Some updates on this issue:
We no longer get lockups as of kernel v5.17-rc1.
The console however says things like this:
[ 1.979485] mmci-pl18x 80005000.mmc: mmc2: PL180 manf 80 rev4 at 0x80005000 irq 93,0 (pio) [ 1.987943] mmci-pl18x 80005000.mmc: DMA channels RX dma0chan6, TX dma0chan7 [ 3.204496] mmc2: Card stuck being busy! __mmc_poll_for_busy [ 4.284431] mmc2: Card stuck being busy! __mmc_poll_for_busy
And after this it never finds the partitions on the card.
So the issue isn't gone, eMMC is still unusable on Skomer and Codina, but the issue no longer hangs the whole MMC/SD stack which is nicer.
Yours, Linus Walleij