mem: Add tRTP to the DRAM controller

This patch adds the tRTP timing constraint, governing the minimum time
between a read command and a precharge. Default values are provided
for the existing DRAM types.

src/mem/DRAMCtrl.py

@@ -135,6 +135,9 @@ class DRAMCtrl(AbstractMemory):
     # minimum time between a write data transfer and a precharge
     tWR = Param.Latency("Write recovery time")
 
+    # minimum time between a read and precharge command
+    tRTP = Param.Latency("Read to precharge")
+
     # time to complete a burst transfer, typically the burst length
     # divided by two due to the DDR bus, but by making it a parameter
     # it is easier to also evaluate SDR memories like WideIO.
@@ -198,6 +201,7 @@ class DDR3_1600_x64(DRAMCtrl):
     tRP = '13.75ns'
     tRAS = '35ns'
     tWR = '15ns'
+    tRTP = '7.5ns'
 
     # 8 beats across an x64 interface translates to 4 clocks @ 800 MHz.
     # Note this is a BL8 DDR device.
@@ -257,6 +261,7 @@ class DDR3_1333_x64_DRAMSim2(DRAMCtrl):
     tRP = '15ns'
     tRAS = '36ns'
     tWR = '15ns'
+    tRTP = '7.5ns'
 
     # 8 beats across an x64 interface translates to 4 clocks @ 666.66 MHz.
     # Note this is a BL8 DDR device.
@@ -314,6 +319,9 @@ class LPDDR2_S4_1066_x32(DRAMCtrl):
     tRAS = '42ns'
     tWR = '15ns'
 
+    # 6 CK read to precharge delay
+    tRTP = '11.256ns'
+
     # 8 beats across an x32 DDR interface translates to 4 clocks @ 533 MHz.
     # Note this is a BL8 DDR device.
     # Requests larger than 32 bytes are broken down into multiple requests
@@ -365,6 +373,8 @@ class WideIO_200_x128(DRAMCtrl):
     tRP = '18ns'
     tRAS = '42ns'
     tWR = '15ns'
+    # Read to precharge is same as the burst
+    tRTP = '20ns'
 
     # 4 beats across an x128 SDR interface translates to 4 clocks @ 200 MHz.
     # Note this is a BL4 SDR device.
@@ -420,6 +430,9 @@ class LPDDR3_1600_x32(DRAMCtrl):
     tRAS = '42ns'
     tWR = '15ns'
 
+    # Greater of 4 CK or 7.5 ns, 4 CK @ 800 MHz = 5 ns
+    tRTP = '7.5ns'
+
     # Pre-charge one bank 15 ns (all banks 18 ns)
     tRP = '15ns'
 

src/mem/dram_ctrl.cc

@@ -76,7 +76,7 @@ DRAMCtrl::DRAMCtrl(const DRAMCtrlParams* p) :
     writesThisTime(0), readsThisTime(0),
     tWTR(p->tWTR), tRTW(p->tRTW), tBURST(p->tBURST),
     tRCD(p->tRCD), tCL(p->tCL), tRP(p->tRP), tRAS(p->tRAS), tWR(p->tWR),
-    tRFC(p->tRFC), tREFI(p->tREFI), tRRD(p->tRRD),
+    tRTP(p->tRTP), tRFC(p->tRFC), tREFI(p->tREFI), tRRD(p->tRRD),
     tXAW(p->tXAW), activationLimit(p->activation_limit),
     memSchedPolicy(p->mem_sched_policy), addrMapping(p->addr_mapping),
     pageMgmt(p->page_policy),
@@ -560,9 +560,10 @@ DRAMCtrl::printParams() const
             "tWTR      %d ticks\n"                        \
             "tRTW      %d ticks\n"                        \
             "tWR       %d ticks\n"                        \
+            "tRTP      %d ticks\n"                        \
             "tXAW (%d) %d ticks\n",
             name(), tRCD, tCL, tRP, tBURST, tRFC, tREFI, tWTR,
-            tRTW, tWR, activationLimit, tXAW);
+            tRTW, tWR, tRTP, activationLimit, tXAW);
 }
 
 void
@@ -993,12 +994,12 @@ DRAMCtrl::doDRAMAccess(DRAMPacket* dram_pkt)
     // read/write (add a max with tCCD here)
     bank.colAllowedAt = cmd_at + tBURST;
 
-    // If this is a write, we also need to respect the write
+    // If this is a write, we also need to respect the write recovery
-    // recovery time before a precharge
+    // time before a precharge, in the case of a read, respect the
-    if (!dram_pkt->isRead) {
+    // read to precharge constraint
     bank.preAllowedAt = std::max(bank.preAllowedAt,
+                                 dram_pkt->isRead ? cmd_at + tRTP :
                                  dram_pkt->readyTime + tWR);
-    }
 
     // increment the bytes accessed and the accesses per row
     bank.bytesAccessed += burstSize;

src/mem/dram_ctrl.hh

@@ -486,6 +486,7 @@ class DRAMCtrl : public AbstractMemory
     const Tick tRP;
     const Tick tRAS;
     const Tick tWR;
+    const Tick tRTP;
     const Tick tRFC;
     const Tick tREFI;
     const Tick tRRD;