mem: Fix address interleaving bug in DRAM controller

This patch fixes a bug in the DRAM controller address decoding. In
cases where the DRAM burst size (e.g. 32 bytes in a rank with a single
LPDDR3 x32) was smaller than the channel interleaving size
(e.g. systems with a 64-byte cache line) one address bit effectively
got used as a channel bit when it should have been a low-order column
bit.

This patch adds a notion of "columns per stripe", and more clearly
deals with the low-order column bits and high-order column bits. The
patch also relaxes the granularity check such that it is possible to
use interleaving granularities other than the cache line size.

The patch also adds a missing M5_CLASS_VAR_USED to the tCK member as
it is only used in the debug build for now.

2 files changed, 41 insertions, 16 deletions

diff --git a/src/mem/dram_ctrl.cc b/src/mem/dram_ctrl.cc
index 2b053d78e..1d96e274c 100644
--- a/src/mem/dram_ctrl.cc
+++ b/src/mem/dram_ctrl.cc
@@ -67,6 +67,7 @@ DRAMCtrl::DRAMCtrl(const DRAMCtrlParams* p) :
     burstSize((devicesPerRank * burstLength * deviceBusWidth) / 8),
     rowBufferSize(devicesPerRank * deviceRowBufferSize),
     columnsPerRowBuffer(rowBufferSize / burstSize),
+    columnsPerStripe(range.granularity() / burstSize),
     ranksPerChannel(p->ranks_per_channel),
     banksPerRank(p->banks_per_rank), channels(p->channels), rowsPerBank(0),
     readBufferSize(p->read_buffer_size),
@@ -122,6 +123,7 @@ DRAMCtrl::DRAMCtrl(const DRAMCtrlParams* p) :
 
     rowsPerBank = capacity / (rowBufferSize * banksPerRank * ranksPerChannel);
 
+    // a bit of sanity checks on the interleaving
     if (range.interleaved()) {
         if (channels != range.stripes())
             fatal("%s has %d interleaved address stripes but %d channel(s)\n",
@@ -129,18 +131,34 @@ DRAMCtrl::DRAMCtrl(const DRAMCtrlParams* p) :
 
         if (addrMapping == Enums::RoRaBaChCo) {
             if (rowBufferSize != range.granularity()) {
-                fatal("Interleaving of %s doesn't match RoRaBaChCo "
+                fatal("Channel interleaving of %s doesn't match RoRaBaChCo "
                       "address map\n", name());
             }
-        } else if (addrMapping == Enums::RoRaBaCoCh) {
-            if (system()->cacheLineSize() != range.granularity()) {
-                fatal("Interleaving of %s doesn't match RoRaBaCoCh "
-                      "address map\n", name());
+        } else if (addrMapping == Enums::RoRaBaCoCh ||
+                   addrMapping == Enums::RoCoRaBaCh) {
+            // for the interleavings with channel bits in the bottom,
+            // if the system uses a channel striping granularity that
+            // is larger than the DRAM burst size, then map the
+            // sequential accesses within a stripe to a number of
+            // columns in the DRAM, effectively placing some of the
+            // lower-order column bits as the least-significant bits
+            // of the address (above the ones denoting the burst size)
+            assert(columnsPerStripe >= 1);
+
+            // channel striping has to be done at a granularity that
+            // is equal or larger to a cache line
+            if (system()->cacheLineSize() > range.granularity()) {
+                fatal("Channel interleaving of %s must be at least as large "
+                      "as the cache line size\n", name());
             }
-        } else if (addrMapping == Enums::RoCoRaBaCh) {
-            if (system()->cacheLineSize() != range.granularity())
-                fatal("Interleaving of %s doesn't match RoCoRaBaCh "
-                      "address map\n", name());
+
+            // ...and equal or smaller than the row-buffer size
+            if (rowBufferSize < range.granularity()) {
+                fatal("Channel interleaving of %s must be at most as large "
+                      "as the row-buffer size\n", name());
+            }
+            // this is essentially the check above, so just to be sure
+            assert(columnsPerStripe <= columnsPerRowBuffer);
         }
     }
 
@@ -228,7 +246,8 @@ DRAMCtrl::decodeAddr(PacketPtr pkt, Addr dramPktAddr, unsigned size,
     // always the top bits, and check before creating the DRAMPacket
     uint64_t row;
 
-    // truncate the address to the access granularity
+    // truncate the address to a DRAM burst, which makes it unique to
+    // a specific column, row, bank, rank and channel
     Addr addr = dramPktAddr / burstSize;
 
     // we have removed the lowest order address bits that denote the
@@ -255,11 +274,14 @@ DRAMCtrl::decodeAddr(PacketPtr pkt, Addr dramPktAddr, unsigned size,
         row = addr % rowsPerBank;
         addr = addr / rowsPerBank;
     } else if (addrMapping == Enums::RoRaBaCoCh) {
+        // take out the lower-order column bits
+        addr = addr / columnsPerStripe;
+
         // take out the channel part of the address
         addr = addr / channels;
 
-        // next, the column
-        addr = addr / columnsPerRowBuffer;
+        // next, the higher-order column bites
+        addr = addr / (columnsPerRowBuffer / columnsPerStripe);
 
         // after the column bits, we get the bank bits to interleave
         // over the banks
@@ -278,6 +300,9 @@ DRAMCtrl::decodeAddr(PacketPtr pkt, Addr dramPktAddr, unsigned size,
         // optimise for closed page mode and utilise maximum
         // parallelism of the DRAM (at the cost of power)
 
+        // take out the lower-order column bits
+        addr = addr / columnsPerStripe;
+
         // take out the channel part of the address, not that this has
         // to match with how accesses are interleaved between the
         // controllers in the address mapping
@@ -292,9 +317,8 @@ DRAMCtrl::decodeAddr(PacketPtr pkt, Addr dramPktAddr, unsigned size,
         rank = addr % ranksPerChannel;
         addr = addr / ranksPerChannel;
 
-        // next the column bits which we do not need to keep track of
-        // and simply skip past
-        addr = addr / columnsPerRowBuffer;
+        // next, the higher-order column bites
+        addr = addr / (columnsPerRowBuffer / columnsPerStripe);
 
         // lastly, get the row bits
         row = addr % rowsPerBank;
diff --git a/src/mem/dram_ctrl.hh b/src/mem/dram_ctrl.hh
index 720383542..fc1a6115b 100644
--- a/src/mem/dram_ctrl.hh
+++ b/src/mem/dram_ctrl.hh
@@ -453,6 +453,7 @@ class DRAMCtrl : public AbstractMemory
     const uint32_t burstSize;
     const uint32_t rowBufferSize;
     const uint32_t columnsPerRowBuffer;
+    const uint32_t columnsPerStripe;
     const uint32_t ranksPerChannel;
     const uint32_t banksPerRank;
     const uint32_t channels;
@@ -469,7 +470,7 @@ class DRAMCtrl : public AbstractMemory
      * Basic memory timing parameters initialized based on parameter
      * values.
      */
-    const Tick tCK;
+    const Tick M5_CLASS_VAR_USED tCK;
     const Tick tWTR;
     const Tick tRTW;
     const Tick tBURST;