UVM设计模式之状态模式介绍
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描述
软件设计中，FSM(Finite-State Machine)分为3部分：状态（State)，事件（Event)，动作（Action)。
状态模式(State Pattern)是行为型(Behavioral)设计模式，将软件主机端的行为归类为各个状态，状态之间可以互相转化，每种状态的行为不相同；统一交给一个Context类型的模块负责调度各个状态的跳转；
硬件设计中的FSM，不仅是一种电路的描述工具，而且也是一种思想方法；数字逻辑本质上都可以归一化为FSM；RTL描述FSM可以归类为常用的几种方法，通常采用三段式的描述；
在我们的验证环境中，有时也需要一个组件专门负责FSM的建模；例如验证USB Device DUT时，验证环境需要模拟USB Host的行为；对于USB协议复杂的状态机，使用专门的FSM组件模拟，可以减少组件间的耦合；也可以将FSM组件的状态赋值到virtual interface上，通过波形协助debug；
并不是所有DUT模块中包含FSM，验证环境中就需要对应的FSM建模；RTL的硬件电路是cycle级的时序电路，采用FSM可以很好的描述算法运算；而验证环境都是事务级的基于事件的高级抽象模型，是否需要采用FSM根据验证环境而定；对于简单的设计，不需要模拟FSM；对于复杂的标准协议，VIP中都会采用FSM建模来完成，具有高内聚低耦合的好处。
Simple example
本篇对一个示例，分别使用两种方式来描述：
一个简单的FSM如下，分为4种状态；对于状态的跳转条件，本篇通过uvm_event类型的事件触发，验证环境中的实际情况可以是事件，某一个signal状态，或者if的判断等；不同状态下的Action，仅使用一句display代表，验证环境中的实际情况可以调用某一个task，对signal的驱动，或者调用其他组件的API等；仅做结构上的演示；
通过randsequence产生激励sequence，遍历状态机跳转条件；
class client;
state_machine FSM;
uvm_event_pool events_pool;
uvm_event to_idle,to_state_a,to_state_b,to_state_c;
function new();
events_pool = uvm_event_pool::get_global_pool();
to_idle = events_pool.get("to_idle");
to_state_a = events_pool.get("to_state_a");
to_state_b = events_pool.get("to_state_b");
to_state_c = events_pool.get("to_state_c");
endfunction
task rand_simulate();
for (int i=0;i<2;i++) begin
bit FLAG = 0;
randsequence (stream)
stream : first second third last;
first : state_a;
second : state_b {FLAG = 1;} | state_c;
third : if (FLAG ==1) state_c else state_b;
last : state_idle;
state_idle: {`INTERVALTIME;to_idle.trigger();};
state_a : {`INTERVALTIME;to_state_a.trigger();};
state_b : {`INTERVALTIME;to_state_b.trigger();};
state_c : {`INTERVALTIME;to_state_c.trigger();};
endsequence
end
endtask
......
......
use task
类state_machine包含四个状态的task；通过request_state_change函数实现状态跳转；每进入一个状态，对应一个线程，当跳出状态时，注意线程需要disable掉；
class state_machine;
typedef enum {
IDLE,STATE_A,STATE_B,STATE_C
} state_t;
uvm_event_pool events_pool;
uvm_event to_idle,to_state_a,to_state_b,to_state_c;
local state_t cur_state;
extern function new();
extern function void start();
extern function void request_state_change(state_t cur_state);
extern task do_idle();
extern task do_state_a();
extern task do_state_b();
extern task do_state_c();
endclass
function state_machine::new();
events_pool = uvm_event_pool::get_global_pool();
to_idle = events_pool.get("to_idle");
to_state_a = events_pool.get("to_state_a");
to_state_b = events_pool.get("to_state_b");
to_state_c = events_pool.get("to_state_c");
endfunction
function void state_machine::start();
cur_state = IDLE;
request_state_change(cur_state);
endfunction
function void state_machine::request_state_change(state_t cur_state);
case(cur_state)
IDLE:begin
fork
begin
$display("Enter %s state!",cur_state.name());
do_idle();
end
join_none
return;
end
STATE_A:begin
fork
begin
$display("Enter %s state!",cur_state.name());
do_state_a();
end
join_none
return;
end
STATE_B:begin
fork
begin
$display("Enter %s state!",cur_state.name());
do_state_b();
end
join_none
return;
end
STATE_C:begin
fork
begin
$display("Enter %s state!",cur_state.name());
do_state_c();
end
join_none
return;
end
default : begin
$display("Enter unknow state!");
$finish;
end
endcase
endfunction
task state_machine::do_idle();
state_t cur_state;
$display("IDLE : nothing to do!
