pmm: implement FrameFromAddress

This is equivalent to vmm.PageFromAddress but returns back a pmm.Frame

src/gopheros/kernel/mem/pmm/frame.go

@@ -24,3 +24,12 @@ func (f Frame) Valid() bool {
 func (f Frame) Address() uintptr {
 	return uintptr(f << mem.PageShift)
 }
+
+// FrameFromAddress returns a Frame that corresponds to
+// the given physical address. This function can handle
+// both page-aligned and not aligned addresses. in the
+// latter case, the input address will be rounded down
+// to the frame that contains it.
+func FrameFromAddress(physAddr uintptr) Frame {
+	return Frame((physAddr & ^(uintptr(mem.PageSize - 1))) >> mem.PageShift)
+}

src/gopheros/kernel/mem/pmm/frame_test.go

@@ -23,3 +23,21 @@ func TestFrameMethods(t *testing.T) {
 		t.Error("expected InvalidFrame.Valid() to return false")
 	}
 }
+
+func TestFrameFromAddress(t *testing.T) {
+	specs := []struct {
+		input    uintptr
+		expFrame Frame
+	}{
+		{0, Frame(0)},
+		{4095, Frame(0)},
+		{4096, Frame(1)},
+		{4123, Frame(1)},
+	}
+
+	for specIndex, spec := range specs {
+		if got := FrameFromAddress(spec.input); got != spec.expFrame {
+			t.Errorf("[spec %d] expected returned frame to be %v; got %v", specIndex, spec.expFrame, got)
+		}
+	}
+}