Last week NVIDIA provided an update on its Tegra SoC roadmap. Kal-El, its third generation SoC (likely to launch as Tegra 3) has been delayed by a couple of months. NVIDIA originally expected the first Kal-El tablets would arrive in August, but now it's looking like sometime in Q4. Kal-El's successor, Wayne, has also been pushed back until late 2012/early 2013. In between these two SoCs is a new part dubbed Kal-El+. It's unclear if Kal-El+ will be a process shrink or just higher clocks/larger die on 40nm.

In the smartphone spirit, NVIDIA is letting small tidbits of information out about Kal-El as it gets closer to launch. In February we learned Kal-El would be NVIDIA's first quad-core SoC design, featuring four ARM Cortex A9s (with MPE) behind a 1MB shared L2 cache. Kal-El's GPU would also see a boost to 12 "cores" (up from 8 in Tegra 2), but through architectural improvements would deliver up to 3x the GPU performance of T2. Unfortunately the increase in GPU size and CPU core count doesn't come with a wider memory bus. Kal-El is still stuck with a single 32-bit LPDDR2 memory interface, although max supported data rate increases to 800MHz.

We also learned that NVIDIA was targeting somewhere around an 80mm^2 die, more than 60% bigger than Tegra 2 but over 30% smaller than the A5 in Apple's iPad 2. NVIDIA told us that although the iPad 2 made it easier for it to sell a big SoC to OEMs, it's still not all that easy to convince manufacturers to spend more on a big SoC.

Clock speeds are up in the air but NVIDIA is expecting Kal-El to run faster than Tegra 2. Based on competing A9 designs, I'd expect Kal-El to launch somewhere around 1.3 - 1.4GHz.

Now for the new information. Power consumption was a major concern with the move to Kal-El but NVIDIA addressed that by allowing each A9 in the SoC to be power gated when idle. When a core is power gated it is effectively off, burning no dynamic power and leaking very little. Tegra 2 by comparison couldn't power gate individual cores, only the entire CPU island itself.

In lightly threaded situations where you aren't using all of Kal-El's cores, the idle ones should simply shut off (if NVIDIA has done its power management properly of course). Kal-El is built on the same 40nm process as Tegra 2, so when doing the same amount of work the quad-core chip shouldn't consume any more power.

Power gating idle cores allows Kal-El to increase frequency to remaining active cores resulting in turbo boost-like operation (e.g. 4-cores active at 1.2GHz or 2-cores at 1.5GHz, these are hypothetical numbers of course). Again, NVIDIA isn't talking about final clocks for Kal-El or dynamic frequency ranges.

Five Cores, Not Four


Courtesy NVIDIA

Finally we get to the big news. There are actually five ARM Cortex A9s with MPE on a single Kal-El die: four built using TSMC's 40nm general purpose (G) process and one on 40nm low power (LP). If you remember back to our Tegra 2 review you'll know that T2 was built using a similar combination of transistors; G for the CPU cores and LP for the GPU and everything else. LP transistors have very low leakage but can't run at super high frequencies, G transistors on the other hand are leaky but can switch very fast. Update: To clarify, TSMC offers a 40nm LPG process that allows for an island of G transistors in a sea of LP transistors. This is what NVIDIA appears to be using in Kal-El, and what NV used in Tegra 2 prior.

The five A9s can't all be active at once, you either get 1 - 4 of the GP cores or the lone LP core. The GP cores and the LP core are on separate power planes.

NVIDIA tells us that the sole point of the LP Cortex A9 is to provide lower power operation when your device is in active standby (e.g. screen is off but the device is actively downloading new emails, tweets, FB updates, etc... as they come in). The LP core runs at a lower voltage than the GP cores and can only clock at up to 500MHz. As long as the performance state requested by the OS/apps isn't higher than a predetermined threshold, the LP core will service those needs. Even with your display on it's possible for the LP core to be active, so long as the performance state requested by the OS/apps isn't too high.


Courtesy NVIDIA

Once it crosses that threshold however, the LP core is power gated and state is moved over to the array of GP cores. As I mentioned earlier, both CPU islands can't be active at the same time - you only get one or the other. All five cores share the same 1MB L2 cache so memory coherency shouldn't be difficult to work out.

Android isn't aware of the fifth core, it only sees up to 4 at any given time. NVIDIA accomplishes this by hotplugging the cores into the scheduler. The core OS doesn't have to be modified or aware of NVIDIA's 4+1 arrangement (which it calls vSMP). NVIDIA's CPU governor code defines the specific conditions that trigger activating cores. For example, under a certain level of CPU demand the scheduler will be told there's only a single core available (the companion core). As the workload increases, the governor will sleep the companion core and enable the first GP core. If the workload continues to increase, subsequent cores will be made available to the scheduler. Similarly if the workload decreases, the cores will be removed from the scheduling pool one by one.