");
fork: disable_fork
begin
to_state_a.wait_trigger();
//$display("do something!
");
cur_state = STATE_A;
end
join_any
request_state_change(cur_state);
endtask
task state_machine::do_state_a();
state_t cur_state;
$display("STATE_A : do something!
");
fork: disable_fork
begin
to_state_b.wait_trigger();
//$display("do something!
");
cur_state = STATE_B;
end
begin
to_state_c.wait_trigger();
//$display("do something!
");
cur_state = STATE_C;
end
join_any
disable fork;
request_state_change(cur_state);
endtask
task state_machine::do_state_b();
state_t cur_state;
$display("STATE_B : do something!
");
fork: disable_fork
begin
to_state_c.wait_trigger();
//$display("do something!
");
cur_state = STATE_C;
end
begin
to_idle.wait_trigger();
//$display("do something!
");
cur_state = IDLE;
end
join_any
disable fork;
request_state_change(cur_state);
endtask
task state_machine::do_state_c();
state_t cur_state;
$display("STATE_C : do something!
");
fork: disable_fork
begin
to_state_b.wait_trigger();
//$display("do something!
");
cur_state = STATE_B;
end
begin
to_idle.wait_trigger();
//$display("do something!
");
cur_state = IDLE;
end
join_any
disable fork;
request_state_change(cur_state);
endtask
use Sate Pattern
采用状态模式的设计，每个状态继承于virtual class state，实现各自的do_something和request_state_change；state_machine通过宏REGISTER_STATE创建各个state实例；state_machine中forvever执行；状态模式和策略模式的实现类似，都是使用OOP的组合 + 多态实现；
virtual class state;
state_machine FSM;
uvm_event_pool events_pool;
uvm_event to_idle,to_state_a,to_state_b,to_state_c;
function new();
events_pool = uvm_event_pool::get_global_pool();
to_idle = events_pool.get("to_idle");
to_state_a = events_pool.get("to_state_a");
to_state_b = events_pool.get("to_state_b");
to_state_c = events_pool.get("to_state_c");
endfunction
pure virtual task do_something();
pure virtual task request_state_change();
endclass
class state_idle extends state;
task do_something();
$display("STATE_IDLE : nothing to do!
");
endtask
task request_state_change();
state_t cur_state;
fork: disable_fork
begin
to_state_a.wait_trigger();
//$display("do something!
");
cur_state = STATE_A;
end
join_any
FSM.set_state(cur_state);
endtask
endclass
class state_a extends state;
task do_something();
$display("STATE_A : do something!
");
endtask
task request_state_change();
state_t cur_state;
fork: disable_fork
begin
to_state_b.wait_trigger();
//$display("do something!
");
cur_state = STATE_B;
end
begin
to_state_c.wait_trigger();
//$display("do something!
");
cur_state = STATE_C;
end
join_any
disable fork;
FSM.set_state(cur_state);
endtask
endclass
class state_b extends state;
task do_something();
$display("STATE_B : do something!
");
endtask
task request_state_change();
state_t cur_state;
fork: disable_fork
begin
to_state_c.wait_trigger();
//$display("do something!
");
cur_state = STATE_C;
end
begin
to_idle.wait_trigger();
//$display("do something!
");
cur_state = STATE_IDLE;
end
join_any
disable fork;
FSM.set_state(cur_state);
endtask
endclass
class state_c extends state;
task do_something();
$display("STATE_C : do something!
");
endtask
task request_state_change();
state_t cur_state;
fork: disable_fork
begin
to_state_b.wait_trigger();
//$display("do something!
");
cur_state = STATE_B;
end
begin
to_idle.wait_trigger();
//$display("do something!
");
cur_state = STATE_IDLE;
end
join_any
disable fork;
FSM.set_state(cur_state);
endtask
endclass
class state_machine;
local state state_m;
state state_pool[state_t];
function void set_state(state_t state);
$display("Enter %s state!",state.name());
state_m = state_pool[state];
endfunction
function void fsm_init();
`REGISTER_STATE(IDLE,idle)
`REGISTER_STATE(A,a)
`REGISTER_STATE(B,b)
`REGISTER_STATE(C,c)
this.set_state(STATE_IDLE);
endfunction
task run();
fsm_init();
forever begin
state_m.do_something();
state_m.request_state_change();
end
endtask
endclass
note
state_machine中还可以加入reset stop函数控制FSM的更多行为；封装更多API供其他模块调用；加入assertion做基于cycle的条件判断；加入covergroup收集功能覆盖率；
审核编辑：刘清
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