mem: Option to toggle DRAM low-power states

Adding an option to enable DRAM low-power states. The low power
states can have a significant impact on application performance
(sim_ticks) on the order of 2-3x, especially for compute-gpu apps.
The options allows for it to easily be enabled/disabled to compare
performance numbers. The option is disabled by default.

Change-Id: Ib9bddbb792a1a6a4afb5339003472ff8f00a5859
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18548
Reviewed-by: Wendy Elsasser <wendy.elsasser@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

mem: Make DRAMCtrl::decodeAddr const

DRAMCtrl's decodeAddr does not need to modify the packet it
receives, nor should it modify the contents of the class,
and therefore both the packet and the function are made const.

Change-Id: I577f48d9a43611ba54878a9a793cb7b4fbb326f4
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17540
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>

arch, cpu, dev, gpu, mem, sim, python: start using getPort.

Replace the getMasterPort, getSlavePort, and getEthPort functions
with getPort, and remove extraneous mechanisms that are no longer
necessary.

Change-Id: Iab7e3c02d2f3a0cf33e7e824e18c28646b5bc318
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17040
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

mem: Make DRAMCtrl a QoS-aware Memory Controller

This patch is turning DRAMCtrl a QoS-aware Memory Controller with "no
policy" as a default policy.

Change-Id: I48163da8c8208498cf0398b07094cb840272507f
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11973
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>

mem: Add support for more flexible DRAM timing and topologies

This patch has 2 main aspects:
1) Add new parameter to adjust write-to-write delay
2) Enable support of more than 64 banks per controller

Changes for new parameter:
Incorporated a new parameter, tCCD_L_WR, which defaults to tCCD_L.
This parameter can be used to set a unique delay between writes and
between reads.

To incorporate this parameter in the controller, modified the DRAMCtrl
class to have separate variables for read and write column delays.
Used these variables to account for tRTW, tWTR, tBURST, tCCD_L, and tCS
as well as the new tCCD_L_WR parameter.

Changes to support more than 64 banks:
Modified the logic selecting the next command (reorderQueue
and minBankPrep functions). Replaced the unint64_t variables with
a vector of uint32_t elements. There is a uint32_t element defined
per ranks to allow up to 32 banks per rank. This will automatically
scale with ranks without issue.
Change will allow analysis of memory sub-systems beyond the current
landscape.

Change-Id: I0ce466efed58276f843ad90e9ecc0ece6c37d646
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10103
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

mem: Optimize self-refresh entry

Self-refresh is entered during a refresh event, when the
rank was previously in a precharge power-down state.
The original code would enter self-refresh after a refresh
was issued. The device subsequently will issue a refresh
on self-refresh entry. On self-refresh exit, the controller
will issue another refresh command.

Devices require at least one additional refresh to be issued
between self-refresh exit and re-entry. This ensures that enough
refreshes occur in the case when the device narrowly missed a
refresh on self-refresh exit.

To minimize the number of refresh operations and still maintain
the device requirement, the current logic does the following:
1) The controller will still enter self-refresh from a refresh
event, when the previous state was precharge power-down.
However, the refresh itself will be bypassed and the controller
will immediately issue a self-refresh entry.
2) On a self-refresh exit, the controller will immediately
issue a refresh command (per the original logic). This ensures
the devices requirements are met and is a convenient way to
kick off the command state machine.

Change-Id: I1c4b0dcbfa3bdafd755f3ccd65e267fcd700c491
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10102
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

ext, mem: Pull DRAMPower SHA 90d6290 and rebase

This patch syncs the DRAMPower library of gem5 to the
external github (https://github.com/ravenrd/DRAMPower).

The version pulled in is the commit:
90d6290f802c29b3de9e10233ceee22290907ce6
from 30th Oct. 2016.

This change also modifies the DRAM Ctrl interaction with the
DRAMPower, due to changes in the lib API in the above version.

Previously multiple functions were called to prepare the power
lib before calling the function that would calculate the enery. With
the new API, these functions are encompassed inside the function to
calculate the energy and therefore should now be removed from the
DRAM controller.

The other key difference is the introduction of a new function called
calcWindowEnergy which can be useful for any system that wants
to do measurements over intervals. For gem5 DRAM ctrl that means we
now need to accumulate the window energy measurements into the total
stat.

