[Bug 236067] [em] Intel 82574L can not turn off flow control

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236067

            Bug ID: 236067
           Summary: [em] Intel 82574L can not turn off flow control
           Product: Base System
           Version: CURRENT
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: lev at FreeBSD.org

I have two systems with 82574L NICs.

One, name it G (for "generator"), is powerful enough to saturate almost 1G
links with small packets (more than 1100Kpps).

Other, name it DUT is underpowered and can not process such packet rate.

Both G and dut have "set dev.em.0.fc=0".

G try to generate traffic to DUT's MAC with netmap & pkt-gen. If G and DUT are
connected (no matter how: with direct patchcord or via dedicated switch) G
could generate only as much packets as DUT could process! If I remove DUT from
dedicated switch, G generates almost-line-rate PPS. If I replace DUT with other
system with I210 NIC, G generates same high PPS even if system with I210 can
not process it.

Looks like 82574L can not turn off flow control.

Once again:

82574L -> Switch -> 82574L - pkt-gen generates PPS limited by receiver.
82574L -> 82574L - pkt-gen generates PPS limited by receiver.
82574L -> Switch -> "blackhole" (no other devices attached to switch) - pkt-gen
generates line-rate PPS.
82574L -> I210 - pkt-gen generates line-rate PPS.
82574L -> Switch -> I210 - pkt-gen generates line-rate PPS.

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