High-performance tablet processor designs: Moving away from big.LITTLE and Cortex-A1x?

Rockchip and Allwinner are both Chinese fabless semiconductor companies that for a number of years have seen success supplying SoCs for tablets. During most of the last few years, either Rockchip or Allwinner has had the highest market share for tablet processors worldwide in terms of unit shipments, being used in high volume and low cost tablet models from Chinese manufacturers, often with a substantial lead on this metric over leading high-end tablet SoC providers such as Apple or Samsung.

Rockchip's RK3288 and Allwinner's A80 processor, both ambitious, high-performance application processors at least originally targeted at tablets, seem to have suffered from design and production issues, mostly related to the high power consumption of the ARM Cortex-A12 (or less likely A17) and A15 cores they implement and other hardware-related issues. Instead of appearing in mass-market tablets in volume, the chips are currently pitched mainly towards lower volume applications like media boxes and development boards, which because of their low volumes are unlikely to make the investment that went into the design and mass production of the chips profitable for the respective companies. Even for these niche markets, there appear to be problems related to performance, heat production, hardware design and related issues when using these chips.

Allwinner A80: Not quite ready

In early testing of the A80-based A80 OptimusBoard, benchmark results were disappointing given the CPU specification (quad-core Cortex-A15 up to 1.8 GHz and quad-core 1.2 GHz Cortex-A7 in big.LITTLE configuration), with evidence of either aggressive throttling or low actual clock frequencies (or even non-utilization in many circumstances) of the Cortex-A15 cores. CPU-specific test results are lower than expected, with the Vellamo benchmark's Multicore result being in line with a quad-core 1.2 GHz Cortex-A7 Snapdragon 400, and a particularly low Physics score in the 3DMark Ice Storm Extreme test, with evidence of very conservative CPU scheduling from kernel logs. In fact, these CPU-specific scores would be consistent with only the four Cortex-A7 cores running at about 1.2GHz (similar to the Snapdragon 400) actually being used. However, the firmware (from July) was relatively early, and preliminary test results of the Phoronix Test Suite in a Linux environment generally show better performance than Cortex-A9 based processors such as the RK3188, despite the A80's Cortex-A15 cores reported to be running at only 1.2 GHz, and falling short of the performance that would normally expected from competitive Cortex-A15-based CPUs such as Samsung's Exynos Octa family. All things being equal, a Cortex-A15 core should be significantly times faster than a Cortex-A9 core at the same clock speed.

Allwinner announces A83T, an octa-core Cortex-A7 tablet processor

In fact, Allwinner on September 4 already announced a new processor, the A83T, containing an octa-core Cortex-A7 configuration, which is expected to be commercially available in devices before the end of the year. According to the announcement, all cores can run simultaneously at up to 2.0 GHz, although it also mentions big.LITTLE, which appears to be an inappropriate term for this type of architecture, although it would be possible to, for example, optimize one four-core cluster for performance and the other for power efficiency. Allwinner prominently advertises the low power consumption of the chip despite high performance, which makes sense given the characteristics of Cortex-A7 and the contrast with the problematic power consumption of Allwinner's A80 (which is a true big.LITTLE chip that includes power-consuming Cortex-A15 cores). The A83T is also advertised as having a PowerVR GPU (probably a Series 6 Rogue GPU, as used by other chip designers including MediaTek and Apple).

The CPU architecture of the A83T appears to be strikingly similar to MediaTek's octa-core Cortex-A7 MT6592, which has already been proven and has been shipping since the end of 2013, and more specifically the MT8392 variant targeting tablets. However, both MediaTek chips incorporate a built-in 3G baseband, and MediaTek currently does not appear to be offering a similar solution for WiFi-only tablets without baseband (probably not intentionally. maybe because it had expected its big.LITTLE MT8135 to be viable). Because of the use of extremely power-efficient Cortex-A7 cores, MediaTek's octa-core chips such as MT6592 have proven to be fast but power-efficient in practical use while remaining relatively cheap and economical to manufacture.

New 28HPC process at TSMC provides cost, power and performance benefits

The Allwinner A83T uses TSMC's new 28 nm HPC (28HPC) process, which is reported to have been widely adopted for new designs by many chip design companies because of performance, power, and cost benefits. According to TSMC, it provides 10% smaller die size and 30% lower power at all levels of speed, or over 20% speed improvement at the same power when compared to its 28LP process. When compared to its currently popular leading-edge 28HPM process, it offers comparable performance but smaller die size.

More evidence of big.LITTLE being superseeded by low-power "true" octa-core designs

Allwinner appears to be on a path to quickly replace the big.LITTLE architecture in the A80 (with appears to have limited potential for tablets) with the much more efficient octa-core Cortex-A7 configuration of the A83T for higher-performance tablets. MediaTek has already made a similar move for its upcoming high-end smartphone platforms, with the big.LITTLE MT6595 looking likely to be quickly superseeded by the much more efficient MT6795 using Cortex-A53 CPU cores.

Allwinner has plenty of experience with Cortex-A7 cores, being one of the first chip companies to adopt it in its A31 quad-core tablet processor manufactured at 40 nm starting from the end of 2012. Its quad-core A33, which has been announced to be currently ramping, also uses a quad-core Cortex-A7 CPU. Despite this fact, Allwinner, like most of the rest of the industry, seems to have underestimated the potential of octa-core configurations of the Cortex-A7 (and subsequently Cortex-A53), because otherwise the new chip would already have been available and the A80 might not even have existed.

