mem: add tCCD to DRAM controller

Information
Submitter:	Amin Farmahini
Repository:	gem5
Branch:
Bugs:
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Description

This patch adds support for tCCD to the DRAM controller.
After changeset 10211: e084db2b1527 (Merge DRAM latency calculation and bank state update), DRAM latency calculations has changed and that changeset provides a rather simple way to incorporate the tCCD parameter into latency calculations.

Testing Done

None

Thanks for the input Amin. One high-level question: what is the main aim of the patch? Until now we have tried to keep the timing constraints to a minimum (without sacrificing fidelity). In general you could argue that tBURST can be used, rather than adding tCCD and then taking the max. Do you envision any use-cases where this is not the case?

Amin Farmahini July 29, 2014, 12:31 p.m. (July 29, 2014, 12:31 p.m.)

Yes, generally you can use tBUSRT without sacrificing accuracy since usually tBURST >= tCCD. 
But in DDR4 tBURST could be smaller than tCCD. For example, in DDR4-2400-17-17-17 model in gem5, Samsung K4A4G085WD, tCCD_S is 4 cycles (3.33ns) and tCCD_L is 6 cycles (5.0ns). So the average tCCD is 4.16ns, whereas tBURST is 3.33ns. Also, we have internally developed some DRAM architectures where tBURST is smaller than tCCD. So in those cases taking the max increases accuracy. 
Anyways, I don't mind at all if you would like to ignore tCCD to keep the timing constraints to a minimum. That makes sense.

Andreas Hansson July 29, 2014, 2:52 p.m. (July 29, 2014, 2:52 p.m.)

We have some patches adding bank groups, and that should address the issue of _S and _L. The argument I was making regarding your patch is that the user could rather do the max(tCCD, tBURST) in advance and set tBURST to this value. Agreed?

Amin Farmahini July 29, 2014, 3:04 p.m. (July 29, 2014, 3:04 p.m.)
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Agree :).
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Andreas Hansson Sept. 17, 2014, 12:31 p.m. (Sept. 17, 2014, 12:31 p.m.)

I suggest to discard this patch in favour of: http://reviews.gem5.org/r/2387/

You have a pending review.

Review Board 2.0.15

This change has been discarded.

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Status: Discarded