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

bugzilla-daemon
2021-02-26T21:18:20Z

--- Comment #14 from David Coe (david.coe@live.co.uk) ---
Good evening, all!

Two extra results from the Ryzen 2400G:

1. 5 x 25 msec loop on Ubuntu 21.04, kernel 5.10.11 and Suravee's original
patch. Works fine!

2. 10x 20 msec loop on Ubuntu 20.10, kernel 5.8.18 with both Suravee and Alex's
patches. Also fine!

As expected,
AMD-Vi: IOMMU performance counters supported
moves to after
Adding to iommu group ...

This means that, for this CPU, the power-gating needs between 100 - 125 msec to
disable. A logging printf would have got me there faster (not overly fond of
kernel rebuilds) but the missing information is the maximum settling-time
needed across the full Ryzen range. Paul's suggestion of increasing the loop
count to 10 gives 200 msec maximum headroom. Surely must be enough?

Now, all we need is a work-around for the PCI INT A: not connected diagnostic
:-).

Many thanks good people. Keep safe.

David