Change-Id: I3570fff2805962e166ff2a1a3217ebf2d5a197fb
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5724
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

mem: Replace EventWrapper use with EventFunctionWrapper

NOTE: With this change there is a possibility for `DRAMCtrl::Rank`s
event names to not properly match the rank they were generated by. This
could occur if the public rank member is modified after the Rank's
construction. A patch would mean refactoring Rank and `DRAMCtrl`b to
privatize many of the members of Rank behind getters.

Change-Id: I7b8bd15086f4ffdfd3f40be4aeddac5e786fd78e
Signed-off-by: Sean Wilson <spwilson2@wisc.edu>
Reviewed-on: https://gem5-review.googlesource.com/3745
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

mem: Move the Rank construction logic to the Rank constructor

This change was made so Rank objects have their name assigned
when they are instantiated. Therefore, they can initialize their
member objects with their name and it is less likely to change during
runtime.

(NOTE: I would recommend hiding the fields which would cause the name to
change behind getters. Since modification of `Rank.rank` during runtime
will cause the `name()` to change.)

Change-Id: Id51c3553b40e489792c57950e18b8ce927e43173
Signed-off-by: Sean Wilson <spwilson2@wisc.edu>
Reviewed-on: https://gem5-review.googlesource.com/3742
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>

mem: Add DRAM low-power functionality

Added power-down state transitions to the DRAM controller model.

Added per rank parameter, outstandingEvents, which tracks the number
of outstanding command events and is used to determine when the
controller should transition to a low power state.
The controller will only transition when there are no outstanding events
scheduled and the number of command entries for the given rank is 0.

The outstandingEvents parameter is incremented for every RD/WR burst,
PRE, and REF event scheduled. ACT is implicitly covered by RD/WR
since burst will always issue and complete after a required ACT.
The parameter is decremented when the event is serviced (completed).

The controller will automatically transition to ACT power down,
PRE power down, or SREF.

Transition to ACT power down state scheduled from:
1) The RespondEvent, where read data is received from the memory.
ACT power-down entry will be scheduled when one or more banks is
open, all commands for the rank have completed (no more commands
scheduled), and there are no commands in queue for the rank

Transition to PRE power down scheduled from:
1) respondEvent, when all banks are closed, all commands have
completed, and there are no commands in queue for the rank
2) prechargeEvent when all banks are closed, all commands have
completed, and there are no commands in queue for the rank
3) refreshEvent, after the refresh is complete when the previous
state was ACT power-down
4) refreshEvent, after the refresh is complete when the previous
state was PRE power-down and there are commands in the queue.

Transition to SREF will be scheduled from:
1) refreshEvent, after the refresh is completes when the previous
state was PRE power-down with no commands in queue

Power-down exit commands are scheduled from:
1) The refreshEvent, prior to issuing a refresh
2) doDRAMAccess, to wake-up the rank for RD/WR command issue.

Self-refresh exit commands are scheduled from:
1) The next request event, when the queue has commands for the rank
in the readQueue or there are commands for the rank in the
writeQueue and the bus state is WRITE.

Change-Id: I6103f660776e36c686655e71d92ec7b5b752050a
Reviewed-by: Radhika Jagtap <radhika.jagtap@arm.com>

mem: Add callback to compute stats prior to dump event

The per rank statistics are periodically updated based on
state transition and refresh events.

Add a method to update these when a dump event occurs to
ensure they reflect accurate values.
Specifically, need to ensure that the low-power state
durations, power, and energy are logged correctly.

Change-Id: Ib642a6668340de8f494a608bb34982e58ba7f1eb
Reviewed-by: Radhika Jagtap <radhika.jagtap@arm.com>

mem: Modify drain to ensure banks and power are idled

Add constraint that all ranks have to be in PWR_IDLE
before signaling drain complete

This will ensure that the banks are all closed and the rank
has exited any low-power states.

On suspend, update the power stats to sync the DRAM power logic

The logic maintains the location of the signalDrainDone
method, which is still triggered from either:
1) Read response event
2) Next request event

This ensures that the drain will complete in the READ bus
state and minimizes the changes required.

Change-Id: If1476e631ea7d5999fe50a0c9379c5967a90e3d1
Reviewed-by: Radhika Jagtap <radhika.jagtap@arm.com>

mem: Sort memory commands and update DRAMPower

Add local variable to stores commands to be issued.
These commands are in order within a single bank but will be out
of order across banks & ranks.

A new procedure, flushCmdList, sorts commands across banks / ranks,
and flushes the sorted list, up to curTick() to DRAMPower.
This is currently called in refresh, once all previous commands are
guaranteed to have completed. Could be called in other events like
the powerEvent as well.

