[Bug 262316] em(4) does not autonegotiate when fixed media is set

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=262316

--- Comment #5 from Stefan Eßer <se@FreeBSD.org> ---
(In reply to J.R. Oldroyd from comment #3)

I have located part of the IEEE 802.3 standard, specifically for Clause 37.

it says in 37.1.4.4:

-------------------------------------------------------------------
37.1.4.4 User Configuration with Auto-Negotiation

Rather than disabling Auto-Negotiation, the following behavior is suggested in
order to improve interoperability with other Auto-Negotiation devices. When a
device is configured for one specific mode of operation (e.g. 1000BASE-X Full
Duplex), it is recommended to continue using Auto-Negotiation but only
advertise the specifically selected ability or abilities. This can be done by
the Management agent only setting the bits in the advertisement registers that
correspond to the selected abilities.
-------------------------------------------------------------------

I.e., a device manually set to operate in a fixed mode may signal the mode
parameters to a partner, but it must not change its mode depending on what the
other side supports (even if this was required for a working data link).

If manually set to full-duplex 1000base-X for example, the device may signal
this setting and thus allows the other device to use a compatible
configuration. But the manually configured device must not switch to some other
mode, even if the other interface does not support the manually configured
mode.

Modifying the manually set parameters is in violation of the standard, and I'll
close this PR again, unless there is proof of a standard clause that supports
the change you are proposing.

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