[Bug 201753] AMD-Vi: Unable to write to IOMMU perf counter

bugzilla-daemon
2021-03-19T20:44:24Z

--- Comment #32 from David Coe (david.coe@live.co.uk) ---
I'm pretty sure this is because IOMMU (for some Ryzens) unlatches after 5 tries
on cold boot and 6 tries on warm. Suravee, alas, has set his patch at 5 x 20
msecs just on the 'dither' point!

Some time mid-next week I'll lay my sticky hands on a 4700U laptop. I have
images re-compiled for Ubuntu's kernel 5.8.0-45 (20.10) and 5.11.0-11 (21.04)
patched at 25 x 20 msecs and logged. Ditto with Alex's oh-so-simple patch.

If all goes well(!), I'll draw up a nice, comparative table of mine and
everybody else's data so we can draw some conclusions. If you scan the
Debian/Ubuntu kernel changelog, you'll find a long list of 'cherry-picked'
patches for both AMD and Intel for IOMMU. It is very much a work-in-progress
still!

Best regards and many thanks all for input.

David