By only flushing commands up to curTick(), will not get out of sync
when flushed at a periodic stats dump (done in subsequent patch).

Change-Id: I4ac65a52407f64270db1e16a1fb04cfe7f638851
Reviewed-by: Radhika Jagtap <radhika.jagtap@arm.com>

mem: add DRAM powerdown timing

ext: Update DRAMPower

Sync DRAMPower to external tool

This patch syncs the DRAMPower library of gem5 to the external
one on github (https://github.com/ravenrd/DRAMPower) of which
I am a maintainer.

The version used is the commit:
902a00a1797c48a9df97ec88868f20e847680ae6
from 07. May. 2016.

Committed by Jason Lowe-Power <jason@lowepower.com>

mem: Unify delayed packet deletion

This patch unifies how we deal with delayed packet deletion, where the
receiving slave is responsible for deleting the packet, but the
sending agent (e.g. a cache) is still relying on the pointer until the
call to sendTimingReq completes. Previously we used a mix of a
deletion vector and a construct using unique_ptr. With this patch we
ensure all slaves use the latter approach.

misc: Add explicit overrides and fix other clang >= 3.5 issues

This patch adds explicit overrides as this is now required when using
"-Wall" with clang >= 3.5, the latter now part of the most recent
XCode. The patch consequently removes "virtual" for those methods
where "override" is added. The latter should be enough of an
indication.

As part of this patch, a few minor issues that clang >= 3.5 complains
about are also resolved (unused methods and variables).

misc: Remove redundant compiler-specific defines

This patch moves away from using M5_ATTR_OVERRIDE and the m5::hashmap
(and similar) abstractions, as these are no longer needed with gcc 4.7
and clang 3.1 as minimum compiler versions.

sim: Refactor and simplify the drain API

The drain() call currently passes around a DrainManager pointer, which
is now completely pointless since there is only ever one global
DrainManager in the system. It also contains vestiges from the time
when SimObjects had to keep track of their child objects that needed
draining.

This changeset moves all of the DrainState handling to the Drainable
base class and changes the drain() and drainResume() calls to reflect
this. Particularly, the drain() call has been updated to take no
parameters (the DrainManager argument isn't needed) and return a
DrainState instead of an unsigned integer (there is no point returning
anything other than 0 or 1 any more). Drainable objects should return
either DrainState::Draining (equivalent to returning 1 in the old
system) if they need more time to drain or DrainState::Drained
(equivalent to returning 0 in the old system) if they are already in a
consistent state. Returning DrainState::Running is considered an
error.

Drain done signalling is now done through the signalDrainDone() method
in the Drainable class instead of using the DrainManager directly. The
new call checks if the state of the object is DrainState::Draining
before notifying the drain manager. This means that it is safe to call
signalDrainDone() without first checking if the simulator has
requested draining. The intention here is to reduce the code needed to
implement draining in simple objects.

mem: Update DRAM command scheduler for bank groups

This patch updates the command arbitration so that bank group timing
as well as rank-to-rank delays will be taken into account. The
resulting arbitration no longer selects commands (prepped or not) that
cannot issue seamlessly if there are commands that can issue
back-to-back, minimizing the effect of rank-to-rank (tCS) & same bank
group (tCCD_L) delays.

The arbitration selects a new command based on the following priority.
Within each priority band, the arbitration will use FCFS to select the
appropriate command:

1) Bank is prepped and burst can issue seamlessly, without a bubble

2) Bank is not prepped, but can prep and issue seamlessly, without a
bubble

3) Bank is prepped but burst cannot issue seamlessly. In this case, a
bubble will occur on the bus

Thus, to enable more parallelism in subsequent selections, an
unprepped packet is given higher priority if the bank prep can be
hidden. If the bank prep cannot be hidden, the selection logic will
choose a prepped packet that cannot issue seamlessly if one exist.
Otherwise, the default selection will choose the packet with the
minimum bank prep delay.

mem: Avoid DRAM write queue iteration for merging and read lookup

This patch adds a simple lookup structure to avoid iterating over the
write queue to find read matches, and for the merging of write
bursts. Instead of relying on iteration we simply store a set of
currently-buffered write-burst addresses and compare against
these. For the reads we still perform the iteration if we have a
match. For the writes, we rely entirely on the set. Note that there
are corner-cases where sub-bursts would actually not be mergeable
without a read-modify-write. We ignore these cases and opt for speed.

mem: Split port retry for all different packet classes

This patch fixes a long-standing isue with the port flow
control. Before this patch the retry mechanism was shared between all
different packet classes. As a result, a snoop response could get
stuck behind a request waiting for a retry, even if the send/recv
functions were split. This caused message-dependent deadlocks in
stress-test scenarios.

