dev: Add a "resetter" callback to the typed register class.

When using the typed register template, most functionality of the class
can be controlled using callbacks. For instance, callbacks can be
installed to handle reads or writes to a register without having to
subclass the template and override those methods using inheritance.

The recently added reset() method did not follow this pattern though,
which has two problems. First, it's inconsistent with how the class is
normally used. Second, once you've defined a subclass, the reader,
writer, etc, callbacks still expect the type of the original class.
That means these have to either awkwardly use a type different from the
actual real type of the register, or use awkward, inefficient, and/or
dangerous casting to get back to the true type.

To address these problems, this change adds a resetter(...) method
which works like the reader(...) or writer(...) methods to optionally
install a callback to implement any special reset behavior.

Change-Id: Ia74b36616fd459c1dbed9304568903a76a4b55de
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/67203
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/dev/reg_bank.hh

@@ -270,6 +270,12 @@
  * is an alternative form of update which also takes a custom bitmask, if you
  * need to update bits other than the normally writeable ones.
  *
+ * Similarly, you can set a "resetter" handler which is responsible for
+ * resetting the register. It takes a reference to the current Register, and
+ * no other parameters. The "initialValue" accessor can retrieve the value the
+ * register was constructed with. The register is simply set to this value
+ * in the default resetter implementation.
+ *
  * = Read only bits =
  *
  * Often registers have bits which are fixed and not affected by writes. To
@@ -554,6 +560,7 @@ class RegisterBank : public RegisterBankBase
         using WriteFunc = std::function<void (This &reg, const Data &value)>;
         using PartialWriteFunc = std::function<
             void (This &reg, const Data &value, int first, int last)>;
+        using ResetFunc = std::function<void (This &reg)>;
 
       private:
         Data _data = {};
@@ -564,6 +571,7 @@ class RegisterBank : public RegisterBankBase
         WriteFunc _writer = defaultWriter;
         PartialWriteFunc _partialWriter = defaultPartialWriter;
         PartialReadFunc _partialReader = defaultPartialReader;
+        ResetFunc _resetter = defaultResetter;
 
       protected:
         static Data defaultReader(This &reg) { return reg.get(); }
@@ -587,6 +595,12 @@ class RegisterBank : public RegisterBankBase
                                                  mask(first, last)));
         }
 
+        static void
+        defaultResetter(This &reg)
+        {
+            reg.get() = reg.initialValue();
+        }
+
         constexpr Data
         htoreg(Data data)
         {
@@ -721,6 +735,30 @@ class RegisterBank : public RegisterBankBase
             return partialWriter(wrapper);
         }
 
+        // Set the callables which handle resetting.
+        //
+        // The default resetter restores the initial value used in the
+        // constructor.
+        constexpr This &
+        resetter(const ResetFunc &new_resetter)
+        {
+            _resetter = new_resetter;
+            return *this;
+        }
+        template <class Parent, class... Args>
+        constexpr This &
+        resetter(Parent *parent, void (Parent::*nr)(Args... args))
+        {
+            auto wrapper = [parent, nr](Args&&... args) {
+                return (parent->*nr)(std::forward<Args>(args)...);
+            };
+            return resetter(wrapper);
+        }
+
+        // An accessor which returns the initial value as set in the
+        // constructor. This is intended to be used in a resetter function.
+        const Data &initialValue() const { return _resetData; }
+
 
         /*
          * Interface for accessing the register's state, for use by the
@@ -817,7 +855,7 @@ class RegisterBank : public RegisterBankBase
         }
 
         // Reset our data to its initial value.
-        void reset() override { get() = _resetData; }
+        void reset() override { _resetter(*this); }
     };
 
   private:

src/dev/reg_bank.test.cc

@@ -868,6 +868,56 @@ TEST_F(TypedRegisterTest, PartialWriterReaderWriter)
     EXPECT_EQ(write_value, 0x0344);
 }
 
+// Use the default resetter for a register.
+TEST_F(TypedRegisterTest, DefaultResetter)
+{
+    BackingType initial_value = reg.get();
+
+    reg.get() = initial_value + 1;
+    EXPECT_EQ(reg.get(), initial_value + 1);
+
+    reg.reset();
+
+    EXPECT_EQ(reg.get(), initial_value);
+}
+
+// Set a custom resetter for a register.
+TEST_F(TypedRegisterTest, Resetter)
+{
+    RegisterBankLE::Register<BackingType> *reg_ptr = nullptr;
+
+    reg.resetter([&reg_ptr](auto &r) {
+        reg_ptr = &r;
+    });
+
+    reg.reset();
+
+    EXPECT_EQ(reg_ptr, &reg);
+}
+
+// Set a custom resetter for a register which is a class method.
+TEST_F(TypedRegisterTest, ResetterMF)
+{
+    using Reg = RegisterBankLE::Register<BackingType>;
+
+    struct ResetStruct
+    {
+        Reg *reg_ptr = nullptr;
+
+        void
+        resetter(Reg &r)
+        {
+            reg_ptr = &r;
+        }
+    } reset_struct;
+
+    reg.resetter(&reset_struct, &ResetStruct::resetter);
+
+    reg.reset();
+
+    EXPECT_EQ(reset_struct.reg_ptr, &reg);
+}
+
 TEST_F(TypedRegisterTest, Serialize)
 {
     std::ostringstream os;