Update, 9/7/19, 4:38 a.m. PT: Some extra information about Gen12 has come out. According to a GitHub merge request, Gen12 will be one of the biggest ISA updates in the history of the Gen architecture and removal of data coherency between register reads and writes: “Gen12 is planned to include one of the most in-depth reworks of the Intel EU ISA since the original i965. The encoding of almost every instruction field, hardware opcode and register type needs to be updated in this merge request. But probably the most invasive change is the removal of the register scoreboard logic from the hardware, which means that the EU will no longer guarantee data coherency between register reads and writes, and will require the compiler to synchronize dependent instructions anytime there is a potential data hazard.”Twitter user @miktdt also noted that Gen12 will double the amount of EUs per subslice from 8 to 16, which likely helps with scaling up the architecture.
Original Article, 9/6/19, 9:34 a.m. PT:
Some information about the upcoming Gen12 (aka Xe) graphics architecture from Intel has surfaced via recent Linux kernel patches. In particular, Gen12 will have a new display feature called the Display State Buffer. This engine would improve Gen12 context switching.
Phoronix reported on the patches on Thursday. The patches provide clues about the new Display State Buffer (DSB) feature of the Gen12 graphics architecture, which will find its way to Tiger Lake (and possibly Rocket Lake) and the Xe discrete graphics cards in 2020. In the patches, DSB is generically described as a hardware capability that will be introduced in the Gen12 display controller. This engine will only be used for some specific scenarios for which it will deliver performance improvements, and after completion of its work, it will be disabled again.
Some additional (technical) documentation of the feature is available, but the benefits of the DSB are described as follows: “[It] helps to reduce loading time and CPU activity, thereby making the context switch faster.” In other words, it is new engine that offloads some work from the CPU and helps to improve context switching time.
Of course, the bigger picture here is the enablement for Gen12 that has been going on in the Linux kernel (similar to Gen11), which is especially of interest given that it will mark the first graphics architecture from Intel to get released as a discrete GPU. To that end, Phoronix reported in June that the first Tiger Lake graphics driver support was added to the kernel, with more batches in August.
Tiger Lake and Gen12 Graphics: What we know so far
With the first 10th Gen (10nm) Ice Lake laptops nearly getting into customers’ hands after almost a year of disclosures, Intel has already provided some initial information about what to expect for the 11th-Gen processors next year, codenamed Tiger Lake (with Rocket Lake on 14nm still in the rumor mill). Ice Lake focused on integration and a strong CPU and GPU update, and with the ‘mobility redefined’ tag line, Tiger Lake looks to be another 10nm product solely for the mobile market.
On the CPU side, Tiger Lake will incorporate the latest Willow Cove architecture. Intel has said that it will feature a redesigned cache, transistor optimizations for higher frequency (possibly 10nm++), and further security features.
While the company has been teasing its Xe discrete graphics cards for even longer than it has talked about Ice Lake, details remain scarce. Intel said it had split the Gen12 (aka Xe) architecture in two microarchitectures, one that is client optimized, and another one that is data center optimized, intending to scale from teraflops to petaflops. From a couple of leaks from 2018, it is rumored that the Arctic Sound GPU would consist of a multi-chip package (MCP) with 2-4 dies (likely using EMIB for packaging), and was targeted for qualification in the first half of next year. The leak also stated that Tiger Lake would incorporate power management from Lakefield.
The MCP rumor is also corroborated by some recent information from an Intel graphics driver, with the (Discrete Graphics) DG2 family coming in variants of what is presumably 128, 256 and 512 executions units (EUs). This could indicate one, two and four chiplet configurations of a 128EU die. Ice Lake’s integrated graphics (IGP) has 64EUs, and the small print from Intel’s Tiger Lake performance numbers revealed that it would have 96EUs.
A GPU with 512EUs would have in the neighborhood of 10 TFLOPS, which does not look sufficient to compete with 2020 GPU offerings from AMD and NVIDIA in the high-end space. However, not all the gaps are filled yet. A summary chart posted by @KOMACHI_ENSAKA talks about three variants of Gen12:
- Gen12 (LP) in DG1, Lakefield-R, Ryefield, Tiger Lake, Rocket Lake and Alder Lake (the successor of Tiger Lake)
- Gen12.5 (HP) in Arctic Sound
- Gen12.7 (HP) in DG2
How those differ is still unclear. For some speculation, the regular Gen12 probably refers simply to the integrated graphics in Tiger Lake and other products. However, the existence of DG1 and information about Rocket Lake could indicate that Intel has also put this IP in a discrete chiplet. This chiplet could then serve as the graphics for Rocket Lake by packaging it together via EMIB. If we assume Arctic Sound is the mainstream GPU, then Gen12.5 would refer to the client optimized version and Gen12.7 to the data center optimized version of Xe. In that case, the amount of EUs Intel intends to offer to the gaming community remains unknown.
Moving to the display, it remains to be seen if the Display State Buffer is what Intel referred to with the ‘latest display technology’ bullet point, or if the DSB is just one of multiple new display improvements. Tiger Lake will also feature next-gen I/O, likely referring to PCIe 4.0.
Given the timing of Ice Lake and Comet Lake, Tiger Lake is likely set for launch in the second half of next year.
Display Improvements in Gen11 Graphics Engine
With display being one of the key pillars of Tiger Lake, it is worth recapping the big changes in Gen11’s display block (we covered the graphics side of Gen11 previously).
As the name implies, the display controller controls what is displayed on the screen. In Ice Lake’s Gen11, it is part of the system agent, and it had some hefty improvements. The Gen11 display engine introduced support for Adaptive-Sync (variable display refresh rate technology) as well as HDR and a wider color gamut. The Gen11 platform also integrated the USB Type-C subsystem, and the display controller has specific outputs for Type-C, and it can also target the Thunderbolt controller.
Intel also introduced some features for power management, most notably Panel Self Refresh (PSR), a technology first introduces in the smartphone realm. With PSR, a copy of the last frame is stored in a small frame buffer of the display. In the case of a static screen, the panel will refresh out of the locally stored image, which allows the display controller to go to a low power state. As another power-saving feature, Intel added a buffer to the display controller, to fetch pixels for the screen into. This allows the display engine to concentrate its memory accesses into a burst, meanwhile shutting down the rest of display controller. This is effectively a form of race to halt, reminiscent of the duty cycle Intel introduced in Broadwell’s Gen8 graphics engine.
Lastly, on the performance side, in response to the increasing monitor resolutions, the display controller now has a two-pixel-per-clock pipeline (instead of one). This reduces the required clock rate of the display controller by 50%, effectively trading die area for power efficiency (since transistors are more efficient at lower clocks as voltage is reduced). Additionally, the pipeline has also gained in precision in response to HDR and wider color gamut displays. The Gen11 controller now also supports a compressed memory format generated by the graphics engine to reduce bandwidth.
