Samsung Updates Exynos 5 Octa (5420), Switches Back to ARM GPU

In the first part of our series on ARM, we mentioned that with every major microprocess design ARM tries to choose 3 licensees to get early access to technology. It's very clear that Samsung was among the early three to get ahold of Cortex A15 IP. Samsung was first on the mobile market with a Cortex A15 based SoC: the Exynos 5250 (aka Exynos 5 Dual). Featuring two cores running at up to 1.7GHz paired with an ARM Mali-T604 GPU, we first met the Exynos 5250 in Samsung's own Chromebook XE303 last October.

The next logical step would be a quad-core version, which we sort of got with the Exynos 5410 - or as it's more commonly known: Exynos 5 Octa. This part features four ARM Cortex A15 cores running at up to 1.6GHz and four ARM Cortex A7 cores running at up to 1.2GHz in a configuration ARM calls big.LITTLE. The specific implementation of big.LITTLE on Exynos 5410 is known as Cluster Migration; either the four Cortex A15 cores or four Cortex A7 cores can be active, but not both and not an arbitrary combination of cores from each island. They're either all on or all off. This is by far the easiest to implement from a software perspective, but is obviously the less interesting option from a heterogeneous SMP perspective. I'll  be talking more about this in an upcoming ARM piece.

On the graphics front, Samsung moved to Imagination Technologies for the Exynos 5410 - implementing a PowerVR SGX 544MP3 setup. The Exynos 5410 saw limited use, appearing in some international versions of the Galaxy S 4 and nothing else. Part of the problem with the design was a broken implementation of the CCI-400 coherent bus interface that connect the two CPU islands to the rest of the SoC. In the case of the 5410, the bus was functional but coherency was broken and manually disabled on the Galaxy S 4. The implications are serious from a power consumption (and performance) standpoint. With all caches being flushed out to main memory upon a switch between CPU islands. Neither ARM nor Samsung LSI will talk about the bug publicly, and Samsung didn't fess up to the problem at first either - leaving end users to discover it on their own. 

Last week Samsung teased a new, improved Exynos 5 Octa - the Exynos 5420. Today we got the first details of the new SoC. The base CPU architecture remains unchanged. Samsung outfitted the Exynos 5420 with four A15s and four A7s, presumably in the same Cluster Migration big.LITTLE configuration. Clock speeds on both clusters are a bit higher now, 1.8GHz is the top speed for the Cortex A15 cores while 1.3GHz is where the A7s top out. Note that on the Cortex A15 side this exceeds where even ARM recommends clocking Cortex A15 for smartphones as far as power efficiency is concerned, but it should be fine for tablets. There's no word on whether or not the CCI-400 bug has been fixed, but I can only assume that it has been otherwise it'd be senseless to do another Exynos 5 Octa release this close to the original. Update: It looks like the CCI bug has been fixed.

For the GPU Samsung switches back to ARM, this time using the Mali-T628 GPU in a 6-core configuration. Mali-T628 is actually a second generation implementation of ARM's Midgard GPU architecture first demonstrated with the T604. The second generation brings higher IPC and higher clocks in the same physical area as the first-gen cores, the combination of the two results in up to a 50% increase in performance. The T604 was a four-core implementation, so we should see another 50% on top of that with the move to 6-cores in the 5420. A six-core configuration is a bit odd in that we've never seen one before, but the T628 is scalable from 4 - 8 cores so it's a valid config.

On the memory interface front the Exynos 5420 retains a dual-channel LPDDR3 interface (2 x 32-bit) with support for up to 1866MHz memory, resulting in peak theoretical memory bandwidth of 14.9GBps. 

The biggest question about the new Exynos 5420 is whether or not the cache coherency issues have been worked out. The solution remains a bit on the large side for most price sensitive tablets, but it could make for an interesting use case in a higher-end tablet. In smartphones I'm still not sold on the idea of having four Cortex A15s running at up to 1.8GHz. Although big.LITTLE is one answer to the problem of getting the best of both worlds (low power and high performance), Qualcomm seems to have a pretty good solution with its Krait 300/400 cores. If Samsung were to enable one of the more interesting big.LITTLE scheduling models in its products however (e.g. big.LITTLE MP, all cores visible at once, intelligent scheduling based on perf needs) I'd be more interested.