freebsd stable - Feb 2021 - Problems with SDHCI on Denverton SoC on stable/12 and stable/11

On a Denverton SoC platform with eMMC, under heavy load I see a
"Controller timeout" error, followed by a register dump, and then
every
operation performed after returns a timeout from the MMC (Error
indicated: 1 Timeout).  The only way to recover is to reboot the
machine.  

This also occurs when sending some vendor commands while untarrring a
large tarball on the eMMC.

I've analyzed the code, compared quirks against Linux, which doesn't
seem to even be affected, and so far have come up empty.

So, my questions become: 
What can cause it to get into this state?  And why would it be unable
to recover?

I've seen this on both mmcsd and mmccam.  I haven't tested on HEAD, but
have no reason to expect a difference, given there haven't been many
changes that have not been MFC'd back to stable/12 in the SDHCI and MMC
areas.

Thanks,
Justin