The patch splits the retry into one per packet (message) class. Thus,
sendTimingReq has a corresponding recvReqRetry, sendTimingResp has
recvRespRetry etc. Most of the changes to the code involve simply
clarifying what type of request a specific object was accepting.

The biggest change in functionality is in the cache downstream packet
queue, facing the memory. This queue was shared by requests and snoop
responses, and it is now split into two queues, each with their own
flow control, but the same physical MasterPort. These changes fixes
the previously seen deadlocks.

mem: Ensure DRAM controller is idle when in atomic mode

This patch addresses an issue seen with the KVM CPU where the refresh
events scheduled by the DRAM controller forces the simulator to switch
out of the KVM mode, thus killing performance.

The current patch works around the fact that we currently have no
proper API to inform a SimObject of the mode switches. Instead we rely
on drainResume being called after any switch, and cache the previous
mode locally to be able to decide on appropriate actions.

The switcheroo regression require a minor stats bump as a result.

mem: Add rank-wise refresh to the DRAM controller

This patch adds rank-wise refresh to the controller, as opposed to the
channel-wide refresh currently in place. In essence each rank can be
refreshed independently, and for this to be possible the controller
is extended with a state machine per rank.

Without this patch the data bus is always idle during a refresh, as
all the ranks are refreshing at the same time. With the rank-wise
refresh it is possible to use one rank while another one is
refreshing, and thus the data bus can be kept busy.

The patch introduces a Rank class to encapsulate the state per rank,
and also shifts all the relevant banks, activation tracking etc to the
rank. The arbitration is also updated to consider the state of the rank.

mem: Add DRAM device size and check against config

This patch adds the size of the DRAM device to the DRAM config. It
also compares the actual DRAM size (calculated using information from
the config) to the size defined in the system. If these two values do
not match gem5 will print a warning. In order to do correct DRAM
research the size of the memory defined in the system should match the
size of the DRAM in the config. The timing and current parameters
found in the DRAM configs are defined for a DRAM device with a
specific size and would differ for another device with a different
size.

mem: DRAMPower integration for on-line DRAM power stats

This patch takes the final step in integrating DRAMPower and adds the
appropriate calls in the DRAM controller to provide the command trace
and extract the power and energy stats. The debug printouts are still
left in place, but will eventually be removed.

At the moment the DRAM power calculation is always on when using the
DRAM controller model. The run-time impact of this addition is around
1.5% when looking at the total host seconds of the regressions. We
deem this a sensible trade-off to avoid the complication of adding an
enable/disable mechanism.

mem: Add DDR4 bank group timing

Added the following parameter to the DRAMCtrl class:
- bank_groups_per_rank

This defaults to 1. For the DDR4 case, the default is overridden to indicate
bank group architecture, with multiple bank groups per rank.

Added the following delays to the DRAMCtrl class:
- tCCD_L : CAS-to-CAS, same bank group delay
- tRRD_L : RAS-to-RAS, same bank group delay

These parameters are only applied when bank group timing is enabled. Bank
group timing is currently enabled only for DDR4 memories.

For all other memories, these delays will default to '0 ns'

In the DRAM controller model, applied the bank group timing to the per bank
parameters actAllowedAt and colAllowedAt.
The actAllowedAt will be updated based on bank group when an ACT is issued.
The colAllowedAt will be updated based on bank group when a RD/WR burst is
issued.

At the moment no modifications are made to the scheduling.

mem: Add memory rank-to-rank delay

Add the following delay to the DRAM controller:
- tCS : Different rank bus turnaround delay

This will be applied for
1) read-to-read,
2) write-to-write,
3) write-to-read, and
4) read-to-write
command sequences, where the new command accesses a different rank
than the previous burst.

The delay defaults to 2*tCK for each defined memory class. Note that
this does not correspond to one particular timing constraint, but is a
way of modelling all the associated constraints.

The DRAM controller has some minor changes to prioritize commands to
the same rank. This prioritization will only occur when the command
stream is not switching from a read to write or vice versa (in the
case of switching we have a gap in any case).

