config: Expose the DRAM ranks as a command-line option

This patch gives the user direct influence over the number of DRAM
ranks to make it easier to tune the memory density without affecting
the bandwidth (previously the only means of scaling the device count
was through the number of channels).

The patch also adds some basic sanity checks to ensure that the number
of ranks is a power of two (since we rely on bit slices in the address
decoding).

configs/common/MemConfig.py

@@ -197,9 +197,15 @@ def config_mem(options, system):
     # address mapping in the case of a DRAM
     for r in system.mem_ranges:
         for i in xrange(nbr_mem_ctrls):
-            mem_ctrls.append(create_mem_ctrl(cls, r, i, nbr_mem_ctrls,
-                                             intlv_bits,
-                                             system.cache_line_size.value))
+            mem_ctrl = create_mem_ctrl(cls, r, i, nbr_mem_ctrls, intlv_bits,
+                                       system.cache_line_size.value)
+            # Set the number of ranks based on the command-line
+            # options if it was explicitly set
+            if issubclass(cls, m5.objects.DRAMCtrl) and \
+                    options.mem_ranks:
+                mem_ctrl.ranks_per_channel = options.mem_ranks
+
+            mem_ctrls.append(mem_ctrl)
 
     system.mem_ctrls = mem_ctrls
 

configs/common/Options.py

@@ -90,6 +90,8 @@ def addCommonOptions(parser):
                       help = "type of memory to use")
     parser.add_option("--mem-channels", type="int", default=1,
                       help = "number of memory channels")
+    parser.add_option("--mem-ranks", type="int", default=None,
+                      help = "number of memory ranks per channel")
     parser.add_option("--mem-size", action="store", type="string",
                       default="512MB",
                       help="Specify the physical memory size (single memory)")

src/mem/dram_ctrl.cc

@@ -92,6 +92,11 @@ DRAMCtrl::DRAMCtrl(const DRAMCtrlParams* p) :
     busBusyUntil(0), prevArrival(0),
     nextReqTime(0), activeRank(0), timeStampOffset(0)
 {
+    // sanity check the ranks since we rely on bit slicing for the
+    // address decoding
+    fatal_if(!isPowerOf2(ranksPerChannel), "DRAM rank count of %d is not "
+             "allowed, must be a power of two\n", ranksPerChannel);
+
     for (int i = 0; i < ranksPerChannel; i++) {
         Rank* rank = new Rank(*this, p);
         ranks.push_back(rank);