The new custom V002 T Bios IBECC has the support for In-Band ECC: https://docs.radxa.com/en/x/x4/bios/update-bios
The option is a little bit hidden: Chipset, System Agent (SA) Configuration > Memory Configuration > In-Band ECC Support and leave In-Band ECC Operation mode 2.
The In-Band ECC uses some of the free RAM for the ECC (1/32?) so on my 8GB X4 there is total 7649220 with IBECC and 7907268 without IBECC. It is about 3%.
Interestingly, in both cases kernel is reporting ECC support:
dmesg | grep -i edac
without ECC:
[ 0.606423] EDAC MC: Ver: 3.0.0
[ 8.592191] caller igen6_probe+0x157/0x8a0 [igen6_edac] mapping multiple BARs
[ 8.592303] EDAC MC0: Giving out device to module igen6_edac controller Intel_client_SoC MC#0: DEV 0000:00:00.0 (INTERRUPT)
[ 8.592378] EDAC igen6 MC0: HANDLING IBECC MEMORY ERROR
[ 8.592381] EDAC igen6 MC0: ADDR 0x7fffffffe0
[ 8.592407] EDAC igen6: v2.5.1
with ECC:
[ 0.625684] EDAC MC: Ver: 3.0.0
[ 8.804176] caller igen6_probe+0x157/0x8a0 [igen6_edac] mapping multiple BARs
[ 8.809573] EDAC MC0: Giving out device to module igen6_edac controller Intel_client_SoC MC#0: DEV 0000:00:00.0 (INTERRUPT)
[ 8.812027] EDAC igen6: v2.5.1
The same with edac-utils --report=short
without ECC:
mc0: Correctable errors: 0
mc0: Uncorrectable errors: 0
Total CE: 0
Total UE: 0
with ECC:
mc0: Correctable errors: 0
mc0: Uncorrectable errors: 0
Total CE: 0
Total UE: 0
There is a slight decrease in the memory speed. As measured by stream/triad.
Without ECC 12450.9 MB/s
With ECC 9918.1 MB/s
It is a 20% decrease.
So as a conclusion, you can now switch on the In-band ECC on X4. It will cost you about 20% of memory bandwidth and 3% of total RAM.