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RISC-V gains momentum as it moves from MCUs to Linux-friendly SoCs

Feb 8, 2018 — by Eric Brown — 1629 views

The open source RISC-V ISA has evolved quickly into silicon, thanks to help from companies like SiFive and Microsemi. SiFive’s HiFive Unleashed board should arrive less than two years after SiFive announced its first Linux-driven Freedom SoCs.

It’s been two years since the open source RISC-V architecture emerged from computer labs at UC Berkeley and elsewhere and began appearing in soft-core implementations designed for FPGAs, and over a year since the first commercial silicon arrived. So far, the focus has primarily been on MCU-like processors, but last October, SiFive announced the first Linux-driven RISC-V SoC with its quad-core, 64-bit bit Freedom U540 (AKA U54-MC Coreplex). A few days ago at FOSDEM, SiFive opened pre-sales for an open source HiFive Unleashed SBC that showcases the U540.



HiFive Unleashed (left) and Freedom U540 block diagram
(click images to enlarge)

The 28nm fabricated U540 SoC that drives the SBC is not only the first multi-core and Linux-ready RISC-V processor, but the first to provide cache coherence. In addition to the four 1.5GHz U54 RV64GC CPU cores, the SoC includes an E51 RV64IMAC management core. What you won’t find is a GPU or VPU. However, third parties are encouraged to integrate coprocessors and peripherals with the help of an open source TileLink interface bus.

The $999 HiFive Unleashed is available on Crowd Supply, with shipments due on June 30. The HiFive Unleashed board integrates a U540 SoC, 8GB of DDR4 RAM, and 32MB quad SPI flash. You also get a microSD slot, a GbE port, and an FMC expansion. Schematics and layout files will be available.

 
RISC-V jumps out to a fast start

It’s too early to say whether RISC-V will ever rival ARM or x86, let alone match the reach of fading architectures such as MIPS and PowerPC. So far, however, there has been a surprising willingness on the part of major computer and semiconductor vendors to experiment with the new ISA. RISC-V Foundation Platinum members include heavy hitters like Draper, Google, Hewlett Packard Enterprise, IBM, Microsemi, Oracle, Microsoft, Nvidia, and Qualcomm.


RISC-V logo

The widespread interest is partially due the fact that RISC-V has a free and permissive license that enables third parties to use the ISA to develop proprietary implementations. The expectation, however, is that most RISC-V SoCs will follow the early players’ leads in providing open source implementations.

RISC-V’s timing seems to be right, as the growing acceptance of open source software and hardware logically leads to a desire to open up of the processor. Opaque chip designs often create obstacles and blind spots — not only for open source projects, but also semiconductor vendors. Conceivably, hidden issues such as Intel’s Spectre and Meltdown security problems might have come to light more quickly in the open source spotlight.

There’s also a sense that the dominance of two closed source architectures — ARM and Intel x86 — is limiting innovation, slowing time to market, and increasing cost. In addition, RISC-V advocates claim that both ARM and x86 platforms are burdened by legacy code. By comparison, RISC-V is starting with a clean slate of modern components.


Krste Asanović

SiFive was founded by RISC-V inventors including Yunsup Lee, Andrew Waterman, and Krste Asanović, the current Chairman of the RISC-V Foundation, based in part on two earlier open source RISC ISAs: SPARC and OpenRISC. In 2014, Asanović and fellow UC Berkeley professor David Patterson, who coined the term RISC, posted a white paper on RISC-V, and development progressed rapidly from there.

SiFive has been the major RISC-V hardware player, while Microsemi has led the way in developing soft-core implementations that can run on FPGAs for prototyping. In Nov. 2016, SiFive announced an open source, Arduino compatible HiFive1 development board for its Freedom E300 — an MCU-like RISC-V design with an E31 Coreplex core designed to run FreeRTOS. Developers could also use Microsemi’s soft-core SmartFusion 2 SoC FPGA to develop for the E300.


Arduino Cinque

The HiFive1 was followed last May by an Arduino Cinque board based on the HiFive1, jointly developed by SiFive and Arduino. The key addition was an Espressif ESP32 SoC that supplies WiFi and Bluetooth.

SiFive and Microsemi aren’t the only vendors invested in RISC-V. Andes, Bluespec, Codasip, and Cortus sell RISC-V core IP that can be used to develop MCU-like SoCs. (SiFive also sells IP in addition to silicon and development services.) Like Microsemi, Rumble and Development and VectorBlox offer soft cores that run on FPGAs.

According to a recent summary of RISC-V developments posted on Electronic Design by Microsemi’s Ted Marena, Vice Chair of the RISC-V Foundation’s RISC-V Marketing Committee, software support is also evolving. In the Linux realm, RISC-V support was added to the GNU/GCC and GNU/GDB toolchains last May. In addition, writes Marena, “several flavors of Linux are supported, including Yocto,” based on Linux 4.6. RISC-V support appears to be headed for a merge into kernel 4.14, which “means RISC-V will soon be a mainline platform in Linux.”

The HiFive Unleashed Crowd Supply page does not have much to say about software aside from noting Linux compatibility, and SiFive did not respond to our queries for more details. When the U540 SoC was announced, however, SiFive said the SoC would be supported by “a rich SDK with demo software and an easy-to-install binary toolchain.” Standard development and debug tools such as OpenOCD, GDB, and an Eclipse IDE, were also said to be in the works.

Performance questions should be partially answered when the HiFive Unleashed appears this Spring. According to an EETimes story posted in October, the “single issue” U54 core is expected to lag the performance of the “dual issue” Cortex-A53. Marena, however, claims that “the modularity of the RISC-V ISA design enables implementations to be more efficient than legacy ISAs such as x86 or ARM.”

In conjunction with the Embedded Linux Conference + OpenIoT event to be held in Portland, Oregon on Mar. 12-14, SiFive will host the first hackathon for the HiFive Unleashed. Registered SiFive Developers will be able to attend the Portland event to try out the SBC. (More information may be found on the HiFive Unleashed product page.) The ELC conference itself includes a presentation by Comcast’s Khem Raj called OpenEmbedded Yocto on RISC-V — New Kid on the Block.

This article is copyright © 2018 Linux.com and was originally published here. It has been reproduced by this site with the permission of its owner. Please visit Linux.com for up-to-date news and articles about Linux and open source.

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