An attempt to port MMU Linux to S31's architecture (RV32IMAFBCNSUX).
Module tested: ESP32-S31-WROOM-3 E1H16R16V (ESP32-S31 Core Board).
- Busybox init
- USB UART I/O
Everything else not listed are not working (for now). Also check out these command outputs.
- [] PSRAM cache
- [] Flash driver(?)
Todo: add more todos
(For more hardware references, see docs/ folder)
This port was done before S31 TRM is available, therefore these guessworks were made:
S31 uses CLIC and CLINT similar to P4. Linux expects PLIC. Therefore a custom CLIC driver is needed. I referenced this CLIC patch from TODO: I forgot where the patch come from to get the CLIC working.
Also, standard RISC-V interrupt CSRs are not usable, presumably because, from P4's TRM, CLINT interrupts are routed to CLIC and mtvec.MODE is hardwired to 0x3 (CLIC mode). Patches needed to make OpenSBI interrupts work.
S31's supervisor mode is not standard and has absolutely no usage in ESP-IDF so a lot of these CSR uses were mostly guessed from either P4's TRM or CSR probing (see docs/). What is sclicbase? sie is illegal instr but mie is not, but hardwired?
S31 implemented SCLIC (Supervisor CLIC?) which is confusing to say at least. According to all laws of esp-idf, mcliccfg.NMBITS is not writable. IT IS NOT! And setting it to 0b01 enables writes to the clicintattr[i].MODE field and thus enabling the use of S-mode interrupts.
To save the precious 16MB PSRAM memory, OpenSBI was modified to use XIP in flash and internal SRAM (hence the 3915901 KB firmware size (obviously wrong) in OpenSBI banner).
In mainline linux, XIP support on RISC-V was removed, so 6.12 was used instead which has proper XIP support.
(as of 7/3/26)
# cat /proc/cpuinfo
processor : 0
hart : 0
isa : rv32imafc_zicntr_zicsr_zifencei_zca_zcf_zbb
mmu : sv32
uarch : espressif,esp32s31
mvendorid : 0x612
marchid : 0x80000003
mimpid : 0x1
hart isa : rv32imafc_zicntr_zicsr_zifencei_zca_zcf_zbb
# uname -a
Linux (none) 6.12.0-00036-gdccf9c1b2c4f-dirty #30 Thu Jul 2 23:48:23 CST 2026 riscv32
# free -h
total used free shared buff/cache available
Mem: 15004 1892 12784 0 328 12108
Swap: 0 0 0
# cat /proc/cpuinfo
processor : 0
hart : 0
isa : rv32imafc_zicntr_zicsr_zifencei_zca_zcf_zbb
mmu : sv32
uarch : espressif,esp32s31
mvendorid : 0x612
marchid : 0x80000003
mimpid : 0x1
hart isa : rv32imafc_zicntr_zicsr_zifencei_zca_zcf_zbb
# ps -aux
1 0 380 S /bin/sh -i
2 0 0 SW [kthreadd]
3 0 0 SW [pool_workqueue_]
4 0 0 IW [kworker/0:0-eve]
5 0 0 IW< [kworker/0:0H]
6 0 0 IW [kworker/u4:0-ev]
7 0 0 IW< [kworker/R-mm_pe]
8 0 0 SW [ksoftirqd/0]
9 0 0 SW [kdevtmpfs]
10 0 0 IW< [kworker/R-inet_]
11 0 0 SW [oom_reaper]
12 0 0 IW< [kworker/R-write]
13 0 0 SW [kcompactd0]
14 0 0 IW< [kworker/R-kbloc]
15 0 0 SW [kswapd0]
16 0 0 IW [kworker/0:1-pm]
17 0 0 IW [kworker/u4:1-ev]
18 0 0 SW [khvcd]
19 0 0 IW< [kworker/R-mld]
20 0 0 IW< [kworker/R-ipv6_]
34 0 380 R ps -aux
# cat /proc/meminfo
MemTotal: 15004 kB
MemFree: 12788 kB
MemAvailable: 12112 kB
Buffers: 0 kB
Cached: 328 kB
SwapCached: 0 kB
Active: 4 kB
Inactive: 32 kB
Active(anon): 4 kB
Inactive(anon): 32 kB
Active(file): 0 kB
Inactive(file): 0 kB
Unevictable: 328 kB
Mlocked: 0 kB
SwapTotal: 0 kB
SwapFree: 0 kB
Dirty: 0 kB
Writeback: 0 kB
AnonPages: 52 kB
Mapped: 208 kB
Shmem: 0 kB
KReclaimable: 0 kB
Slab: 756 kB
SReclaimable: 0 kB
SUnreclaim: 756 kB
KernelStack: 352 kB
PageTables: 24 kB
SecPageTables: 0 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 7500 kB
Committed_AS: 304 kB
VmallocTotal: 524288 kB
VmallocUsed: 12 kB
VmallocChunk: 0 kB
Percpu: 32 kB