To prioritize commands to the same rank, the model will determine if there are
any commands queued (same type) to the same rank as the previous command.
This check will ensure that the 'same rank' command will be able to execute
without adding bubbles to the command flow, e.g. any ACT delay requirements
can be done under the hoods, allowing the burst to issue seamlessly.

mem: Update DRAM controller comments

Update comments and add a reference for more information.

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.

mem: DRAMPower trace output

This patch adds a DRAMPower flag to enable off-line DRAM power
analysis using the DRAMPower tool. A new DRAMPower flag is added
and a follow-on patch adds a Python script to post-process the output
and order it based on time stamps.

The long-term goal is to link DRAMPower as a library and provide the
commands through function calls to the model rather than first
printing and then parsing the commands. At the moment it is also up to
the user to ensure that the same DRAM configuration is used by the
gem5 controller model and DRAMPower.

mem: Add bank and rank indices as fields to the DRAM bank

This patch adds the index of the bank and rank as a field so that we can
determine the identity of a given bank (reference or pointer) for the
power tracing. We also grab the opportunity of cleaning up the
arguments used for identifying the bank when activating.

mem: Extend DRAM row bits from 16 to 32 for larger densities

This patch extends the DRAM row bits to 32 to support larger density
memories. Additional checks are also added to ensure the row fits in
the 32 bits.

mem: Add DRAM cycle time

This patch extends the current timing parameters with the DRAM cycle
time. This is needed as the DRAMPower tool expects timestamps in DRAM
cycles. At the moment we could get away with doing this in a
post-processing step as the DRAMPower execution is separate from the
simulation run. However, in the long run we want the tool to be called
during the simulation, and then the cycle time is needed.

mem: Simplify DRAM response scheduling

This patch simplifies the DRAM response scheduling based on the
assumption that they are always returned in order.

mem: Remove printing of DRAM params

This patch removes the redundant printing of DRAM params.

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.

mem: Merge DRAM latency calculation and bank state update

This patch merges the two control paths used to estimate the latency
and update the bank state. As a result of this merging the computation
is now in one place only, and should be easier to follow as it is all
done in absolute (rather than relative) time.

As part of this change, the scheduling is also refined to ensure that
we look at a sensible estimate of the bank ready time in choosing the
next request. The bank latency stat is removed as it ends up being
misleading when the DRAM access code gets evaluated ahead of time (due
to the eagerness of waking the model up for scheduling the next
request).

mem: Add tWR to DRAM activate and precharge constraints

This patch adds the write recovery time to the DRAM timing
constraints, and changes the current tRASDoneAt to a more generic
preAllowedAt, capturing when a precharge is allowed to take place.

The part of the DRAM access code that accounts for the precharge and
activate constraints is updated accordingly.

mem: Add DRAM power states to the controller

This patch adds power states to the controller. These states and the
transitions can be used together with the Micron power model. As a
more elaborate use-case, the transitions can be used to drive the
DRAMPower tool.

At the moment, the power-down modes are not used, and this patch
simply serves to capture the idle, auto refresh and active modes. The
patch adds a third state machine that interacts with the refresh state
machine.

mem: Ensure DRAM refresh respects timings

This patch adds a state machine for the refresh scheduling to
ensure that no accesses are allowed while the refresh is in progress,
and that all banks are propely precharged.

As part of this change, the precharging of banks of broken out into a
method of its own, making is similar to how activations are dealt
with. The idle accounting is also updated to ensure that the refresh
duration is not added to the time that the DRAM is in the idle state
with all banks precharged.

mem: Make DRAM read/write switching less conservative

This patch changes the read/write event loop to use a single event
(nextReqEvent), along with a state variable, thus joining the two
control flows. This change makes it easier to follow the state
transitions, and control what happens when.

With the new loop we modify the overly conservative switching times
such that the write-to-read switch allows bank preparation to happen
in parallel with the bus turn around. Similarly, the read-to-write
switch uses the introduced tRTW constraint.

mem: Track DRAM read/write switching and add hysteresis

This patch adds stats for tracking the number of reads/writes per bus
turn around, and also adds hysteresis to the write-to-read switching
to ensure that the queue does not oscilate around the low threshold.

mem: Rename SimpleDRAM to a more suitable DRAMCtrl

This patch renames the not-so-simple SimpleDRAM to a more suitable
DRAMCtrl. The name change is intended to ensure that we do not send
the wrong message (although the "simple" in SimpleDRAM was originally
intended as in cleverly simple, or elegant).

As the DRAM controller modelling work is being presented at ISPASS'14
our hope is that a broader audience will use the model in the future.