Courtesy NVIDIA

NVIDIA can switch between the companion and main cores in under 2ms. There's also logic to prevent wasting time flip flopping between the LP and GP cores for workloads that reside on the trigger threshold.

NVIDIA expects pretty much all active work to be done on the quad-core GP array, it's really only when your phone is idle and dealing with background tasks that the LP core will be in use. As a result of this process dichotomy NVIDIA is claiming significant power improvements over Tegra 2, despite an increase in transistor count:


Courtesy NVIDIA

NVIDIA isn't talking about GPU performance today but it did reveal a few numbers in a new white paper:


Courtesy NVIDIA

We don't have access to the benchmarks here but everything was run on Android 3.2 at 1366 x 768 with identical game settings. The performance gains are what NVIDIA has been promising, in the 2 - 3x range. Obviously we didn't run any of these tests ourselves so approach with caution.

Final Words

What sold NVIDIA's Tegra 2 wasn't necessarily its architecture, but timing and the fact that it was Google's launch platform for Honeycomb. If the rumors are correct, NVIDIA isn't the launch partner for Ice Cream Sandwich, which means Kal-El has to stand on its own as a convincing platform.


Courtesy NVIDIA

The vSMP/companion core architecture is a unique solution to the problem of increasing SoC performance while improving battery life. This is a step towards heterogenous multiprocessing, despite the homogenous implementation in Kal-El. It remains to be seen how tangible is the companion core's impact on real world battery life.

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  • Draiko - Tuesday, September 20, 2011 - link

    PS - The MSM8660 was volume sampling in June, 2010. Reply
  • jjj - Tuesday, September 20, 2011 - link

    You might be confusing comercial sampling dates or volume production with actual devices in stores.In any case the fact is that Qualcomm said early next year for devices on a number of ocasions,including IR events and there isn't anything more accurate that this at this time.
    If Nvidai had Wayne ready for devices hitting retail next year there would be no need for Kal-El plus.

    PS: we could also see some surprises from Samsung and Apple.(ofc what Apple does is less relevant here)
    Reply
  • Draiko - Wednesday, September 21, 2011 - link

    Kal-el plus might be a lower-cost part for mid-range devices and other markets.

    Samsung won't pull out any surprises. They'll either stick with Mali 400 (possibly MP2 or MP4 configs) or jump directly to a Mali-T604.

    Apple is just going to keep using the SGX series GPUs but I doubt they'll start using a "Rogue" GPU for a while longer.
    Reply
  • Lucian Armasu - Wednesday, September 21, 2011 - link

    My guess is Kal-El+ is a mid-life kicker for Kal-El, to keep up with the new competition mid year. So it will probably be an upgraded Kal-El to 2 Ghz or even 2.5 Ghz per core. Reply
  • z0mb13n3d - Tuesday, September 20, 2011 - link

    Highly unlikely we'll see anything Wayne-related at CES 2012, especially since nvidia will expect a lot of Kal-El -product announcements at CES since it's now been pushed to Q4'11.

    It will be very interesting to see if nvidia is able to out-execute the competition in the SoC industry, much the same way they did in the GPU industry early on. Interesting times lay ahead in the mobile space!
    Reply
  • Draiko - Wednesday, September 21, 2011 - link

    Kal-el tablets are due out next month. Reply
  • Lucian Armasu - Wednesday, September 21, 2011 - link

    Kal-El wasn't showed at CES either, like Tegra 2 was. It was showed at MWC. But it's still coming to tablets this year, and phones early next year. I figure Wayne would do the same. It's too bad Nvidia couldn't keep up with their original schedule, though, so we could see both next-gen chips in tablets and phones by Christmas.

    But it probably has mostly to do with manufacturers. It's very possible they delayed Nvidia's schedule by 2 months, maybe to wait for Ice-Cream Sandwich, or who knows.
    Reply
  • metafor - Tuesday, September 20, 2011 - link

    Cost of the chip is primarily determined by die area, not how expensive the R&D for the process is.

    And 28nm Snapdragons will be in devices Q1 of 2012.
    Reply
  • z0mb13n3d - Tuesday, September 20, 2011 - link

    Unlikely. But even if they do manage to push out 28nm on shipping devices by Q1, the possibility of them being quad-core are almost impossible. Reply
  • metafor - Tuesday, September 20, 2011 - link

    It's not quad-core, but why does it need to be? 2x A15-class > 4x A9 class any day of the week. Reply

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