uc-rv32ima

Run linux on various MCUs with the help of RISC-V emulator. This project uses CNLohr's mini-rv32ima RISC-V emulator core to run Linux on various MCUs such as ESP32C3, RP2040, STM32F103 etc. Although, only ESP32C3 is tested now. In theory, ESP series SoCs are supported, but except ESP32C3 I don't have other SoCs' dev boards, so I can't test them.

Why

Just for fun
When CNLohr wrote mini-rv32ima, he said he wanted to run linux on ESP32C3, I was interested in that idea too.

What's missing if we want to run linux on ESP32C3

A single RV32-IMC cpu core, well, this can be solved by patching the linux kernel to remove the 'A' extension usage
No MMU, well, this can be solved by using NOMMU
No enough memory, only 4MB flash and 400KB sram, well, this can be solved by adding one SPI PSRAM chip. However,
I can't make ESP32C3 directly executing code on PSRAM work as ESP32S3 does. If anyone knows the howto, kindly tell me, I really appreciate it;) Then we can directly run linux on ESP32C3!

So far, the idea solution is to use a RISC-V IMA emulator, and use the PSRAM as the emulator's main system memory.

How it works

It uses one 8MB SPI PSRAM chip as the system memory. On startup, it initializes the PSRAM, and load linux kernel Image(an initramfs is embedded which is used as rootfs) and device tree binary from flash to PSRAM, then start the booting.

To improve the performance, a simple cache is implemented, it turns out we acchieved 95.1% cache hit during linux booting:
- 4KB cache
- two way set associative
- 64B cacheline
- LRU

Difference from tvlad1234's pico-rv32ima

esp32c3 VS rp2040, although rp2040 will be supported too in uc-rv32ima
only one 8MB SPI PSRAM is needed
a simple cache mechanism is implemented, thus much better performance
no need sdcard

Requirements

one ESP32C3 development board
one 8 megabyte (64Mbit) SPI PSRAM chip (I used PSRAM64H) and solder it on a SOP8 adapter board, then properly connect the adapter board with MCU dev board with dupont line.

How to use

Ensure the SPI PSRAM chip pins are properly connected with your MCU board. E.g the PSRAM is connected with below GPIOs of ESP32C3, and don't forget VCC and GND pins:
- MOSI: GPIO7
- MISO: GPIO2
- CS: GPIO10
- SCLK: GPIO6
build uc-rv32ima with esp idf env
write imgs to the board's flash as following with esptool then reset the board
- 0x10000 build/uc-rv32ima.bin
- 0x8000 build/partition_table/partition-table.bin
- 0x200000 main/Image
- 0x3ff000 main/uc.dtb
In no less than 1 sec, Linux kernel messages starts printing on the USB CDC console. The boot process from pressing reset button to linux shell takes about 1 minute and 20 seconds.

mmexport1682090995693.mp4