Compute Module • New CM4 batch with alternative eMMC (PCN 50, Rayson/Foresee) fails to boot legacy 5.10 image – "Unable to mount root fs"

Hi all,

we have a custom CM4-based product that has been shipping for years with a
legacy 32-bit image (kernel 5.10.81-v7l+, firmware from Nov 2021). All CM4
modules received until recently boot this image without problems.

With the latest batch of CM4 modules (i think KW42 2025) the same image
fails to boot with a kernel panic:

VFS: Unable to mount root fs on unknown-block(179,2)

According to PCN 50 ("Alternative sources of eMMC", June 2026) this batch
appears to use the new eMMC parts (Rayson RS70B08G3S03F / RS70B16G4M15G,
Foresee FEMDNN064G-C9A61). The PCN states "Software/Firmware Changes
Required: None", which does not match our observations.

What we have verified so far:
- Current Raspberry Pi OS Bookworm (kernel 6.12.93) boots fine on the new
modules, so the hardware is OK.
- Updating only the boot firmware on our image (start4.elf, fixup4*.dat,
bcm2711-rpi-cm4.dtb, overlays/) does NOT fix it - same panic with our
5.10.81 kernel.
- cmdline.txt contains rootwait; root= uses PARTUUID and is correct.
- Old-batch modules still boot the same image fine

Questions:
1. Is there a known minimum kernel version that supports the new eMMC
parts? Ideally something in the 5.x series, since we would like to
minimize changes to our validated image.
2. Is there a specific commit (mmc core / sdhci-iproc) that could be
backported to 5.10.y?
3. If a kernel update is unavoidable, is 6.1.y (Bullseye legacy) confirmed
to work with these eMMC parts?

Thanks !

Statistics: Posted by MartinNPN — Wed Jul 29, 2026 12:15 pm

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