My attempt to boot mainline Linux on a stock Dabao board. It works: the kernel executes in place out of RRAM, the whole system lives in 2 MiB of on-chip SRAM, and you get an interactive BusyBox shell over a serial console. It is also only that. There are basically no drivers.
| Subsystem | State |
|---|---|
| XIP kernel from RRAM, Sv32 | yes |
| Serial console | yes - SBI API over UART2 |
| Timer / clocksource / clockevent | yes - TICKTIMER via SBI, 100 kHz |
| Interactive shell, job control | yes - BusyBox as PID 1 |
| Userspace ELF, fork, mmap, /proc | yes |
| Storage of any kind | no |
| Interrupt controller driver | no - the timer IRQ is handled in M-mode |
| UART driver | no - console goes through SBI |
| USB, SDIO, SPI, GPIO, BIO, RRAM MTD | no |
| Networking | no |
| Suspend, power management | no |
| Repo | What's in it |
|---|---|
pkoscik/linux (rv32-xip) |
patches on v6.19-rc5 that make XIP work on 32-bit RISC-V |
pkoscik/xous-core (linux-mmode-stub) |
the M-mode SBI stub that boots it |
The Baochip-1x is a Rust-first SoC, and the Dabao is its evaluation board. It is designed to run Xous, a pure-Rust embedded OS, and it is not a Linux target in any sense - which is most of the fun.
| Component | Detail |
|---|---|
| CPU | VexRiscv, RV32IMAC @ 700 MHz, M/S/U privilege, MMU |
| MMU | Sv32 |
| Caches | 16 KiB I + 16 KiB D, write-through |
| RRAM | 4 MiB @ 0x6000_0000 |
| SRAM | 2 MiB @ 0x6100_0000 |
| Package | WLCSP-71 |
| Interrupts | no PLIC; a CSR-mapped array, 20 banks x 16 lines |
| Timer | no CLINT, no mtime, MTIP tied to zero in the RTL |
| Console | UDMA UART2 on PB13/PB14 |
boot0 -> boot1 -> M-mode SBI stub -> Linux (S-mode)
ROM xous xous-core XIP from RRAM
updater baremetal app data in SRAM
SBI is emulated. There is no OpenSBI port. The bao1x has no CLINT, no PMP, one hart and no time CSR, so nearly everything OpenSBI provides is either absent or unused, and a port would mean stubbing most of it out. Instead there is a small M-mode stub built as a target of the baremetal app, which inherits its linker script, signing and UF2 packaging. It also emulates the missing time/timeh CSRs, and turns TICKTIMER's alarm into an injected mip.STIP, so Linux drives its own stock timer-riscv with no kernel patch.
The kernel has to run XIP, it is not an optimisation. Code and rodata are 1463 K against 2048 K of SRAM, before page tables, slab, or a byte of userspace, so a conventional build needs more RAM for its own text than the chip has in total. Executing it out of RRAM leaves 179 K of rwdata and bss in SRAM, and about 650 K free with a shell running.
Which is awkward, because RISC-V XIP was deleted from mainline in 9b3a2be84803 on the grounds that it kept breaking and therefore had no users. This is pinned to v6.19-rc5 and will not follow the tree forward.
You need clang/lld, zig, rustup (riscv32imac-unknown-none-elf), dtc, python3, cpio and curl.
./setup.sh check tells you what is missing and refuses to continue if anything is.
git clone --recurse-submodules https://github.com/pkoscik/baochip-linux
cd baochip-linux
./setup.sh # check tools, fetch submodules, write the kernel config
./flash.sh busybox # bin/busybox-rv32-min, from source
./flash.sh demo # bin/demo-rv32, from initramfs/demo.c
./flash.sh all # stub + device tree + kernel, onto the boardUserspace has to be built first: the initramfs is compiled into the kernel image.
Note
The BAOCHIP volume only exists in bootloader mode. Hold BOOT while plugging USB or rebooting the board, and flash.sh will find it. Set VOL=out to write the images to a directory instead.
Flashing is just copying UF2 files onto a mass-storage volume - boot1 programs RRAM as the blocks arrive.
Two consoles. The Xous REPL is on /dev/ttyACM0 over USB. Linux talks on UART2, which needs a 3.3 V USB-TTL:
| Header pin | Signal | Adapter |
|---|---|---|
| 15 | PB14 / UART2TX | RX |
| 16 | PB13 / UART2RX | TX |
| 13 | GND | GND |
picocom /dev/ttyACM0 # Xous REPL
picocom -b 1000000 /dev/ttyUSB0 # Linux console[ 0.000000] Linux version 6.19.0-rc5 (clang version 22.1.8, LLD 22.1.8)
[ 0.000000] Machine model: Baochip-1x Dabao
[ 0.000000] SBI specification v1.0 detected
[ 0.000000] clocksource: riscv_clocksource: mask: 0xffffffffffffffff
[ 0.000000] sched_clock: 64 bits at 100kHz, resolution 10000ns
[ 0.005470] Memory: 1468K/2040K available (1283K kernel code, 111K rwdata,
180K rodata, 150K init, 68K bss, 340K reserved)
[ 0.038600] Run /init as init process
=== Linux on Baochip-1x ===
Linux (none) 6.19.0-rc5 riscv32 GNU/Linux
total used free shared buff/cache available
Mem: 1756 792 656 0 308 396
/bin/demo is a separate statically linked ELF rather than a BusyBox applet, because applets run inside one already-resident binary and prove much less:
== this process's memory map ==
00010000-00012000 r--p 00000000 00:02 24 /bin/demo
00012000-0001b000 r-xp 00001000 00:02 24 /bin/demo
0001b000-0001c000 rw-p 00009000 00:02 24 /bin/demo
86355000-86356000 r-xp 00000000 00:00 0 [vdso]
9d0da000-9d0fb000 rw-p 00000000 00:00 0 [stack]
== fork ==
child: pid 37, parent 36
parent: pid 36, child 37 exited with 7
== timer from userspace ==
slept 1s, clock says 1003 ms
- Baochip: https://baochip.com/ and the Dabao on Crowd Supply
- Xous Book: https://betrusted.io/xous-book/
- Dabao board files, BOM and pinout: https://github.com/baochip/dabao
- Baremetal SDK for the Dabao: https://github.com/samblenny/baochip-sdk
linux-on-litex-vexriscv: https://github.com/litex-hub/linux-on-litex-vexriscv- The unfortunate XIP removal:
9b3a2be84803