase);
…
31 uvm_config_db#(int)::set(uvm_root::get(),
32 “uvm_test_top.env.i_agt.drv”,
33 “pre_num”,
34 100);
35 `uvm_info(“my_env”, “in my_env, env.i_agt.drv.pre_num is set to 100”,UVM_LOW)
36 endfunction
In this case ,driver Got pre_num The value of is 100. because set The first argument to the function is uvm_root::get（）, So the sender became uvm_top. under these circumstances , We can only compare the time of sending letters .UVM Of build_phase It's top-down ,my_case0 Of build_phase Precede my_env Of build_phase perform . therefore my_env Yes pre_num The setting of is after , Its setting becomes the final setting .
If uvm_test_top in set The first argument to the function is this, and env in set The first argument to the function is uvm_root::get（）, that driver Got
pre_num The value of is also 100. This is because env in set The sender of the function becomes uvm_top, stay UVM The highest priority in the tree .
therefore , in any case , Calling set The first parameter of a function should try to use this. When you can't get this In case of pointer （ If in top_tb in ）, Use null
perhaps uvm_root::get().

Multiple settings at the same level

When looking at problems across levels , It's high-level set Set priority ; When at the same level , As mentioned in the previous section , Time first .

uvm_config_db#(int)::set(this, "env.i_agt.drv", "pre_num", 100);
uvm_config_db#(int)::set(this, "env.i_agt.drv", "pre_num", 109);

When the above two statements appear in the test case at the same time build_phase In the middle of the day ,driver The final value obtained will be 109. Usage like the one above seems to be completely
That's monkey business , It doesn't make any sense .
A principle of writing code in validation is that the same statement only appears in one place , Try to avoid appearing in multiple places . The solution to this problem is to base_test Of
build_phase Use in config_db：：set Set it up , such , When base_test Derived from case1～case99 In execution super.build_phase（phase） when , Will be set ：

classs base_test extends uvm_test;
function void build_phase(uvm_phase phase);
super.build_phase(phase);
uvm_config_db#(int)::set(this, "env.i_agt.drv", pre_num_max, 7);
endfunction
endclass
class case1 extends base_test;
function void build_phase(uvm_phase phase);
super.build_phase(phase);
endfunction
endclass
…
class case99 extends base_test;
function void build_phase(uvm_phase phase);
super.build_phase(phase);
endfunction
endclass

But for the first 100 Test cases , It still needs to be written like this ：

class case100 extends base_test;
function void build_phase(uvm_phase phase);
super.build_phase(phase);
uvm_config_db#(int)::set(this, "env.i_agt.drv", pre_num_max, 100);
endfunction
endclass

case100 Of build_phase It is equivalent to two consecutive settings as shown below ：

uvm_config_db#(int)::set(this, "env.i_agt.drv", "pre_num", 7);
uvm_config_db#(int)::set(this, "env.i_agt.drv", "pre_num", 100);

According to the principle of time first , Back config_db：：set The value of will eventually be driver obtain .