Looking ahead, it seems like that companies like Allwinner will transition to the efficient 64-bit ARM Cortex-A53 core (primarily because it is a new and faster successor of the Cortex-A7, rather than its support ARM's 64-bit ARMv8 instruction set, which is not yet important) sooner rather than later, and Cortex-A53 has already been widely adopted for smartphone processors by leading smartphone SoC companies MediaTek and Qualcomm across several segments ranging from entry-level to premium (see my earlier articles).

Based on a reported roadmap of upcoming chips, even Rockchip is finally moving to more cost-effective and less power-consuming chip architectures. Rockchip's  RK3126 and RK3218 are low-cost quad-core Cortex-A7-based tablet processors with a Mali GPU, closely matching Allwinner's new A33 chip. Additionally, the future "MayBach" SoC will contain an octa-core ARM Cortex-A53 CPU, the type of configuration that has already started to show  impressive performance test results in early benchmarks while likely relatively low power-consumption, which is likely to be much more viable than Cortex-A1x and big.LITTLE for the higher-performance segment, while still allowing the option of a big.LITTLE-like power saving technique because of the ability to optimize a portion (e.g. one cluster) of the procesor cores for power consumption rather than performance.

RK3288 appears to have limited potential for tablets

As evidenced by the review of a an early RK3288-based TV box at CNXSoft, Rockchip's RK3288 is fast but there still seem to be issues that will affect its viability. Some stability problems were noted, and the device became relatively hot. Such heat production would make the chip problematic for tablets, which is normally the main market generating high sales volumes for Rockchip. Without tablets, it is doubtful that production of this chip will be profitable at all.

Widespread early design activity for alternative devices like TV boxes, with indications of hardware and possibly software issues related to the chip, could point to a relatively large inventory of RK3288 chips with certain hardware defects held by Rockchip, which because of the unviability for tablet production, Rockchip is trying to unload onto the TV box market (which has a greater tolerance for heat production and greater ability to work around hardware problems).

Rockchip not transparent about presence of Cortex-A12 cores

The CPU-Z app for Android was run on the device, and shows fairly conclusive evidence that the CPU cores in the RK3288 are really Cortex-A12 cores. This is not at all surprising, given that Rockchip's foundry GlobalFoundries was the first to adopt the Cortex-A12 core and announced volume production a few months ago (very likely for the RK3288)  and the Cortex-A17 core is currently more closely associated with TSMC, although chips using it seems to have had little success ramping to full production so far.

Earlier reports (1) (2) already suggested that the chip in fact contains Cortex-A12 cores, as was originally reported based on information by Rockchip in 2013. It appears that Rockchip at some point made a marketing decision that the chip should be advertised as having Cortex-A17 cores, despite actually having Cortex-A12 cores, because it sounds better, is a higher number and is known to be faster than the Cortex-A12. This behaviour is questionable at best, and raises questions about Rockchip's company structure and culture (for example, many engineers would probably feel uncomfortable with blatantly incorrect marketing information, while a senior manager might insist on it, especially if that manager had earlier embarked on commitments with overstated specifications).

There are some similarities with the behaviour of Actions Semiconductor, a smaller Chinese player in the tablet processor market, at the start of 2013, when the ATM7029, one of the first affordable quad-core tablet processor chips, was for a long time advertised as having more attractive Cortex-A9 cores instead of the actually present, much slower (but power-efficient) Cortex-A5 cores. Actions went as far as to modify the OS kernel to hide the processor core information, although they later became more open about the actual CPU specifications. Hopefully, Rockchip will not go as far.

Despite problems, RK3288 offers high CPU and GPU performance

The early review of a TV box product with a RK3288 shows that the chip has great performance potential, even with Cortex-A12 cores, which are not significantly slower than Cortex-A17. In the TV box, the clock frequency went up to 1.8 GHz, which given the high performance per cycle of the Cortex-A12 (being in the same family as the Cortex-A15 and Cortex-A17) leads to impressive CPU performance. Performance is also helped significantly by the dual-channel memory interface, providing significantly more memory bandwidth when compared to previous Rockchip products.

Performance reported by benchmarks was not entirely consistent, with Antutu 4.x reporting a high score (but relatively low CPU integer score), while it shows strongly in the most recent Vellamo benchmark with a strong Multicore test result and Browser and Metal test results close to that of the fastest Qualcomm Snapdragon 801-based devices.

Although performance in games in practice appeared to be excellent (but with a drop-off when going from 720p to 1080p resolution), the 3DMark Ice Storm Extreme result was disappointing compared to high-end Qualcomm Snapdragon platforms. Since the RK3188 is one of the first chips to be benchmarked with (as far as can be confirmed) an Mali-T764 GPU (which is presumably equivalent to Mali-T760 MP4), this could reflect performance characteristics of the Mali-T760 GPU architecture.

Ice Storm Extreme is an OpenGL ES 2.0 benchmark that renders at 1080p and uses relatively demanding (large) textures and post-processing effects. It is possible that the memory subsystem (including the amount of memory bandwidth and the efficiency of the L2 cache and memory interface) is a bottleneck in the tested RK3288 device, as that would result in significantly lower performance when using high resolutions and large textures. Size and speed of the GPU cache memory (which is usually configurable by the chip design company) could also be involved. That practical game performance appeared to be excellent is probably helped by the fact that OpenGL ES 2.0 games tend to have limited texture size and shader complexity in order to be compatible with a wider range of devices. Ice Storm Unlimited test results (which tests performance offscreen using a fixed 1080p resolution but standard textures) would provide better comparison material.

Sources: CNXSoft, TSMC (28HPC process announcement), Allwinner (A83T announcement), CNXSoft (Rockchip product roadmap)
 
Updated September 30, 2014.